NLC - The Next Linear Collider Project

JLC/NLC Baseline Design and TRC Report

Tor Raubenheimer & Kaoru Yokoya

ISG9

KEK, December 10th, 2002

X-band Linear Collider Goals

Stage 1: Initial operation (length ~ 18 km)

500 GeV cms (650 GeV at lower luminosity)

L = 25 ~ 20x1033 in JLC and NLC

Stage 2: Add additional X-band rf components (length ~ 32 km)

1 TeV cms (1.3 TeV at lower luminosity)

L = 25 ~ 30x1033 in JLC and NLC

Higher Energy Upgrades:– 1.5 TeV with upgrade of linac rf system or length increase

• NLC injector and beam delivery built for >1.5 TeV– 3 TeV+ with advanced rf system and upgraded injector

• See CLIC parameters: “A 3-TeV e+e- Linear Collider Based on CLIC Technology,” CERN-2000-008

• Beam delivery sized for 3 to 5 TeV collisions

Beam Parameters

CMS Energy (GeV)Site US Japan US Japan

Luminosity (1033) 20 25 30 25Repetition Rate (Hz) 120 150 120 100

Bunch Charge (1010)Bunches/RF PulseBunch Separation (ns)Loaded Gradient (MV/m)

Injected x / y (10-8)

x at IP (10-8 m-rad)

y at IP (10-8 m-rad)

x / y at IP (mm)

x / y at IP (nm)

x / y at IP (nm)

z at IP (um)

avePinch Enhancement

Beamstrahlung B (%)Photons per e+/e-Two Linac Length (km)

High Energy IP ParametersStage 1 Stage 2

500 1000

0.75

1.450

192

300 / 2

360

4

8 / 0.11

219 / 2.1

17 / 20

1.51

13.81.35.4

243 / 3.0

32 / 28

110

0.14

300 / 2

360

4

13 / 0.11

1.327.6

0.75

110

0.291.47

8.9

1921.450

• Stage 2 has ~5x1033 at 1.3 TeV

Trade cms energyfor beam current

• Parameters also exist for operation at the Z, W, low-mass Higgs, and top

Change to Baseline Design

• Initial NLC design presented to the TRC was too difficult(3.2 s klystrons and 8x DLDS pulse compression)– 3.2 s klystrons were not on the horizon– DLDS demonstration was years away– TESLA has been making rapid progress both technically and politically

• However with progress on the structure gradient, the potential for X-band was clearer than it had been for years!

• Needed an rf configuration that could be conclusively demonstrated in ~1 year timescale to compete with TESLA– 1.6 s klystrons were making great progress at KEK– SLED-II only requires 2 klystrons rather than 8 klystrons for DLDS– Concept was already demonstrated with NLCTA operation– At 4x pulse compression, compression efficiency is still quite good

Stolen from Nobu Toge!

Desired Energy Range

• Would like to cover the Z-pole and W-production and then have continuous energy coverage at energies above LEP (>220 GeV)

• Upper energy reach needs are not known– Opinions vary around the world (HEPAP and ACFA specify 1 ~ 1.5 TeV

while ECFA calls for 400 ~ 800 GeV)

• Lower energy operation is limited by e+ production schemes– Current TESLA scheme does not work between cms 200 ~ 300 GeV

• Highest energy reach can be attained by trading beam loading for energy– TESLA has 50% loading 50% greater energy reach (providing cavity

limits are not exceeded)– NLC has 30% beam loading – cavities are qualified at unloaded gradients

Energy Reach

NLC Stage 2

Trade cms energyfor beam current

Luminosity (1034

)

0 0.5 1 1.5 2 2.5 31000

1100

1200

1300 25 Bunches

192 Bunches

Both designs can trade luminosityfor energy in Stage I TESLA reaches ~ 750 GeV NLC reaches ~ 650 GeV

NLC can do the same in Stage IIto reach 1.3 TeV

NLC trades tighter tolerances for a greater energy reach

Upgrade Routes and Costs

• NLC and TESLA costs are similar in value for 500 GeV– FNAL review of TESLA costing methods estimated 6.1 B$ with US accounting

and 20% contingency– NLC (and JLC?) costs are estimated at ~7 B$ with 30% contingency– The error on the costs is much greater than any differences!

