LCLS Bunch Length Monitor Conceptual Design Review Instrument Design Considerations February 23,...

2/23/2006 LCLS Bunch Length Monitor Marc Ross - SLAC

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LCLS Bunch Length Monitor Conceptual Design Review

Instrument Design ConsiderationsFebruary 23, 2006


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LCLS Bunch length monitor system:

• Two subsystems:– Deflecting structure ‘LOLA’

• Accurate, calibrated, complex• Destructive – ‘pulse stealing mode ok’• Expensive• Tested

– Radiation monitors• Dipole radiation• Gap radiation• Simple sensors• Cheap• Tested

Complementary devices


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Bunch length monitor system

LOLAGap radiation

Dipole radiation

Single signal device

Video signal

Pyro-electric detectorHigh frequency diode

WGmm waveoptics


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Two coherent radiation monitors for BC1• Simple ceramic gap surrounded by mm-wave

diode detectors (paired)– Total radiated energy is 2 uJ

• For 1nC, 200 micron bunch. • (energy scales as Q^2/(bunch length)

– Tested at SLC and ESA (actually many years…)

• Annular reflector directs dipole radiation onto mm-wave ‘optics’ – pyroelectric detector– similar total radiated energy– Tested at FFTB/SPPS

Bunch charge distribution • Simple indicator: central frequency of radiated

energy


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Coherent radiation detection strategy:

• Each individual detector has ~ factor 2 range

• Long bunches: use diode sequence– (100, 200, 400, 1000 GHz)– Down to 100 um rms

• Short bunches: use reflector and pyro-electric detectors– Below 150 um rms

• RD required to match – see 2007 testing plan

CERN Bunch length RF gap


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ESA 100 GHz Gap and Detector

gap

ESA gap monitor and detector

• Gap / horn / WG-10 closeup

gap


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400 GHz diode

ESA 100 GHz System – Jan 8, 2006Sig_z_min ~300 um

gap


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Multi-frequency diode ‘xylophone’

Frequency (GHz)

Waveguide Attenuation (20 cm)

Energy in (J) Energy out

100 WR-10 0.8 dB 5 e-9 5e-9 200 WR-4.3 3 dB 1 e-9 500 e-12 400 WR-2.2 8 dB 200 e-12 30 e-12 1000 WR -1.0 22 dB 50 e-12 300 e-15

• 1nC / 200 um example– (Total radiated energy 2uJ)

• “Energy in” assumes catalog item waveguide horn• Detector sensitivity is 2.3e-15J into 50 Ohm out.• Good S/N for 100, 200, 400… 1000GHz ?

gap


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‘Beam view’ of multi-diode / waveguide

assembly

gap


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Reflector• (in use at FFTB for plasma wake exp. – Hogan)

– Thin Ti foil ‘in the beam’ – polished.– Si window– Simple, direct, detector optics– Typically shorter than LCLS BC1 (not BC2)

• LCLS annular reflector will be 30mm diameter with 14 mm aperture– Capture 50% ~ 1 uJ – at best

• Low frequency performance reported to be poor (<300 GHz)

e-

VariablePositionMirror

∆z

Interferometer Pyro Detector

12.5 µm MylarBeam Splitters

RT≈0.17

12.5 µm Mylar1mm HDPE

Vacuum Window(3/4” dia)

Reference Pyro Detector

Alignment Laser1 µm Titanium Foil at 45º

• Interference signal normalized to the reference signal

CTR MICHELSON INTERFEROMETER

• Motion resolution ∆zmin=1 µm or ≈14 fs (round trip)

x=60 µm, y=170 µm N≈1.91010 e-

• Mylar: R≈22%, T≈78%, RT≈0.17 reflector

CTR Energy Correlates withBunch Length

Rel

ativ

e E

nerg

y @

end

of l

inac

reflector

0

5000

1 104

1.5 104

2 104

2.5 104

3 104

3.5 104

-2.5 -2 -1.5 -1 -0.5 0 0.5 1

XCompressionfrom544X OverOpitimumUnder

Am

plitu

de (a

.u.)

