The International Linear Collider: A Detector Physicist ... · George Gollin, The International...

George Gollin, The International Linear Collider

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IPhysicsPIllinois 1 ....

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The International Linear Collider: A Detector Physicist Describes Accelerator

Physics to String Theorists

George GollinDepartment of Physics

University of Illinois at Urbana-Champaign


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Doing something different


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Classical physics as a research tool

I am really a detector physicist. My usual tool set:

• engineering (mixed-signal electronics and mechanical devices)• software (large analysis codes)• theory/phenomenology (results from quantum field theory)

I have been concentrating on ILC accelerator physics since mid-2002. The tools are different:

• engineering (power RF and acoustics)• software (small modeling codes and test beam data analyses)• theory/phenomenology (classical mechanics, electrodynamics)


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Doing something different

The work is well-suited to undergraduate participation. In particular, small codes and classical physics permit inexperienced students to work productively after only a few weeks.

So far: • eight undergraduates, working on two separate projects• one senior thesis, with a second in the pipeline

The students participate in planning, managing our Fermilab testbeam running, and developing models for systems we study.

It is a great deal of fun, but now we need serious support.


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We have an impact

We have been (we were the first) proponents for a small ILC damping ring.

The original damping ring design had a circumference of 17 km.

The UIUC/Fermilab/Argonne design has a circumference of 6 km.

The baseline design report specifies 6 km damping rings.

!!!


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A linear collider


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A highly simplified view of an e+e- linear collider

e± source

main linac

120 keVpreaccelerator

damping ring

bunch compressor

5 GeV

5 GeV

final focus

detector

dump

250 - 500 GeV

collimation


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Fields of a charge in uniform motion

and ?E E E Eγ⊥ ⊥′ ′= =

What do we mean when we say


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A group of moving observers prepares to measure EA grid of sensors moves past a stationary charge. Sensor clocks are synchronized in their rest frame; each will measure E when t = 0.

qframe F´

? ?

?

?

frame Fβ = 0.8

Sensor clocks are NOT synchronized in the rest frame of q.


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Moving observer clocks are not synchronizedMoving sensor clocks are not synchronized in the rest frame of the charge. Aft sensor clock is the first to read zero. It is far from q.

this sensor records E now

frame F´

frame F

Recall thatE E′=

q


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Some measurements are closer to q than others

this sensor records E

this sensor records E

q

Moving transverse clocks are closer to q at t = 0 when they record E.

Recall thatE Eγ⊥ ⊥′=


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frame F´

frame Fq

E E′=

Unequal distances to charge, , and all contribute. E E′= E Eγ⊥ ⊥′=

E Eγ⊥ ⊥′=

Note that the transverse clocks had been closer to q when they made their measurements.

E = E′ doesn’t mean E is unchanged along v


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Locations of the measurements, from q rest frame

Locations of field measurements, as viewed from rest frame of q.

q

This (green) clock is far away when it makes a measurement

This (green) clock is close to q when it makes a measurement


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The electric field in the frame in which q is in motion is seen to be

( )( )3 22 2 2 20

14 1 sin

qErπε γ β θ

=−

2 20

1 as 04

qEr

θγ πε

→ → 20

as 904

qEr

γ θπε

→ →

q

θ

E ~ 1/γ2 and E ~ γ


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Fields of a relativistic charge in uniform motionField lines, schematically:

Note that the total number of field lines is the same in both cases.


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ILC bunches


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ILC bunch parameters

Bunch parameters continue to evolve. Here’s a typical set.

bunch energy 250 GeV

bunch charge 2×1010 e± (3.2 nC)

σy 5.7 nmσz 300 μm

σx 655 nm

number of bunches 2820

y

xz

…and 458 times longer than it is wide.

top view: beam traveling to the right

A bunch is 115 times wider than it is thick… y

xzbeam coming out of the page


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Fields near an ILC bunchBunch is highly relativistic. (In its rest frame it is ~147 m long!)

EB

EB

e-

The fields just above/below the bunch are large in the lab frame:11~ 1.5 10 V/m = 150 V/nm

~ 500 TEB

×

Energy density in the fields is appreciable: u ~ 1 keV/nm3


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Bunches focus each other during collision

Inside a bunch the focusing force increases with (vertical) distance from the horizontal median plane of the bunches since B increases

force is appreciable and compresses the bunches as they collide.v B×

The effect is significant: a naïve estimate shows an e± will oscillate through the median plane a few times while the bunches cross.

e+.. . . . .. ... . ...

. . . ... ... ... ........ .... .... ...... .. ... ... ....... . . .. . . .. . . ....... .... ... ...... . ... ........ .... ... ..... . . .. . . .....

.. .... .... .. ...... .. .... .... .... ........ .. ..... . . . ........ .... .. .. .... .... .... . ........

........ .... .... . .... .... ... .. . .. .... ..

.... .... .. ...... .. .... .... .... ........ .. ...... .. .... .... .... .... .... .... .... . ................ .... .... . .... .... ... .