• JLC/NLC upgrade requires adding structures, klystrons, etc. in the 2nd half of the linac tunnel– Cost to upgrade to 1 TeV is roughly 30% of initial TPC

• TESLA upgrade route: install 35 MV/m cavities at onset, double rf system, upgrade cryo-plant– Assuming initial installation of 35 MV/m cavities, cost to upgrade to 800 GeV

cms is 20% of initial project cost– Upgrade from 800 GeV to 1 TeV is another 25% for a total of 45% of the initial

project cost

SLED-II Implications

• Changing the baseline rf system to SLED-II:– Faster demonstration of both power source and later a full rf sub-unit,

i.e. 1, 2, or 4 klystron pairs powering structures

• Depends on final modulator configuration

– Doubled number of klystrons, modulators, and PC systems and increased the linac length by 8%

• Obvious cost impact; perhaps some advantage in reliability

– Decreased unloaded gradient by 7%

• Reduced breakdown rate

– Very small effect on tolerances and emittance preservation

• Tolerances scale with the square root of the gradient

– Makes 8-pack failure slightly more invasive to beam operation

• With DLDS 8-pack failure probably did not have to scale quadrupole lattice (LEM) with SLED-II almost certainly will

Why X-Band?

• Why X-band instead of superconducting?– Superconducting has loose tolerances and stability however these are not necessary

nor are they necessarily easier to achieve in cryostats– Entering new regime with massive superconducting facility—will limiting effects be

found that cannot be seen in TTF?– Many sub-systems are very different from anything demonstrated

• Particularly true for damping ring complex– Are the cost of TESLA and NLC similar for comparable energy reach?

• Why X-band instead of lower rf frequencies?– Support higher gradients: 70 100 MV/m for cheaper collider– The technology exists and the tolerances are attainable

• Present prototypes are better than tolerances• Directly use knowledge from SLC: “It’s the diagnostics, stupid!”

– X-band provides essential knowledge-base for continued progress in e+/e- colliders at higher gradients and possibly higher rf frequencies

LC Technical Review Committee

Energy & TechnologyDaniel Boussard (Chair)

Chris Adolphsen, SLACHans Braun, CERNYong-Ho Chin, KEKHelen Edwards, FNALKurt Hubner, CERNLutz Lilje, DESY(Pavel Logatchov, BINP)Ralph Pasquinelli, FNALMarc Ross, SLAC(Tsumoru Shintake, KEK)Nobu Toge, KEKHans Weise, DESYPerry Wilson, SLAC

LuminosityGerald Dugan (Chair)

Ralph Assmann, CERNWinnie Decking, DESYJacques Gareyte, CERNWitold Kozanecki, SaclayKiyoshi Kubo, KEKNan Phinney, SLACJoe Rogers, CornellDaniel Schulte, CERNAndrei Seryi, SLACRon Settles, MPIPeter Tenenbaum, SLACNick Walker, DESYAndy Wolski, LBNL

Two working groups (plus reliability)

LC Technical Review Committee

• The Technical Review Committee (TRC) has led to extensive review of many JLC/NLC and TESLA subsystems– Many JLC/NLC people are involved directly or indirectly

– Energy/technology Group: present status is being studied (compared)

– Luminosity Group: breaking new ground (collaborative effort)

• Mainly damping ring studies and DR IP luminosity studies

• Benchmarking simulation codes against each other

• Pushing to include ‘all’ relevant effects– Static alignment procedures

– Vibration and stability effects with feedback systems

– Beam-beam effects at IP

– Integrated luminosity performance very difficult to evaluate

– Reliability Group: pushing everybody to arrive at realistic reliability goals with necessary changes to designs

– Leading to improvements in all designs: TESLA and JLC/NLC

TRC Conclusions

• Greg Loew reported at ICFA Seminar– Four levels of R&D priority: R1 = feasibility; R2 = required for design; R3 =

before final engineering; R4 = optimization.

• Energy and technology issues were not unexpected• Lots of work on luminosity issues

– Conclusions not very sharp

– Two unexpected items found in JLC/NLC design

• Importance of collimator wakefields

• Jitter amplification of parasitic crossings

– Many unexpected items found in TESLA

• 10x better vacuum and BPM resolution required in TESLA DR

• Wigglers severely limit dynamic aperture in TESLA DR

• 100 ~ 150% emittance growth in TESLA linac versus 3% in TDR

• Large difficulties with head-on collisions

– Also learned of things that work in TESLA design apply to NLC?

TRC Energy & Technology Conclusions

Test of complete accelerator structure at design gradient with detuning and damping, including study of breakdown and dark current

Demonstration of SLED II pulse compression system at design power level

Test of a complete main linac RF sub-unit (as identified in machine description) with beam

Full test of KEK 75 MW, 1.6 µs PPM klystron at 150 or 120 Hz

Full test of SLAC induction mod.

