NLC Accelerator Physics

1
NLC Accelerator Physics

• NLC Machine Advisory Committee
• SLAC
• October 4th, 2000

2
Outline

• Recent progress
• Layout and optics
• Structure design
• Beam delivery systems
• Vibration program
• Simulations / modeling
• Outstanding issues
• Source modeling
• Damping ring
• Beam based alignment issues
• High luminosity operation
• System-wide tolerance calculations
• System-wide studies (tuning, beam tails, MPS)
• Plans

3
Accelerator Physics Issues in NLC

• Two issues
• Energy (rf technology)
• Luminosity (small spots beam power)
• Beam power (long bunch trains)
• charge from sources
• long-range wakefields
• Small spot sizes
• low emittance damping rings
• final focus system
• alignment and jitter tolerances
• beam-based alignment and feedback
• Both issues (very high charge densities)
• beam collimation and machine protection

4
NLC Project Scope

• Injector Systems
• Main Linac
• Housing with all internal services
• Half filled for initial 500 GeV cms
• Upgrade by adding rf, water, power to the 2nd
  half of the tunnels
• Beam Delivery (high energy IR)
• Two BDS tunnels and IR halls with services
• some components must be upgraded to get from 1
  TeV to 1.5 TeV

1.5 TeV 0.5 - 1.0 TeV (1.5 TeV withincreased
gradientor length) 1.5 TeV(length will
support a 5 TeV FFS)
5
IR Layout Issues

• Final focus aperture is set by low energy beams
  ??1/?? but magnet strength is limited by highest
  energy operation
• Final focus has limited energy range without
  rebuilding magnets and vacuum system
• Simplify design by dedicating one IR to low
  energy operation and one to high energy
  operation
• Low energy range of 90350? GeV (build arcs
  for lt500 GeV?)
• High energy range of 2501000 GeV
• Need to specify reasonable ranges!
• High energy beamline would have minimal bending
  to allow for upgrades to very high collision
  energies
• High energy BDS could be upgraded to multi-TeV
  operation!

6
Dual Energy IR Layout
Site roughly 25 km in lengthwith two 10 km linacs
Optional low energy IP92-350 (500??) GeV
Possible staged commissioning
Low energy (50 - 175 GeV) beamlines
e
e-
Return lines production and possibly drive beam
-share main linac tunnel
High energy IP0.25-5.0 TeVupgraded in stages
Centralized injector systempossibly for TBA
drive beamgeneration also
7
Status of Dual Energy IRs

• Optics for high energy CD0.4 BDS updated as of
  9/00
• Studied muon backgrounds in high energy IR
• more aggressive but looks OK at 1 TeV and 109
  particles collimated with two muon spoilers
• No studies on low energy BDS
• simple scaling from Big Bend arc suggests bending
  is not an issue
• ?arc ? E?3/2 for constant Ncell and De
• Final focus system should be straight-forward
• Studying round beam capabilities of new FFS for
  g-g IR in either low or high energy IR
• To deliver luminosity simultaneously to both IRs,
  need to duplicate end-of-linac emittance
  diagnostics and collimation

8
180 Hz Operation Issues

• 180 Hz operation is decoupled from low/high
  energy IR
• two options 180 Hz at 500 GeV or ?120 Hz at 500
  GeV and 60? Hz at lower (250 GeV) energy
• Choice depends on AC power
• Primary issues are
• power consumption, average heating and radiation
• machine protection (60 Hz minimum operation for
  any low e beam)
• emittance generation
• duplicate BDS beam lines for dual energy
  operation
• Damping redesign is an opportunity to ease design
• either two identical rings or optimize each of
  two rings for damping and emittance, respectively
• probably 200 meter circumference with reduced
  wiggler lengths

9
Centralized Injector

• Discussing different options
• sharing linac tunnels
• accelerating multiple bunch trains in one
  accelerator
• see John Sheppards talk
• Studied BD for multiple bunch train acceleration
• much easier than SLC!
• trains separated by filling time (1ms at S-band)
• requires damped structures at S-band and probably
  L-band
• still operational issues to resolve
• Have parameters for bunch compressors etc. but no
  optics and need further study of transport

10
Parameters for 500 GeV and 1 TeV
11
Low Energy IR Luminosity
12
Possible 1.5 TeV Parameters

• Two cases considered initially
• doubled rf power
• 50 increased length
• Both cases ac power limited to ? 200 MW
• Higher energy parameter sets are based on the
  CLIC parameter sets and require lower emittance
  beams and smaller IP spot sizes