. .. . . ... ... ... ........ .... .... ...... . . . .. .. . . . ..... . . .. . . .. . . .. ..... .... ... ...... . ... ..

..... . . ... . .. . .... . . .. . . .......

...... .. .... ..

.... .... .. ...... .. .... .... .... ........ .. ...... .. .... .... .... .... .... .... .... . ................ .... .... . .... .... ... .

. .. . . ... ... ... ........ .... .... ...... . . . .. .. . . . ..... . . .. . . .. . . .. ..... .... ... ...... . ... ..

..... . . ... . .. . .... . . .. . . ........ ... . ...

. . . ... ... ... ........ .... .... ...... .. ... ... ....... . . .. . . .. . . ....... .... ... ...... . ... ........ .... ... ..... . . .. . . .....

.. .... .... .. ...... .. .... .... .... ........ .. ..... . . . ........ .... .. .. .... .... .... . ........

........ .... .... . .... .... ... .. .

e-

B B B

B B B


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Particles oscillate around the horizontal median plane

From Nick Walker’s USPAS Santa Barbara (2003) material:http://www.desy.de/~njwalker/uspas/coursemat/pp/unit_7.ppt

http://www.desy.de/~njwalker/uspas/coursemat/pp/unit_7.ppt


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Cool bunch collision simulation by Andrei Seryi

http://www.slac.stanford.edu/~seryi/LCD_May28_2002/web/zxy.gif

Bunch geometry here isn’t quite the same as current ILC specs

play the animation…

http://www.slac.stanford.edu/~seryi/LCD_May28_2002/web/zxy.gif


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Beam-beam effect improves luminosity

It also pitches the beams vertically if they don’t hit head-on, and generally makes a mess of things after the bunches cross.

Approximate the vertical restoring force on an electron as linear (note that change of variable to z = ct):

( ) ( )y z ky z′′ ≈ −

“Disruption parameter” Dy characterizes how strong this is:

22

3 so that ( ) ( ) 3

yy z

z

DD k y z y zσ

σ′′≡ ≈ −

It’s a like a spring constant, scaled by σz2 to make it dimensionless.


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Beam position monitors


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Fields of a relativistic bunch inside a beam pipe

γ = 4.9×105 and ( )( )3 22 2 2 2

0

14 1 sin

qErπε γ β θ

=−

Field at ±π/2 is γ times stronger than for a charge at rest.

The field falls off appreciably when θ changes by ~1/γ.

Field lines terminate on the image charge that travels along the beam pipe as a bunch as moves through the accelerator.


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Image charge

Image charge is concentrated in a short cylinder of the same z extent as the bunch. It travels along the vacuum pipe, keeping pace with the bunch.

Magnitudes of image and bunch charges are identical: 3.2 nC.

Field at the inner wall of a beam pipe with 2 cm radius is 2.8 MV/m.

e+

------------------------------------------

r

2×1010 e± (3.2 nC)


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Pickup electrode for a beam position monitor

electrically isolated “button”

image charge traveling along inner wall of

beam pipe with radius r

- - - - - - - -

- - - - - - - -

signal cable

signal current

The 3.2 nC image charge is spread over a band of circumference 2πr.

A BPM button with diameter dwill see ~0.5 nC × d/r if a bunch is perfectly centered.

t

I

~1 kV into 50 Ω


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Position measurement with a BPM

Difference in signals is proportional to offset of the bunch from the center of the beam pipe. Sub-micron resolution is possible.

(x, y)


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Fermilab AØ BPM

We are using the 16 MeV e-

beam at the AØ photoinjector lab for damping ring kicker tests.

Here is one of the ~50 μm resolution BPMs we use.

Our bunch charge is about 10% of an ILC bunch.


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Thinking in new ways

I am used to thinking of systems designed to see single “minimumionizing” particles (singly charged, relativistic particles): wire chambers, silicon tracking devices, scintillation counters, and photomultiplier tubes. Best resolutions of dozens of μm and hundreds of ps are typical for these kinds of devices.

With so much charge present in a bunch, there are other ways to measure position and time that are new to me.


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“Phase detector” for bunch timing


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Using the AØ RF system for timingThe AØ photoinjector lab uses a 1.3 GHz RF system to power its accelerating structure. Everything is nicely synchronized to a master oscillator that generates the original 1.3 GHz signal.

We can determine timing of beam-induced events with respect to the RF clock with (to me) amazing precision. Here’s how…


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Phase detectorThe device produces a very short bipolar signal when a bunch passes through it.

Dispersion in a coaxial cable broadens the signal to a full width of ~1 ns.

t

V

~1 ns

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Phase detector signal path

resonant structure

RF mixer

1240 MHz

1300 MHz

resonant structure

60 MHz + 2540 MHzLow pass

V

phase detector signal

1300 MHz AØ RF

V


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Doing almost nothing yields 100 ps precision

Mixer output contains sum and difference frequencies. A low-pass filter transmits only the 60 MHz component.

