BCD-Main Linac

1
BCD-Main Linac

• RF-Source Modulator, Klystron
• Cavity High Power Input Coupler, Frequency Tuner
• Cryomodule Configuration
• RF Distribution Configuration
• Cryogenic System Configuration
• Tunnel Curvature, Layout

2
Modulator

• Baseline
• To power the 10 MW MBKs, the baseline choice is
  to use the bouncer-compensated pulse transformer
  modulator that was developed initially by FNAL
  and industrialized by DESY.
• The units perform well, however, expensive,
  multi-ton, oil-filled transformers and
  susceptible to single-point failures.
• FNAL recently designed a lower-cost,
  more-reliable, pulse-transformer modulator that
  has 5 ms pulse capability for the Proton Driver
  projenct.
• DESY continues to work with industry to improve
  the reliablity of their modulator design for the
  XFEL.
• Alternatives
• The main alternative is a Marx-style generator
  which is being developed at SLAC.
• This modulator has the design with built-in
  redundancy and it shoud be easier to mass produce
  and repair compared with the baseline modulator.
• Other approaches are a direct-switch by DTI and a
  DC-to-DC converter by LANL.
• If a TDR-like tunnel layout were adopted with the
  modulator separated from the klystron by up to
  2.5 km, the transport and impedance
  matching(cable to klystron) of the 120 kV pulses
  would require further development. (Marx-style is
  the easiest way.)

3
Klystron

• Baseline
• The 10 MW Multi-beam klystrons (MBKs) being
  developed by Thales, CPI and Toshiba are the
  baseline choice.
• If the MBKs do not meet availability
  requirements, the commercial, single beam, 5MW
  tube from Thales could be used. This has been the
  work-horse for L-band testing at DESY and FNAL.
• Alternatives
• 10 MW sheet-beam klystron by SLAC to reduce cost
• 5 MW inductive output tube by CPI to improve
  efficiency
• 10 MW, 12 beam MBK by KEK to reduce the modulator
  voltage and the modulator plus klystron cost.

4
High Power Input Coupler (1)

• BCD Choice
• Twin cylindrical window architecture of the
  TTF-III coupler
• Pro
• Lengthly experience on TTF (100,000 coupler
  hours) .
• Demonstration of use with a cavity at 35 MV/m on
  CHECIA.
• Tested at a power of 1MW, 1.3ms pulse in TW mode.
• In principle, then this power coupler would be
  sufficient even if the cavities were to be run at
  35 MV/m and would meet, at least in TW mode, the
  needs of 2?9 cell superstructure at 35 MV/m.
• Con
• The present unit cost is prohibitive. However,
  the couplers have only been built in small
  numbers.
• The experience with conditioning indicates that
  the conditioning time is rather long.
• The conditioning issue is currently under study
  at Orsay in the DESY-LAL collaboration.
• A period of two years will be necessary to
  complete the conditioning and industrialization
  study.
• Potential Modification
• Increase in the diameter of the cold assemble
  (from 40 mm to 62 mm).
• Technical benefit of pushing multipactor levels
  to higher powers
• Of interest in case of a choice of higher
  gradient (45 MV/m)

5
High Power Input Coupler (2)

• Alternatives
• The coupler designs incorporating two disk type
  window coupler
• The capacitive disk window coupler
• The TRISTAN like window coupler
• The TW60 coupler
• The AMAC window coupler
• Pro
• Relatively free from multipactor.
• Mechanically easy to fabricate and therefore
  cheaper.
• Thin disk windows can be positioned at low value
  of the SW electric field.
• Disk windows should be relatively easy to braze
  into the coupler.
• Con
• The current version of the capacitive coupler
  cannot be DC biased.
• The present capacitive and TRISTAN like couplers
  have no possibility to have their external Q
  variable.
• Disk ceramic are in the line of sight of the
  cavity beam pipe.
• Seen to be a problem on the early CEBAF linac
  design
• The co-axial version may be less problematic
  because of the reducing the solid angle presented
  to the x-
• ray or electrons which might impinge on the
  ceramic.
• Too early to estimate the cost impact.

6
Frequency tuner (1)

• ILC requirements
• Coarse tuning range500kHz (1.6mm at 315 Hz/mm)
• Coarse resolutionlt5Hz
• Fast tuning range (static at 2K)
• ?f 2 ? K ? E2 (factor 2 for dynamic operation
  overhead)
• 2.5kHz (for K1 Hz/(MV/m)2 cavity at 35 MV/m)
• 3.2kHz (for K1 Hz/(MV/m)2 cavity at 40 MV/m)
• 4.0kHz (for K1 Hz/(MV/m)2 cavity at 45 MV/m)
• The requirement on the fast running range is not
  well known at this point.
• Dedicated experiments are needed to define the
  actually required fast tuning range.
• Options under consideration
• Original Saclay/TTF tuner
• Used in TTF/VUVFEL for several years. No fast
  tuning element, though a piezo actuator added for
  proof-of-principle
• tests of Lorentz-force detuning. Range of the
  fast tuner lt500Hz.
• Modified Saclay tuner
• Similar to the original Saclay tuner. Located at
  one end of cavity. Incorporated piezo actuators
  (1kHz tuning range).
• INFN/DESY blade tuner
• No fast actuator(1st version). Tested at TTF with
  the superstructure. Located around the LHe
  vessel.
• The recent version includes piezo
  actuator(1kHz).