13
Multi-TeV Collider Facility

• Need improvements in rf technology to make higher
  energy cost effective
• multi-beam klystrons, active rf pulse
  compression, or TBA
• Need high gradients to keep length reasonable and
  balance cost of rf system
• At 35 MV/m (SC max gradient), 3 TeV linac would
  be 110 km
• Optimum gradient in NLC is between 75 and 100
  MV/m depending on how components scale with rf
  power
• Normal conducting cavities with lower cost rf
  system and higher gradient of 100 ? 200 MV/m
• Need very small beam emittances and small spots
  to achieve luminosity injection complex similar
  to present NLC
• Reuse next-generation LC injection system and
  beam delivery, and use linac tunnels with
  modified components

14
Multi-TeV Energy Upgrade

• High energy FFS could support multi-TeV
  collisions if we deliver the beams (not true of
  present energy collimation)
• Beam parameters studied as part of CLIC project
• significantly smaller beam emittances and spot
  sizes
• similar average currents with smaller bunch
  spacing
• slightly shorter bunches
• need to verify these parameters
• In addition to rf systems, would have to upgrade
  low emittance generation and diagnostics
• Need to consider the likely options to avoid
  precluding a upgrade route!

15
Structure Design ? Wakefields

• Short-range wakefields depend on average iris
  radius
• To avoid BBU structures must cause the long-range
  transverse wakefield to decay in a few bunches
• Four different configurations
• Detuned (DS) - this tends to be insufficient for
  long bunch trains because of the re-coherence (OK
  at L-band and S-band)
• Damped-Detuned (DDS) - route followed by SLAC
  based on a detuned structure with weak damping
  imposed by coupling to a manifold that runs along
  the cells which also serves as BPM. Heavily
  Damped (HDS) - route followed by CERN and
  Shintake where every cell is directly damped.
  Has reduced shunt impedance and possible pulsed
  heating problems.
• Medium Damped (MDS) - combine local damping with
  detuning may provide a good compromise!

16
Structure Design

• Injector linac beam loading compensation based on
  DT
• simplifies rf systems
• match filling time to maximum beam current
• Optimized injector linacs for large apertures and
  low wakefields
• thicker irises and higher phase advance per cell
  (135 or 150 deg.)
• Injector linacs based on RTOP design rather than
  rounded cells -- simpler parameterization with
  similar performance
• L- and S-band linacs based on detuned design
  but C-band needs damping either DDS or MDS

17
S-Band phase advance options
Struct Type t/a1 favd Ncell F1 Tf aave/? GL a RD
S 6.8/4.8 120 114 4.03 592 0.143 17.08 b RDS 6.8/4
.8 120 114 3.94 429 0.156 13.54 c RDS 8.5/5.5 120
114 3.94 509 0.154 15.83 d RDS 9.5/6.0 120 114 3.9
4 579 0.153 16.45 e RTOP 8.0/ - 135 102 3.93 546
0.156 16.42 f RTOP 9.0/ - 135 102 3.92 578 0.157 1
6.63 g RTOP 9.0/ - 135 102 3.93 600 0.156 16.83 h
RTOP 6.5/ - 150 92 3.92 633 0.158 16.83
6 structure/module, 120 MW per module rf power

• Started with RDS structure case (a)
• Needed to be close to zero-crossing for LR
  wakefield case (d)
• Better performance and simpler RTOP structure
  with 135 case (f)

18
L-band Structures

• RTOP cell profile
• Filling time optimized to e booster parameters,
  6-structure/module.
• T (thickness) optimized to maximize amin
• (adjusting R such that GLVbooster/Lacc at full
  power)
• GL for the e booster 12.5 MV/m.