( ) ( )

[ ]( ) [ ]( )1300 1240

1300 1240 1300 1240

sin sin1 1cos cos2 2

t t

t t

ω ω φ

ω ω φ ω ω φ

− =

− + − + −

Note that φ appears unaltered in the 60 MHz term.

φ = 90° in the 1240 MHz signal (202 picoseconds) maps into φ = 90°in the 60 MHz signal (4.17 nsec).

Timing accuracy is 1240/60 times better than scope precision.


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AØ data

timeDow

n-m

ixed

sign

al v

olta

ge

20 ns

How well we can determine the relative start times for each of the displayed signals? (Probably to better than 1 ns.) Divide by 1240/60 to obtain bunch timing precision.


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Radiation from accelerating charges


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Retarded time is (distance to charge) / c

observer: time t( )ˆ tε ′

( )v t′

( )Qr t′ ( )Qr t r( )Retarded time: Qt t r r t c′ ′= − −

Potentials measured by the observer depend on where the movingcharge was, not where it is right now: ( ) ( ), not .Q Qr r t r r t′− −


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Calculating potentials using retarded timeIncluding retardation, the vector and scalar potentials are

( ) ( ) 3

0all space

, 1, 4

r t r r cV r t d r

r rρ

πε

′ ′− −′=

′−∫

( ) ( ) 30

all space

, ,

4J r t r r c

A r t d rr r

μπ

′ ′− −′=

′−∫

It is beautiful in its simplicity. But we differentiate the potentials to calculate E and B, so retardation effects complicate the calculation:

E V A t= −∇ − ∂ ∂


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Differentiating the potentials to obtain E and B

The result of the differentiation is remarkable: the solution includes electromagnetic radiation.

( )321 only0

1ˆ4 1r

Q

QaEc v cr r πε ε

⊥′= −′ ′′ − ⋅−

ε̂ ′

v′

For relativistic motion the term dominates the angular dependence so that radiation is emitted primarily along the direction of motion.

The rms width of the radiated power is θrms = 1/γ.

( )3ˆ1 v cε ′ ′− ⋅


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Calculate power radiated in the usual way. For synchrotron radiation we have .a v⊥

Radiated power (GeV/sec) is4

324.23 10 EP

ρ= ×

E is the electron’s energy (GeV), ρ is its trajectory’s radius of curvature.

Radiated energy (GeV) per orbit is4

58.85 10 EEδρ

−= ×

Electromagnetic radiation, in particular when a v⊥


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If we use the same magnets to build a bigger ring…

• ring size: ρ ∝ E• synchrotron power loss per bunch: P ∝ E4/ρ2

• number of bunches in ring: Ν ∝ ρ

Cost ~ ($¥€ per unit length) × E + ($¥€ per MW RF) × E3

Linac cost scales linearly with E, but bunches only collide once

Linear vs. circular machines


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Beamstrahlung


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Vertical oscillations cause beamstrahlung

Beam-beam induced synchrotron radiation is called beamstrahlung.Let’s estimate its importance using classical electrodynamics:

.. . . . .. ... . .... . . ... ... ... ........ .... .... ...... .. ... ... ....... . . .. . . .. . . ....... .... ... ...... . ... ..

...... .... ... ..... . . .. . . ....... .... .... .. ...... .. .... .... .... ........ .. ..... . . . ....

.... .... .. .. .... .... .... . ................ .... .... . .... .... ... .

. . .. .... ...... .... .. ...... .. .... .... .... ........ .. ...... .. .... .... .... .... .... .... .... . ........

........ .... .... . .... .... ... .. .. . . ... ... ... ........ .... .... ...... . . . .. .. . . . ..... . . .

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..

...... .. .... ...... .... .. ...... .. .... .... .... ........ .. ...... .. .... .... .... .... .... .... .... . ........

........ .... .... . .... .... ... .. .. . . ... ... ... ........ .... .... ...... . . . .. .. . . . ..... . . .

. . . .. . . .. ..... .... ... ...... . ... ....... . . ... . .. . .... . . .. . . .....

... ... . .... . . ... ... ... ........ .... .... ...... .. ... ... ....... . . .. . . .. . . ....... .... ... ...... . ... ..

...... .... ... ..... . . .. . . ....... .... .... .. ...... .. .... .... .... ........ .. ..... . . . ....

.... .... .. .. .... .... .... . ................ .... .... . .... .... ... .

. .B B B

B B B

e+

B ~ 500 T; radius of curvature of a 250 GeV e+ is 1.66 m (!!)


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Beamstrahlung radiated power

Beamstrahlung broadens the energy spectrum at the point of annihilation and adds to the uncertainty in c.m. energy of a collision.