Relative Energy (MeV)

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

400 450 500 550 600 650 700 750 800

544X(Pyro)wwoImage Event with ImageEvent without Image

BP

MS

DR

13 5

44 X

CTR Pyro Amplitude (a.u.)

#61I7

#348I39

#124I14

X-ray

Pyro Is Not The Whole StoryNeed to Look at Details of the Spectra

• Pyro amplitude is ambiguous• Energy spectra are not• They are complimentary diagnostics• Clear correlation between energy spectrum and E-164X outcome

Example: Jitter from North Damping Ring:

Relative Energy [GeV]

reflector

reflectorpyro jitter

distribution – SLC NRTL

stability

Spectra vs pyro-electric signalreflector

reflectorOne pyro vs another

• meets 5 to 10% resolution goal

reflectorPyro for one band vs another

• Pyro response as a function of linac ‘chirp’ (phase - offset)

reflectorpyro response has position correlations:


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MM Wave detector Sensitivity

• Pyroelectric: Basically a charge source, approximately 1.5uC/Joule.• Capacitance is 120pf for 3mm detector. • Charge amplifier (AmpTek A250, external FET), has noise 300

electrons RMS. – Corresponds to 30pJ of mm wave energy

• Typically pyro detectors are supplied with included amplifier, performance tends to be worse.

• Detector is a thermal integrator. Dynamic range is limited by the dynamic range of the amplifier.

– For the AmpTek A250, this is approximately 60,000:1. • Commercial pyroelectric detectors (Scientech PHF02) have noise

level of 3nJ, approx 100uJ maximum signal. • Note, sensitivity is 100X worse than theoretical, dynamic range is

30,000:1


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MM wave diode detectors

• Sensitivity of ~2V/W (into 50 Ohms) with ~100GHz bandwidth (at 300GHz).

• For a 100ps pulse, Bandwidth ~5GHz.– Noise is 2.3e-15J.

• Diodes typically linear to ~30mV output.– 1.5pJ. Dynamic range 700:1

• Expect realistic amplifier (10dB noise figure) to limit dynamic range to 250:1

• Waveguide for 300GHz is WR-2.8, 0.7X0.35mm. – expected attenuation 0.2dB/cm.

• Waveguide for 900MHz is WR-1.0, 0.25X0.13mm. – Expected attenuation 1.1dB/cm

• Need something like 20cm of waveguide for dispersion – The initial pulse is very short, with extremely high peak power. Waveguide

dispersion spreads the pulse in time, while keeping the original frequency content.


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Comparison of pyro and diode detectors

• Pyro detectors have much larger dynamic range (>30,000:1, vs ~200:1).

• Noise energy diodes is 10^4 lower than for pyro detectors• Pyro area (for sample detector) is ~10 mm^2. • Diode (waveguide)

– At 300GHz 0.2mm^2– At 900GHz 0.03mm^2

• Diode Sensitivity / Area is 250x at 300GHz, 30X at 900GHz• Not clear how much gain available from horn antenna. (~10dB?)• Diodes more sensitive than pyros at 300GHz. • At 900GHz, diodes probably slightly less sensitive. • Dynamic range of pyro detectors is better• Diode alignment of waveguide is much easier.


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Controls and data acquisition

• These systems are ‘single signal’ systems, i.e. only a simple gated digitizer is needed*– (Some concerns over gating precision and noise – to be tested)

• BC1 Feedback will require beam intensity normalization and (possibly) steering correction / feedback

• integration with LOLA improves the systematics greatly we strongly recommend an aggressive approach to LOLA data acq./integration.

• Pyro/diode systematics will be different and may require different procedures.

• 2007 testing plan


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Testing plan

• ESA – Minimum bunch length ~200 um ?– Multi-channel resolution test– No independent high accuracy reference– April and July 2006

• LCLS injector– Minimum bunch length ~ 50 um– High accuracy reference (29-4 LOLA Transverse cavity

LCLS Bunch Length Monitor Conceptual Design Review Instrument Design Considerations February 23,...

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