19
C-band Linac Pulse Compression

• SLED-I Q0100000
• Klystron power 65 MW
• Klystron pulse 3.7 ?s
• Module 2 klystron, 4 structures

• DLDS
• Klystron power 65 MW
• Klystron pulse/bin 0.5 ?s
• Module 8 klystron, 32 structures

20
C-band Injector Linac

• Tolerances are tight (similar to X-band)
• 70 looser tolerances with DLDS-like system
  because of shorter fill time and larger iris
  radius but higher cost (John Sheppard)
• Need further optimization on different pulse
  compression techniques

21
Injector Linac Tolerances

• Tolerance estimated for a maximum of 10
  emittance dilution
• One-to-one trajectory correction in e- booster,
  e drive linacs
• Beam based alignment similar to main linac in
  pre-linacs
• Short-range wakefields limit trajectory and
  structure alignment
• Long-range wakefields impose cell-to-cell
  alignment
• Alignment tolerances in ?m

22
X-band Structures

• RF damage problem appears to correlate with group
  velocity
• Choose 150 degree phase advance to reduce vg
• Use thicker irises to further reduce vg and the
  surface field
• Improved coupler design reduces fields in
  couplers
• Same rf power to 6m girder produces the same
  acceleration
• 25 fewer cells per linac but more couplers
• Two options studied thus far
• L 1.4 m, vg0 6.5, 4 structures per 6m girder
• L 0.9 m, vg0 4.4, 6 structures per 6m girder
• Studying the dipole wakefields - may require MDS
• Looking at pulsed heating of MDS and DDS
  structures

23
X-band Structure Parameters
1500/cell RDDS1
24
Dipole Damping versus Shunt Impedance

• Strong damping such as the choke- mode
  structure or the CLIC HDS have more than
  20 reduction in the Rs
• LIAR studies indicate that Qs of 750
  are OK
• Pulsed heating is another limitation at
  the damping waveguide irises

25
RF Pulsed Heating
Peak ?T ? 40C in RDDS ? 70C in MDS with
a dipole Q of 250
MDS structure with standing wave
26
Short-Range Wakefield Calculations

• Linac tolerances are dominated by short-range
  wakefields
• Single bunch energy spread is compensated by
  running 11 off-crest and the BBU is treated
  with BNS damping
• Short-range wakefield models are based on mode
  summation for periodic structures with extension
  for high frequencies
• Measurements on the SLAC linac agree with the
  longitudinal wakefield calculations at the 10
  level
• Correction for group velocity as noted recently
  at CERN will be more important for NLC
  structures
• Linac parameters are chosen so wakefield impact
  on the transverse dynamics is 25 of that in the
  SLC
• Measurements of transverse wakefield in SLAC
  linac are started

27
Long-Range Wakefield Calculations

• Long-range wakefield models are based on two band
  circuit models for the lowest dipole band
• Parameters from OMEGA3P modeling of cells
• Very good modeling of RRDS1 rf and wakefield
  measurements at ASSET
• Starting to model 150 degree MDS structures at
  FNAL and SLAC (KEK)
• Studying higher dipole bands - visible in RDDS1
• Parallel time-domain calculations using TAU3P -
  10 cells calculated recently - full structure
  calculations soon

28
RDDS1 ASSET Measurements

• Wakefield model (with errors) for RDDS1 agree
  well with measurements

• Lowest band is around 15 GHz
• Next band (25 GHz) is visible after 1.4 ns

29
Wakefield Calculations

• Circuit model has been used to study structure
  internal alignment and frequency tolerances for
  RDDS1
• Tolerances are set by detuning and are expected
  to be similar in MDS and DDS - to be verified
• Alignment tolerances
• structure alignment is determined by short-range
  wakefield
• cell-to-cell alignment is determined by
  beam-based alignment requirements and by
  long-range wakefields
• cell-to-cell alignment also limited by
  bookshelf effect
• Frequency tolerances
• fundamental is set by phase error through
  structure and net energy gain
• dipole frequency tolerance is set by long-range
  transverse wakefield - frequency errors break the
  detuning scheme causing BBU and growth of the
  projected emittance

30
Beam Delivery Status

• Working on the collimation system (Peter
  Tenenbaum)
• Pre-linac collimation (provides MPS functionality
  and cleans tails)
• Bunch length collimation in BC2
• Post-linac collimation (provides MPS
  functionality and cleans tails)
• Collimation in final focus
• Need to update beam halo and tail calculations
• Consumable collimator work is progressing nicely
• Collimator wakefield experiment has some great
  data!
• New design for final focus (Pantaleo Raimondi)
• Much shorter length (design is still evolving)
• Scales to high energy very nicely (700 meters for
  5 TeV collisions)
• Background studies are being performed
• Need to study full BDS performance
• Present high energy BDS is 2.5 km instead of 5 km
  per side