A real calculation yields

Radiated power (GeV/sec) is4

3 1224.23 10 6 10 GeV/secEP

ρ= × = ×

An electron takes about 1.7 ps to pass through the positron bunch. So it might lose as much as 10 GeV due to beamstrahlung. (!!)

http://www.slac.stanford.edu/econf/C980914/papers/F-Th24.pdf

7.0 GeV, 1.65E n eγ±Δ = =

http://www.slac.stanford.edu/econf/C980914/papers/F-Th24.pdf


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Characteristic beamstrahlung photon energyMake a rough estimate of the typical beamstrahlung photon energy using classical electrodynamics. 1/γ

1/γ

observer sees all radiation emitted between these two points

cone of synchrotron

radiation

Observable radiation is emitted along an arc of length s = Δθ/r = 2/(γρ). It arrives during a time interval Δt = s(1/v – 1/c) = ρ / (cγ 3).

ωc = 2π/Δt and Ephoton ~ hωc yields Ephoton ~ 8.7 GeV

Exact (QED) expression for ωc is 50% larger.


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Beamstrahlung-induced problems

This is not a good thing: photons mixed in with the colliding bunches allow large numbers of soft e+e- pairs to swirl out from the IP.

e ee e e ee e e e e e

γγ

γ

+ −

± ± + −

+ − + − + −

→

→

→

Use largest possible magnetic field in the detector to confine e± to small radii so they don’t wipe out the vertex detector.

Under discussion: SiD (“Silicon Detector”) with 5 T field, 5 m diameter, 5 m length.


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Beam dynamics, briefly


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Reference orbitA single beam particle with exactly the “right” momentum and initial position travels along the (closed) reference orbit in an ideal accelerator.

Orbits of particles that remain inside the machine’s acceptance will oscillate around the reference orbit.

Local “wavelength” of oscillations varies along the reference orbit.

λ ~ separation between red marks.

reference orbit (gray line)

particle orbit oscillates around reference orbit


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Tune and βThe number of oscillations around the reference orbit in one turn is called the tune of the accelerator. Shown: ν = 22.75.

β ~ λ/2π

reference orbit

particle orbit

ν = 22.75

( )ds

sν

β≡ ∫

for s the path length along the reference orbit.

Μοre precisely:


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Phase space; Liouville’s theoremx is the distance from the reference orbit. x and its first derivative x′will lie on an ellipse in the x, x′ phase plane.

The shape of the ellipse varies from place to place along the reference orbit, but its area remains constant.

x′

xx′

xx′

x

Liouville’s theorem: without dissipative effects the area of the ellipse will not change.

Beam’s emittance is the area of the ellipse.


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Emittance

The beam’s emittance (εx) is the area of the ellipse. Since x′ is dx/ds, x′ is an angle, not a velocity.

Normalized emittance is defined as γεx

The ILC’s normalized (vertical) emittance goal is

γεy = 0.04 mm.mrad

yielding a luminosity L = 2×1034 cm-2 s-1.

x′

x


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Beating Liouville’s theorem…

…is possible if we cause the beam to lose energy. (That makes thetheorem not applicable.)

We can collapse the ellipse considerably, as is done in a damping ring. More about this later.


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Polarized e- source


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e- source overview

This is where everything begins: an intense laser beam blasts a “strained GaAs” photocathode to liberate electrons.

Electrons from the photocathode are accelerated to ~120 kV, thenbunched, accelerated more, and eventually sent to a damping ringto reduce the bunch emittance.

Space charge effects dominate the emittance coming out of the electron source: εy ~ 500× too big.

photocathode

laser beam

electrons

accelerating voltage

~2 cm


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LaserTi:Sapphire, 90% circularly polarized beam.

• 800 nm wavelength to match photocathode band structure

• Epulse ~ 2 μJ (8×1012 γ)

• circularly polarized

• timing structure: 2820 pulses spaced 337 ns apart; repeat this at 5 Hz.

http://www.astec.ac.uk/id_mag/BCD/ILC_source_R&D_plan.pdf



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PhotocathodeBy growing crystals on substrates with slightly different lattice sizes it is possible to do “band gap engineering.” Photon polarization is well preserved by ejected electrons. 80% seems feasible.

http://www.astec.ac.uk/id_mag/BCD/ILC_source_R&D_plan.pdf http://www.slac.stanford.edu/cgi-wrap/getdoc/slac-pub-11686.pdf


http://www.slac.stanford.edu/cgi-wrap/getdoc/slac-pub-11686.pdf


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e+ source


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Helical undulator positron source150 GeV electrons from the main linac will pass through a helical undulator, emitting circularly polarized synchrotron radiation. Each electron loses 3.23 GeV in the process.

Photon beam power is ~150 kW; spot size on a 0.4 radiation length conversion target is 0.75 mm.

100 - 200 m

helical undulator magnet150 GeV e-

conversion

collection

Positrons from γ → e+e- conversions in the target are captured and accelerated to 5 GeV for injection into a damping ring.


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Undulator magnetSeveral designs being considered. Here is an NdFeB permanent magnet version with a 1.4 cm period and 0.8 T field.

http://hep.ph.liv.ac.uk/~ibailey/helical/helical_hep2005_cockcroft.pdf

http://hep.ph.liv.ac.uk/~ibailey/helical/helical_hep2005_cockcroft.pdf


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Photon energy from the undulatorBeam electrons move in narrow helices along the length of the undulator. For 1 cm spatial period, orbit “corkscrew” period is 3.33×10-11 sec.