Building and testing of a cryomodule at 35 MV/m and measurement of dark current

Test of a complete main linac RF sub-unit (as identified in machine description) with beam

Testing of several cryomodules at nominal field (23.4 MV/m) over long enough periods to verify breakdown and quench rates, and measure dark current

Test of RF components at higher powers for 800 GeV operation

R1

R2

R1

R2

JLC/NLC TESLA

TRC Luminosity Conclusions

Electron cloud and ion instabilities need study

Additional simulations and experiments on correction are needed for damping rings

Demonstrate extraction kicker with better than 0.1% stability

Complete static DRIP tuning simulations with dynamic effects

Develop most critical beam instrumentation, including intra-train diagnostics

Develop sufficiently detailed prototype of linac girder/cryostat to provide information on vibration

Further optimization of damping ring dynamic aperture

Study tighter alignment and electron cloud and ion instability requirements for 800 GeV upgrade

Development of TESLA DR kicker

Review trade-offs between head-on and crossing-angle collisions

Detailed analysis of the tradeoffs between one and two-tunnel layouts

Detailed evaluation of critical sub-system reliability

R2 R2

Common (JLC/NLC and TESLA) TESLA Only

Luminosity Issues Only few x 10,000 larger than SLC!

– Increased beam power from long bunch trains • SLC: 120 Hz x 1 bunch @ 3.5x1010

• NLC: 120 Hz x 192 bunches @ 0.75x1010 200x• Generation of uniform multi-bunch trains sets source requirements• Control of long-range wakefields is essential to prevent BBU

– Larger beam cross-sectional densities: N / (x y)

• SLC: 3.5x1010 x 1.6 m x 0.7 m (FFTB: 0.6x1010 x 1.7 m x 0.06 m)• NLC: 0.75x1010 x 250 nm x 3.0 nm 330x SLC• Factor of 5 from energy (adiabatic damping) and factor of 4 from stronger focusing

(similar to Final Focus Test Beam)• Factor of 15 ~ 30 from decrease in beam normalized emittances at IP

Dyxcms

b HN

E

PL

4

2 D

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brepH

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2

4

Luminosity Issues for FY03

• Concentrate on improving source designs– Evaluate undulator-based e+ source– Improve energy spectrum at damping ring injection and review multi-bunch

limitations

• Improve damping ring designs– Evaluate e-cloud and ion instabilities and improve wiggler models– Consider alternate ring designs for better stability

• Improve linac (LET) emittance preservation simulations– Aim towards evaluating integrated luminosity– Must understand reliability issues– Experiments?

• Continue BDS optimization– More inclusive background/collimation studies

• Stabilization studies

Polarized Electron Sources

• JLC/NLC e- source based on successful SLC injector design

• Emittance estimates reasonable from SLC operation

• Charge Limit limited current from SLC polarized photocathode

• Demonstrated charge and polarization during E158 run

14 mm spot in 100ns

20 mm spotin 100 ns

SLC photocathodein 300 ns

• NLC requires ~100 nC per 100 ns

• Greatly exceeded in E-158 tests

• Polarization was ~85%

NLC Req.

Positron Source Targets

• SLC e+ target failed after 5 years at stress levels ~2x lower than previously measured and predicted for WRe targets

• Radiation damage problem may be worse in Ti targets starting R&D program to investigate

SLC e+ target Beam direction• SLC target studied at LANL and modeled at LLNL

• Problem due to embrittlement from radiation damage

Positron Source Concepts

• Baseline design uses abrute force solution toe+ target limitations– Need to investigate

other conventional concepts

• Studying a variation of TESLA undulator source for polarized positron production– Studies of e+ yield as well as helical undulator and targets

– 150 GeV location eases operational limitations

– Possible test in FFTB line at SLAC

Damping Ring Issues

• Beam is stored for a relatively long time in the rings (ms)– More accelerator physics in the rings than elsewhere in LC– Stability is essential for collider performance– JLC/NLC rings are similar to the ATF and the 3rd generation SRS

• Damping rings have beam currents and bunch trains similar to the high operating luminosity factories– However they have much smaller beam sizes (higher densities) and are much more

sensitive to weak instabilities– Tougher dynamic aperture and stability requirements– They also require much better alignment to get flat beams ~ 40 um

• High beam density pushes frontier in electron machines– Space charge tune depression and microwave instabilities– Ion trapping and electron cloud effects– Intrabeam scattering and short Touschek lifetime