31
Vibration Program

• Vibration program (Andrei Seryi)
• Extensive measurements of slow motion
• strong correlation with pressure
• explanation for site dependence in diffusive
  (ATL) motion
• separate out systematic motion from diffusive ATL
  motion
• Studying cultural sources and interaction with
  tunnels
• SLD pit has nearly acceptable vibration
• SLD detector is quite noisy
• Integrated model for whole spectrum including
  fast wave motion, ATL, and systematic motion
• Working at including localized cultural sources
• Implementing model in LIAR for feedback and
  beam-based alignment studies
• Organizing GM2000 in November

32
Simulations and Code Development

• Optics and modeling codes (MAD, DIMAD, and LIAR)
  in good shape (except for space charge effects)
• Structure and wakefield calculations in good
  shape
• Ground motion model is being developed (Andrei
  Seryi)
• Feedback simulations being compared with SLAC
  linac performance
• Parallel FBII code, code development on ECI -
  need exp. comparisons
• Parallel version of LIAR but need somebody to
  drive this program
• No progress on full system tuning simulations
• No progress on tolerance calculations

33
Outstanding Issues

• Source modeling
• Damping ring
• Beam-based alignment issues
• High luminosity operation
• System-wide tolerance calculations
• System-wide studies (tuning, beam tails, MPS)

34
Electron / Positron Sources

• Sources (John Sheppard and David Schultz)
• Polarized electron gun suffers from current limit
  
• SLC positron target damage being investigated
• Brute force can be used in both cases
• Need to understand yield and capture better
• Polarized rf gun would simplify damping ring
  design
• Looking at alternate e sources (undulator and
  laser based systems)
• Radiation / beam tails
• Significant limitation from sources ? DR (60 kW
  beam power)
• Post-DR tails will be removed in pre-linac and
  post-linac collimation systems (Peter Tenenbaum)
• Pre-DR collimation systems are not defined
• Need modeling of source beams with space charge
  and wakefields

35
Damping Rings (John Corlett)

• Damping rings are similar to 3rd generation light
  sources
• ex0 is 1/3 of ALS normalized emittance
• ey0 is 1/150 of ex0 (extracted emittance is 50
  larger)
• tolerances are 100 to 50 mm
• impedance issues are thought understood
• many issues have also been demonstrated at the
  KEK ATF
• Concerns
• new instabilities electron cloud instability
  (ECI) and fast beam-ion instability (FBII)
• large current dependence observed at ATF
  (intrabeam scattering?)
• heavy reliance on wiggler damping (aperture and
  other issues?)
• extreme sensitivity of collider to instabilities

36
Structure Alignment

• The average alignment of the cells in the
  structure is set by the short-range wakefield
• Present emittance budget allocation is 50 in
  NLC-IIb ? 10 ?m structure alignment
• Structures MUST be aligned using beam derived
  information
• Use 3 S-BPMs along structure -- resolution is few
  ?m
• S-BPMs must represent average alignment of the
  structure
• With 3 S-BPMs cell-to-cell alignment must be
  better than 9 ?m assuming a random walk model
• Long-range wakefield in DDS (or MDS?) set a
  tolerance of 3 ?m cell-to-cell alignment assuming
  a random walk model
• Even this looks possible!

37
Beam-Based Alignment Hardware
38
Quadrupole Alignment (Nan Phinney)

• The quadrupole alignment is set by the dispersion
  error
• Present emittance budget allocation is 40 in
  NLC-IIb ? 2 ?m quadrupole alignment
• Quadrupoles MUST be aligned using beam derived
  information
• Tolerance corresponds to roughly 100 ?m
  dispersion error (dispersion is not exact in
  linac with varying energy spread)
• With 1 ?m BPM resolution, 100 ?m dispersion not
  so bad!
• Desire very local correction (align every
  quadrupole perfectly) with a procedure that does
  not interrupt luminosity
• May not work with (permanent) quadrupole center
  shifts
• Investigating alternate routes (DF steering,
  e-bumps, ballistic corr.)