Radiation emitted during one turn arrives in an interval

11 222

1 1 11 cm 3.33 10 1.94 10 sec2v c

τγ

− −⎡ ⎤⎡ ⎤= × − ≈ × = ×⎢ ⎥⎢ ⎥⎣ ⎦ ⎣ ⎦

Using ω = 2π /τ and Ephoton ~ hω yields Ephoton ~ 22 MeV.

Note that Ephoton ~ γ 2 so that high “drive beam” energy is necessary.


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Polarized positrons!

Positrons will tend to exhibit the polarization of the parent photon. The ILC baseline design document discusses the possibility of anearly upgrade (longer undulator) to obtain 60% polarization in the e+ beam.

SLAC E166 took data in 2005 to study production of polarized e+

using an undulator; it worked. “No surprises,” but they are still analyzing data from their runs.

Emittance from the positron source: εy ~ 500,000 × too big.


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Emittance reduction in the ILC damping ring


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Damping ring issues

The damping rings need to reduce the emittances of the e+ and e-

beams considerably in ~ 0.1 second.

Also, the damping ring needs to cool all 2820 bunches at once, then extract them to the main linac with 337 nsec bunch spacing.

injected γ εx, γ εy extracted γ εx extracted γ εy

electrons 10-5 m 8 × 10-6 m 0.02 × 10-6 m

positrons 10-2 m 8 × 10-6 m 0.02 × 10-6 m

The damping ring may be the ILC’s most challenging subsystem.


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Damping via synchrotron radiation

Since synchrotron radiation is nearly parallel to the particle’svelocity, radiation reduces both the longitudinal and transversemomentum components of the particle relative to the reference orbit.

RF system only restores the longitudinal component.

px

pz

RF

before radiation

after radiation

after RF


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Damping time

Damping time is inversely proportional to how rapidly the particle loses energy via radiation:

2

3

2D

EP E

ρτ ≈ ∝

Small radius (hard bend) reduces damping time. But there is a minimum (equilibrium) horizontal emittance that can be obtained due to the discrete nature of photon emission.

Damping ring uses wiggler magnets to speed up the process. Even so, vertical emittance takes 8τD to drop sufficiently.

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Damping ring designs

Original (TESLA) design called for a 17 km circumference dampingring (!!)

“Dogbone” shape would put most of it in the main linac tunnel but:• access for repairs is not possible when linac is powered• stray fields from RF system klystrons may disrupt DR beam

Why is it so long?

Because…


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Original design: 17 km circumferenceILC main linac beam:

• 2820 bunches, 337 nsec spacing (~ 300 kilometers) • Cool an entire pulse in the damping rings before linac injection

Damping ring beam (TESLA TDR):• 2820 bunches, ~20 nsec spacing (~ 17 kilometers) • Eject every nth bunch into linac (adjacent bunches are undisturbed)

Kicker speed determines minimum damping ring circumference.

Large circumference is worrisome. Tricky beam dynamics?

Wigglers used to cool beam: ~400 m of them.


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Thinking differently

What if we figured out how to make a faster kicker?

If so, a smaller damping ring would be possible.

The problem with the kicker: it is hard to turn on/off quickly.

Two approaches we’re exploring (UIUC + FNAL + Cornell):1. brute force: robust, faster HV switch.2. a new kind of kicker that is always running.

In tandem with kicker studies we’ve been investigating a dampingring lattice for a 6 km ring.

The 6 km ring is now the baseline recommendation!


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ILC damping ring stripline kicker


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Brute force: stripline kicker

Fast kicker specs (à la TESLA TDR):• ∫Bdl = 100 Gauss-meter = 3 MeV/c (= 30 MeV/m × 10 cm)• stability/ripple/precision ~.07 Gauss-meter = 0.07%

A bunch “collides” with electromagnetic pulses traveling in the opposite direction inside a series of traveling wave structures. Hard to turn on/off fast enough.


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UIUC/FNAL stripline kicker studies

Start with a simple kicker whose properties are calculable and can be understood independently of those of the A0 electron beam.

Most important: how well can we measure a device’s amplitude andtiming stability with the A0 beam?

A0 runs at 1 Hz, so performance demands on DAQ and offline analysis processing are not severe.

First significant data were written August 11, 2005. That was a very good day!*

Analysis is underway: first are working at understanding the beam and beamline.

*You have no idea…


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Test it in the FNAL AØ photoinjector beam

16 MeV electron beam, good spot size, emittance.

Data in August 2005 and then January 2006.


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We’ve built one and run tests at FNAL

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HV pulsersStart with a Fermilab linac chopper HV pulser: ±750 V.

Chris Jensen and colleagues at FNAL built it for us.

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FID Technology F5201 Pulser Parameters• Dual channel: +/- 1kV• 0.5% - 0.7% amplitude stability• 0.7 ns rise time• 2.0 ns top of pulse• 1.2 ns fall time• 3 MHz max. burst rate• 15 kHz max. average rate• <20 ps timing jitter

FID pulser: much faster, but stability unclear


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FID HV pulser: ±1 kV, few ns length


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Stripline kicker in A0, looking upstream

We test the FID with a stripline kicker in Fermilab’s 16 MeV e- beam.