Operating Ring Comparisons

• Compare random alignment and jitter ‘tolerances’– Uncorrelated misalignments or jitter that would lead to equilibrium

emittance, jitter equal to the beam size, or = 0.001

• These are not specs. on alignment but they are measures of the sensitivity

• Looking for significantly better alignment and stability than has been previously attained

ALS APS ATF SLS NLC DR TESLA DR

Energy 1.9 7 1.3 2.4 2 5Circ 200 1000 140 288 300 17,000x [mm-mrad] 24 34 2.8 15 3 8y [nm-rad] 500 140 28 150 13 14

Yalign [m] 135 74 87 99 38 13

Roll align [r] 860 240 1475 530 400 45

Yjitter [nm] 850 280 320 337 93 91K/K [0.01%] 1.5 1.4 2.1 1.5 1.8 1.1

Dynamic Aperture Studies

• Dynamic aperture in rings is difficult for three reasons:– Strong focusing for small emittance

– Long wigglers for damping time

– Large injected beam size from high-power injector (60 kW)

• Current calculations look good but have narrow energy acceptance

• Want to improve model of incoming beam and reduce E

• Need to improve wiggler model and add field errors– Recent work by Marco Venturini

looks like a good start

Impedance Issues

• Calculated chamber impedance model using MAFIA and Omega3P in 1995 and again in 1998– Ring layout and components have not changed much

• Used Oide-san Vlasov code to calculate thresholds– Close to weak threshold and factor of 3 or below strong

– Very similar results in 1996 ZDR

• Recently started calculating CSR impedance from bends and wigglers (Juhao Wu)– Thresholds look similar to that of chamber impedance

– Need to combine all sources of impedance and update threshold calculations

• Also need to consider methods of increasing p – Increases bunch length and increases threshold

Electron Cloud and Ion Instabilities

• Electron cloud and ion instabilities are important concerns– Designed ring for 1 nTorr vacuum pressure to reduce ion instability growth times

• Analytic estimates still give 100 us

– Will need to reduce SEY to handle electron cloud

– Experiments planned at LBNL to study methods of reducing SEY

– Need to understand e- lifetime

– Need full simulations: ions are easy; electron cloud more difficult.

Calculation by Mauro Pivi

Linac Emittance Preservation

• Improved optics to avoid known SLC problems

• BBU from short- and long-range wakefields must be controlled– Measurements at ASSET verify wakefield reduction procedures

• Tight alignment tolerances– NLC alignment and jitter tolerances are ~10x and 100x tighter than SLC

TESLA alignment and jitter tolerances are ~1x and 10x tighter than SLC– Sets tight requirements on diagnostics and controls

• NLC design includes more controls and diagnostics than FFTBTESLA design include less control and diagnostics than SLC

– Demonstrated NLC-level alignment at FFTB– Demonstrated emittance tuning and BNS damping at SLC

• Will require excellent stability (both vibration and drift)– Need to understand limitations

• Requires improved simulation tools integrated luminosity

Coolant Vibration Studies

• Vibration of accelerating structure caused by turbulent water flow– Measured 300 nm on structure << tolerance– Coupling to quadrupole was measured

at about ~3 nm – tolerance is ~10 nm!– Direct coolant induced quad vibration ~2 nm– Direct effect from rf pulse is negligible!– Next step: model complete girder with quad

300 nm at nominal

flow

Stabilization Studies

• Final doublet tol. ~ 1nm

• Inertial system with 6 d.o.f. tested at SLAC– Limitation due to sensors– Reduced vibration >10x

• Other inertial studies atCERN

• Optical anchor systemstudied at UBC

• Developing new non-magnetic sensorwith better noise characteristics– Working on details before considering a

‘real’ implementation

Inertial Stabilization Test Block

FONT: Feedback On ns Timescales

Magnet assembly and X-band BPM installed onto NLCTA

latency of ~40 ns + cable delay

Measured 10x reduction of beam motion

Feedback loop

Beam direction

Conclusions

• SLED-II demonstration and Gradient R&D are 1st priority• Luminosity R&D has been very broad and addressed most issues

– This is a real strength of the JLC/NLC design– Many test facilities, R&D, and simulation studies– Most sub-systems and diagnostics are modest extrapolations from

operating accelerators or demonstrations at test facilities– Collider is designed with an energy reach from 92 GeV to 1.3 TeV

• Continue to develop models to evaluate reliability and integrated luminosity– Think about next-generation simulation tools

• Work on multi-bunch issues in source studies• Improve damping ring designs and beam injection systems• Continue to work on stability issues and BDS optimization

WG1 (Luminosity Issues) Schedule