7
Frequency tuner (2)
8
Frequency tuner (3)

• Risk and reliability
• All but KEK slide jack tuner have a cold drive
  motor inside the vacuum vessel.
• The main risk is a failure of the motor, the fast
  actuator or the gearing.
• All designs with cold drive can use the same type
  of motor, gearing and fast actuator, so that
  there is no principle difference in risk and
  reliability between these tuner designs.
• The reliability of a single cold drive might not
  be sufficient. The tuners will either have to be
  made very reliable (probable via redundancy) or
  their failure prone components made replaceable
  without warm-up.
• Options for improving the reliability
• Redundant motor and piezo, if inside of vacuum
  vessel
• Improved design with highest reliability for
  motor and/or piezo, if inside of vacuum vessel
• Warm motor
• Fast actuator
• Piezo actuator
• Detailed studies have been done to verify pulsed
  cryogenic operation in radiation environment.
• Magnetrostrictive actuator
• Similar size to piezo, so can be used instead of
  piezo.
• Has a significant larger stroke than a piezo at
  5K, produces less heat and might have a higher
  life time and higher tolerance for preload change
  than piezo.
• Base line
• No existing tuner design fulfill the
  specification on fast tuning range above 30 MV/m.
• The tuner needs to provide 500 kHz slow tuning
  range and more than 3kHz fast tuning range.
• Required RD

9
Frequency tuner (4)

• Close to BCD
• Modified Saclay tuner
• Pros
• Relative simple and compact design
• Redundant design for piezo element
• Original Saclay tuner was tested in detail
• Cons
• Maybe difficult to increase fast tuning range
• Redesign needed with increased fast tuning range
• Poor maintainability of stepping motor and fast
  actuator
• RD necessary
• Design with increased fast tuning range
• Fast actuator RD
• Prototype tests with Lorentz-force compensation
  at 35 MV/m
• Verification of sufficient MTBF for cold motor
• INFN blade tuner
• Pros
• Compact design (not at cavity end)
• High stiffness

10
Frequency tuner (5)

• Alternatives
• TJNAF renascence tuner
• KEK slide jack tuner
• Pros
• Motor outside of vacuum vessel (inexpensive
  motor)
• Piezo can be replaced (cryostat warm-up required)
• High stiffness
• Cons
• Feed-through to outside needed (penetration of
  shields and vacuum vessel)
• Some static losses (0.05 W?)
• Redesign needed with increased fast tuning range
• Poor maintainability of fast actuator no
  redundancy
• RD necessary
• Design with increased fast tuning range
• Fast actuator RD
• Prototype tests with Lorentz-force compensation
  at 35 MV/m
• KEK coaxial ball screw tuner
• Pros
• Wide tuning range

11
Gradient

• Baseline
• The WG5 recommendations call for TESLA-like
  cavities to be used.
• Qualified to operate at a gradient of at least 35
  MV/m with a Qgt0.8?1010 in CW tests.
• Cavities not meeting these requirements would be
  rejected or reprocessed.
• Only a small fraction of cavities and cryomodules
  would be pulsed-power test.
• With such screening, a 31.5 MV/m gradient and Q
  of 1?1010 would be achieved on average in a linac
  made with eight-cavity cryomodules.
• For a future upgrade, the WG5 recommends that
  cavities of the low-loss or reentrant type be
  used and that they be qualified to at least 40
  MV/m with Qgt0.8?1010 in order to achieve 36 MV/m
  and Q 1?1010 on average in the linac.
• Alternatives
• The linac cost is a weak function of gradient in
  the 30-50 MV/m range, and operating close the
  ultimate 45-50 MV/m gradient limit would prevent
  extending the machine energy by lowering the beam
  current (and depending on the cooling overhead,
  lowering the machine repetition rate). Thus, a
  better strategy would be to design for a gradient
  around 30 MV/m, and if the cavities that are
  eventually installed perform better than the
  initial requirement, use this capability to
  extend the machine energy reach (e.g., up to 750
  GeV if 45 MV/m operation is eventually achieved).
• The WG5 ACD gradient recommendation for 500 GeV
  operation is the same as given for the BCD
  upgrade.