100Ω device, 1 cm gap, 15 mrad deflection


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DAQ uses an oscilloscope as a transient digitizer


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Event display, 10 pulses

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It kicks the beam! First FID HV pulser data

data

predictionNothing is calibrated yet, and we have programming artifacts to remove…

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Characterizing kick to beam using UIUC DAQ system

We tried to generate pulse trains with ILC-like timing structure.

Pulser burst mode appears to have failed after allowing the FID to run over night.

• Observed 1st pulse parameters still near nominal

• Trailing pulses do not have stable amplitudes

Cornell met with FID engineer to discuss evaluation and repairs

Present Status

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As measured after the A0 stripline kicker

Pulse train failure


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Fourier series pulse compression kicker


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Since it’s hard to turn on/off, why not leave it ON all the time?

Kicker field needs to be zero when unkicked bunches pass through.

Fields when kicker is empty of beam are irrelevant.

Synthesize kicker impulse from Fourier components of something with good peaks and periodic zeroes.

Kicker is always on.

conventional kicker

kicker field vs. time

damping ring beam

Fourier kicker

kicker field vs. time

damping ring beam


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Three functions with good peaks and zeroes: #1

1. part of the series for a periodic δ function (ω is linac frequency):

“Features” (peaks and zeroes) are evenly spaced.

A problem: field has non-zero time derivative at the zeroes. Bunch head and tail experience different (non-zero) fields.

N=16

Fourier amplitudes

ak

k

k=N


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2. “square” of last page: this way zeroes also have zero slope…

Fourier amplitudes

ak

k

k=N kick ∼sin2 Nωt

sin2 ωt

-150 -100 -50 50 100 150

0.2

0.4

0.6

0.8

1

kicker fields

40 60 80 100 120 140 160

0.01

0.02

0.03

0.04

kicker fields

Better… but frequencies range from 3 MHz to 180 MHz.

A 3 MHz RF device is very different from a 180 MHz device.

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3. high-frequency modulate: this way fractional bandwidth is reduced. bb bbbbbbbbbbbb Fourier amplitudes

ak

k

k=ΓN

2N

This is what we’re actually studying now, but with N = 60 and Γ = 10: ~1.8 GHz ± 10% bandwidth

(Graph uses N = 16, Γ = 4.)

kick ∼sin2 Nωt

sin2 ωt cos ΓNωt

-150 -100 -50 50 100 150

-0.5

0.5

1

kicker fields


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Damping ring operation with an FS kicker

During injection/extraction, deflectors route beam through bypass section. Bunches are kicked onto/off orbit by kicker.

kick

damping

injection/extraction

We don’t want the beam to go through the kicker until we’re ready to extract.

Fourier series kicker would be located in a bypass section.

While damping, beam follows the upper path.

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Fourier series pulse compression kickerConstruct a kicking pulse from a sum of its Fourier components.

Combine this with a pulse compression system to drive a small number of low-Q cavities.

Illinois, Fermilab, Cornell are involved.


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Group velocity vs. frequency

1.3 GHz cutoff frequency wave guide

1× 109 2× 109 3× 109 4× 109 5× 109

5× 107

1× 108

1.5× 108

2× 108

2.5× 108

group velocity vs. frequency

Into wave guide first

Into wave guide last

c

0.5 c

05 GHz30 1.3 42


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1.5× 10-7 2× 10-7 2.5× 10-7 3× 10-7 3.5× 10-7 4× 10-7

-0.015

-0.01

-0.005

0.005

0.01

0.015

upstream end of waveguide fields including cavity response

function

generatorRF

amplifier (dispersive) wave guide

kicker cavity

Field at entrance to the wave guide

Field at upstream end of the wave guide.

Note that peak field is about .018 here, in comparison with 1.0 inside cavity.

Pulse compression, plus energy storage in the cavity!

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Waveguide compresses pulse

Pulse compression!

Maximum amplitudes: •entering ~0.016•exiting ~0.1


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Signal inside the kicker cavity

Kicker cavity field for ~6 ns bunch spacing.

Cavity center-frequency is 600 times linac frequency, 10 times damping ring frequency.

function

generatorRF

amplifier (dispersive) wave guide

kicker cavity

-1.5 × 10-7 -1× 10-7 -5× 10-8 5× 10-8 1× 10-7 1.5× 10-7

-0.75

-0.5

-0.25

0.25

0.5

0.75

kicker fields

-1×10-8 -5×10-9 5×10-9 1×10-8

-1

-0.5

0.5

1

kicker fields , +ê− 10 ns

±10 ns

Field inside cavity


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RF work at UIUC

We’ve been borrowing lab space in an instructional lab in ECE (Electrical and Computer Engineering) for some of our work.

We do not have previous RF engineering experience, so we’re learning as we go.