39
Beam-Based Alignment Hardware

• Stripline BPMs for FFTB have 1mm resolution
  and rf cavity BPMs have 40nm resolution

• Stripline BPM-Quad centers shift but 20 mm
  over 2 years

40
Beam-Based Alignment Hardware

• Movers developed for Final Focus Test Beam
  (FFTB) have steps lt0.5mm
• Alignment procedure at FFTB with statistic
  errors of few mm
• FFTB dispersion measurements set upper bound
  on alignment ?5? statistic errors

Vertical quad-to-bpm alignment 12/95 and 6/96
41
Beam-Based Alignment Studies

• Measure quadrupole center shifts (how accurately
  can this be done)
• Re-visit DF steering alignment procedures
• Study systematic effect of rf deflections
• Study technique of changing energy upstream of
  alignment region
• Study effectiveness of e-bump procedures
• Bumps used in SLC to reduce emittance dilution by
  factor of 10
• Three diagnostic stations along linac to
  facilitate e tuning
• Study stability of e-bump solutions
• Re-balance emittance budgets depending on
  performance
• Study interaction of beam-based feedbacks, ground
  motion and alignment - include terrain following!

42
Parameters for 500 GeV and 1 TeV
43
Route to High Luminosity in NLC

• NLC design has built-in margins to cover nominal
  operating plane including 50 charge overhead and
  300 emittance dilution
• NLC damping rings spec. to produce 0.02 mm-mrad
  although design requires 0.03 mm-mrad
• SLC used emittance bumps to reduce emittance
  dilution from 1000 to 100technique not
  included in NLC emittance budgets
• Use margins to achieve higher luminosity
• Present prototypes and RD results are better
  than initial specs (see RDDS cell frequencies and
  structure alignment S-BPM)
• ??y lt 25 in linac if production rf components
  are similar to prototypes
• Both will lead to increased luminosity (34?1033
  at 880 GeV)

44
RDDS1 Structure Construction

• RDDS1 cells were designed at SLAC and machined at
  KEK final machining performed on
  diamond-turning lathe
• Attained excellent resultsfrequency errors less
  than 1 MHz, i.e. lt1?m errors
• Tolerances for dipole modefrequencies are 5
  times looser!
• Bonding process still needsto be understood!

45
High Luminosity Issues

• High luminosity operation was first described at
  Snowmass 96 but as something to be attained well
  after initial operation, i.e. long learning curve
• Presently there is pressure to verify high
  luminosity operation is possible from the
  beginning
• Need to perform study on beam bumps to verify
  effectiveness (no question in simple simulation!)
  and rebalance emittance budgets
• Need to verify that all requirements are being
  treated consistently for high luminosity
  operation although without the conservatism that
  is presently used

46
System-wide Tolerances

• Tolerances have been updated in piece-wise manner
• Review of tolerances presented at May
  Collaboration / MAC meeting
• Last systematic update was ZDR in 1996!
• Too many tolerances to update individually!
• Most of collider is linac / transport line
• Possible to generate program to calculate
  tolerances from lattice deck and simple inputs
• Must be verified with lots of detailed tracking
• Creating tracking program for linac and BDS (in
  collaboration with Ralph Assmann at CERN)
• Requires implementation of tuning techniques

47
System-wide Simulation Studies

• For simulations, collider can be broken into
  three sections
• sources (upstream of damping rings)
• capture efficiency beam tails multi-bunch
  loading
• linac dynamics non-relativistic beams bunching
  
• damping rings
• capture efficiency dynamic aperture collective
  instabilities beam-based alignment
• storage ring dynamics nonlinearities collective
  instabilities
• linacs and beam delivery
• dynamic aperture collective instabilities
  beam-based alignment and feedback beam tails
• linac dynamics bunching nonlinearities time
  evolution

48
System-wide Simulation Studies

• Have tools for storage ring dynamics (lots)
• Have tools for linac dynamics (LIAR, ELEGANT?)
• Have tools for BDS (DIMAD , ELEGANT?)
• Have tools for bunch compression (but 6-D is
  slow)
• Need to generate a combined program for linac /
  BDS
• Need tool to study interaction of feedbacks,
  ground motion and beam-based alignment
• Do not need source ? IP simulations ever
• Do not need DR ? IP simulation in the near term

49
FY01 Tasks

• Tolerance studies, parameter budgets and BB
  alignment
• Verify consistent parameters for high luminosity
  operation
• Re-evaluate beam tails and loss models for BDS
  and injector
• Re-evaluate DR optics and instability
  calculations
• Continue ground motion and isolation studies
• Continue collimator wakefield experiment
• Continue low vg structure development
• Continue optics development to support costing
  exercises
• Need to increase staff - budgeted for 2.5
  additional people in FY01 and 4 more in FY02