12
Cryomodule Configuration

• Baseline
• A reasonable baseline rf unit is a 10 MW klystron
  driving 24 TESLA cavities.
• The cavities would be divided into three
  cryomodules instead of two.
• This is the configuration that has been used and
  will continue to be used for several years.
• There is no significant cost savings with longer
  cryomodules.
• The cavity gradient variation can be more
  efficiently dealt with if there are less cavities
  per cryomodule.
• Every fourth cryomodule in the linac would
  include a cos(2q)-type quadrupole with horizontal
  and vertical corrector windings.
• The quad helium vessel would be supported above
  by the 300 mm diameter gas return pipe.
• The quad would be located below the center
  (fixed) post, and attached to its upstream face
  would be a BPM.
• The TDR cavity spacing of 283 mm was optimized
  based on the flange connection and bellow scheme.
• The linac packing fraction would be about 75 .
• For 500 GeV operation, 328 such rf units would be
  required. Overall, 71 of the peak rf capability
  would be transformed into beam power.
• Alternatives
• Having up to 12 cavities per cryomodule instead
  of 8 to reduce the number of inter-connections.
• Shortening the distance between cavities to 250
  mm like JLab, or as close as possible to the 180
  limit from cavity cross talk and heat losses at
  the transitions.
• Instrumenting the HOM readouts to provide a
  measure of the average beam position in the
  cryomodule. The cryomodules would either be moved
  manually during down periods or equipped with
  remote-controllable mover for beam centering.
• Putting the quad and BPM in a separate
  cryo-section for better stabilizing them
  vibrationally. No corrector magnets for moving
  quad and BPM independently from cavities.
• Putting the movers on the middle support post of
  the gas return pipe to allow adjustment of the
  quad and BPM.
• Reducing the quad aperture by half (35mm) to
  allow the use of superferric quads, which will
  likely have more stable magnetic centers with
  respect to quad shunting.

13
RF-distribution Configuration

• Baseline
• TDR-like distribution system that includes a
  circulator in each cavity feed followed by a
  tuner(three-stub or E-H type) to allow control
  the cavity phase and Qext.
• DESY uses off-the-shelf components for the
  distribution system.
• Alternatives
• The circulator is the largest cost component at
  about 25 of the rf distribution cost.
• There are several alternative distribution scheme
  that eliminate the circulator.
• More precise cavity-to cavity phasing.
• Harder to deal with the variation in the maximum
  cavity gradients.
• In the event of rf breakdown in a coupler or
  cavity, these schemes would allow some fraction
  of the reflected power to propagate to the other
  cavities.
• KEK plans to test such a distribution scheme at
  the STF in the next year.

14
Cryosystem Configuration

• Baseline
• The refrigerator spacing would depend on the
  choice of operating gradient (the spacing is
  about 5 km in the TDR).
• The length of the 2K, two phase lines depends on
  the tunnel slope.
• For the slopes up to 0.3 mrad, the 167 m long,
  8.5 cm diameter lines specified in the TDR could
  be used.
• For larger slopes, shorter lines.
• For 1-4 mrad, a canal-like system. A
  laser-straight tunnel would have a 3 mrad slope.
• The maintenance length is half of the
  refrigerator spacing.
• The maintenance length is the length that would
  need to be warmed up to repair a cryomodule.
• Alternatives
• The thermal cycling long string of cryomodules
  will be slow and may cause vacuum leaks.
• To reduce the maintenance length, U-tubes and
  turnarounds can be included.
• If such sections (each 1.5 m long) were installed
  every 500 m, the number of cryomodules thermally
  cycled would be reduced by a factor of five.
• The warm-up time is reduced by a factor of two.
• The cool-down time is reduced by a factor of 10.

15
Tunnel Curvature

• Baseline
• Until on-going beam dynamics simulation show
  otherwise, the linac will follow the curvature of
  the earth, unless a site-specific reason (cost
  driven) dictates otherwise.
• Alternatives
• Laser-straight linac or one constructed from
  straight-line segments.
• Final choice depends on the cost consideration.
• Required RD
• Aggressive beam dynamics simulations of emittance
  preservation (beam-based alignment) for the
  continuous curved linac, including tolerance
  studies.
• Studies of viable and cost effective cryogenic
  solution for a tilted linac.

16
Tunnel layout

• Baseline
• The rf sources are to be distributed along a
  second tunnel (or surface gallery) that runs
  parallel and nearby to the beam line tunnel.
• The sources would not be subjected to radiation
  and could be accessed for repairs while the
  machine is running.
• Alternatives
• The beamline is in a near-surface tunnel (lt30 m
  deep) and the modulators, sans transformers, are
  clustered in surface building located every 5 km
  (the beamline tunnel contains the modulator
  transformers and klystrons).
• For both case, locating the beamline near the
  surface would allow easier access and shorten the
  power and cooling distribution lines that connect
  to the surface.
• The main disadvantages would be larger ground
  motion and limited site availability.