We discuss things with Fermilab’s RF group. When we are further along, they’ll drive south to spend a few days at UIUC working with us.


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In the RF lab at UIUC

Cheapest source of L-band waveguide: Lowe’s!

(It’s actually aluminum downspout.)

Since then we’ve borrowed real waveguide from Fermilab.

using aluminum downspout as an inexpensive L-

band waveguide


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Measuring cut-off frequency in “L-band” downspout


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Cut-off frequency and complicated frequency structure

RF signal out: notice cutoff frequency


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Fermilab/Illinois activities

Initial studies: use Fermilab A0 photoinjector beam (16 MeV e-) for studies:

1. concept and design studies of FSPC kicker 2. build a fast, simple strip line kicker 3. use the stripline kicker to study the timing/stability

properties of the AØ beam4. build a single-module pulse compression kicker5. study its behavior at AØ6. perform more detailed studies in a higher energy, low

emittance beam (ATF??)


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Main linac

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ILC main linac

TESLA main linac

(From TESLA TDR)

TTF

Cryogenic “unit length” is ~2.5 km

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ILC rf cavities

(From TESLA TDR)

Accelerating structures:

• 500 GeV requires 23.4 MV/m gradient (theoretical limit is 50 MV/m)

• Niobium, 1.3 GHz cavities


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RF cavity gradientsRecent progress is even better: 42 MV/m now! Single crystal

cavities being manufactured (fewer domain boundaries).(M. Liepe, http://www-conf.slac.stanford.edu/alcpg04/Plenary/Wednesday/Liepe_ColdMachine.pdf )

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Wake fieldsHigh-Q (superconducting) structures: induced fields persist.

Bunch length ~20 picoseconds so lots of modes can be excited.

(click to play movie)Long bunch spacing (337 nanoseconds) is necessary.


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Physics


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Physics at the ILC

We have all heard this many times:

• Electroweak symmetry breaking is mediated by something that almost certainly will reveal itself in the 100 GeV to 200 GeV mass range.

• If nothing is there, the wheels come off the bus. Everything goes crazy.

• Even if there’s a higgs, there are enormous cosmological constant problems without something else (SUSY?)


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Physics at the ILC

Precision measurements and discovery experiments:

Both have been necessary.

The role of LEP in refining our understanding of the Standard Model is an example.


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Our success at predicting the unknown…

…is mixed. Our confidence in our predictions is influenced by our optimism!

• SU(5) and proton decay

• value of ε′/ε in K → ππ decay (direct CP violation)

We will need both LHC and ILC to understand the physics of electroweak symmetry breaking


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We may be closer than we think

In stock and on sale!


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Who are we kidding?

Dark energy: 73%

Dark matter: 22.6%Standard Model Physics: 4.4%

We will need a goodly amount of data to understand things.


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Politics


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Will it ever get built?

1. The evolution of events is chaotic, not smooth. We must be ready to act quickly to take advantage of opportunities that may arise.

2. “The key to 'revolutionary' social change in modern societies does not therefore depend, as Marx had predicted, on the spontaneous awakening of critical class consciousness but upon the prior formation of a new alliance of interests, an alternative hegemony or 'historical bloc', which has already developed a cohesive world view of its own.” (quoted by M. Stillo from Williams, 1992: 27 in http://www.theory.org.uk/ctr-gram.htm)

Antonio Gramsci1891 – 1937

Prison Notebooks(“Hegemony”)

http://www.theory.org.uk/ctr-gram.htm


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State of Illinois efforts1. “RIA for Illinois” task force run by the Illinois Department for

Commerce and Economic Opportunity (DCEO). Argonne, Fermilab, and Illinois Universities had been major participants.

2. Considerable strengthening of the Fermilab – Argonne alliance has taken place in the last year or two.

3. RIA, ILC, proton driver, and APS direct injector linac are all superconducting machines that could be built in Illinois. DCEO appeared to understand that a wider scope for its RIA advocacy is appropriate. This is still evolving.

4. This was a near-miss: DCEO lost interest when RIA RFP was postponed. But we learned something from this.


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The federal scene

Office of Science has now stated that the U.S. proposal to become the ILC host country will put forth Fermilab as the host laboratory.

Apparently OS would like to see a significant reduction in total project cost relative to what we presently expect ILC to cost.

Recent news from Ray Ohrbach, Director, Office of Science of DOE.

If we can:• show that ILC has good support in HEP (and other physical science) community• demonstrate that US share will be ~ $3.5B

Then ILC will become a presidential initiative for a 2011 construction start.


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University R&D


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Can university groups do accelerator physics?

Accelerators are BIG, EXPENSIVE devices.

Many university HEP groups have concentrated on detector projects, perhaps because they believe these are:

• more suitable in scale for a university group• more practical, given their prior experience in detector

development.

Is this really true? Should university groups stay away from accelerator physics projects?


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Of course university groups can do accelerator physics!

There are interesting, important projects whose scope is ideal for a university group.

The (inter)national labs welcome our participation and will help us get started, as well as loaning us instrumentation.

Many projects involve applications of classical mechanics and classical electrodynamics. These are perfect for bright, but inexperienced undergraduate students.

The projects are REALLY INTERESTING. (Also, it’s fun to learn something new.)


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Engaging the university HEP community

January, 2002:• FNAL was focused on Run II problems. LC wasn’t on the lab

directorate’s radar.• most university LC groups were already affiliated with SLAC;

most were doing detector simulations.• there was little planning underway to attract new groups (for

example, with Fermilab orientations).

That wasn’t good!


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We organized ourselves

ALCPG working group leaders offer suggestions for revision, collaboration with other groups, etc.

LCRD (DOE) proponents write short project descriptions

UCLC (NSF) proponents write short project descriptions

UCLC proponents write “project descriptions”

LCRD proponents write “subproposals”

UCLC proponents revise project descriptions

LCRD proponents revise subproposals

proposal coordinators create one unified document combining LCRD and UCLC projects

separate accelerator and detector committees review proposed work for both agencies

proposal coordinators create new document combining revised LCRD and UCLC projects, then transmit to DOE and NSF. 10/02

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The result, first yearThe result:

• 71 new projects• 47 U.S. universities• 6 labs• 22 states• 11 foreign institutions• 297 authors• 2 funding agencies• two review panels• two drafts• 546 pages• 8 months from t0

Funded by NSF* and DOE

*planning grant only

…renewal submitted November 2003…third year submitted February 2005…fourth year submitted January 2006

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It’s time to get serious

Ohrbach’s message was also a warning.

University groups won’t engage until they see that there is genuine interest from DOE and NSF in supporting ILC R&D.

The ILC R&D budget went up $30M this year. An unimaginative approach would be to have all of it flow to the national labs and none to the universities.

The unimaginative approach would be irresponsible and would crash the U.S. HEP program.

Comments of the speaker…

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I do have a jokeThe federal government has announced that all research in the physical sciences will need to be performed in low earth orbit in order to receive federal support.

A world-famous Ex-Director of a national lab travels to Washington DC to meet with the Secretary of Science to protest the radical change in science policy.

Unfortunately, the Ex-Director hails from New York and does not speak the inside-the-beltway language employed in the nation’s capital. As a result, the two men must communicate in sign language…

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…joke……The Ex-Director is ushered into the Secretary’s office and sits down. The Secretary points one finger upwards. The Ex-Director emphatically points towards the ground. The Secretary points up a second time. The Ex-Director holds up three fingers. The men pause, and then the Secretary takes a small sapphire out of a desk drawer. In response, the Ex-Director removes an object made of silver strands from his pocket to show the Secretary. After a moment, the two men shake hands and the Ex-Director leaves.

Later the Secretary describes the meeting to his Undersecretaries this way:

“I met with the Ex-Director this morning. It was curious: I couldn’t understand a word he said, so we ended up communicating in sign language. In spite of the language problem it went well, and I am confident we have reached an accord.

“I pointed upwards to show him how appreciative we are of the synergy between cosmology and particle physics. He gestured downwards to comment that the next breakthrough is likely to come from work done in a tunnel under the Jura, or 150 meters underground in Northern Illinois.

“I held up one finger to describe the unity of the sciences and how physics deepens our understanding of everything else, including biology.

“He held up three fingers to remind me that we still do not have a successful unification of the three basic forces, the strong and electroweak interactions with gravity, and that new data and new experiments are necessary.

“I took out a Ti:Sapphire laser crystal made through a new industrial process to show him that even an abstract field like high energy physics relies on our commercial, applied research base…

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…joke……“He held up a stranded object, I think some sort of superconducting braid from a Tevatron magnet, to remind me that pure research is sometimes the primary driver for productive lines of industrialization.

“We shook hands and he left.

“It was a good meeting and I hope the President’s budget request for an increase in funding for the physical sciences receives Congressional support. Maybe we shouldn’t really force them to launch all their experiments into low earth orbit.”

The Ex-Director returns to his university and describes the meeting to his colleagues over lunch.

“Those Washington guys, they don’t know what they’re doing: they don’t even speak English anymore! We had to wave our arms to communicate. It was ridiculous. Even so, I think it’s going to be OK.

“I went into his nice office and sat down. He said to me ‘If you want to keep doing science, you’d better find a space suit and get your butt up there!’

“I said to him ‘That’s ridiculous. I’m staying right here!’

“He said to me ‘Go climb a tree you old fart!’

“I said to him ‘You go climb three trees, you putz!’…

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…joke…

… “It was odd what happened next. He took out some jewelry, maybe a birthday present for his wife, so I showed him the Navajo bracelet I got at the National Gallery. That seemed to satisfy him: maybe that we both respect Family Values or something like that. We shook hands and I left.

“I think it’s going to be OK. Now if he’d only learn to speak English!”

The International Linear Collider: A Detector Physicist ... · George Gollin, The International...

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Transcript of The International Linear Collider: A Detector Physicist ... · George Gollin, The International...