Cavity BPM studies

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Cavity BPM studies

• Explore uses (and limitations) of uwave cavity
  BPMs
• Develop nanometer resolution
• 10x better than FFTB/Shintake
• Digital mixing / angle control
• Develop beam phase space monitors
• Tilt-meter

ATF is the ideal location for these tests very
stable beam and low emittance
Marc Ross
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Goal ATF Nano-BPM project

• Prove that nanometer sized beams can be kept in
  collision
• short time scales vibration
• long time scales thermal drift
• Steps
• Measure with nanometer resolution
• design and test a BPM that has 1 nanometer
  resolution
• Study beam stability
• study the stability of the ATF extraction line
  beam
• Stabilize with active movers/sensors
• Stabilize the magnets that focus the beam (they
  probably need it)
• Stabilize the BPM itself

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Multi-bunch feedback final step

• There will still be some instability from the
  ring / extraction kicker
• It may be possible to stabilize the trajectory
  within a long pulse train
• need good multibunch BPMs
• FONT experiment at NLCTA
• 4. Use a long extracted pulse and stabilize the
  back section of the train

FONT Feedback On Nanosecond Timescales
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Tilted bunch

• Point charge offset by d
• Centered, extended bunch tilted at slope d/st
• Tilt signal is in quadrature to displacement
• The amplitude due to a tilt of d/s is down by a
  factor of
• with respect to that of a displacement of d
• (bunch length / Cavity Period )

Papers CLIC 244 Measurements for Adjusting
BNS Damping in CLIC 17.08.94 W. Wuensch EPAC
2002 Beam Tilt Signals as Emittance Diagnostic
in the Next Linear Collider Main Linac P.
Tenenbaum
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Example

• Bunch length st 200 mm/c 0.67 ps
• Tilt tolerance d 200 nm
• Cavity Frequency F 11.424 GHz
• Ratio of tilt to position sensitivity ½?f?t
  0.012
• A bunch tilt of 200 nm / 200 mm (1 mrad) yields
  as much signal as a beam offset of 0.012 200 nm
  2.4nm
• Need BPM resolution of 2 nm to measure this
  tilt
• Challenging!
• Getting resolution
• Separating tilt from position
• Use higher cavity frequency?

Need 1 mrad tilt sensitivity for linac tuning
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Angled trajectories

• A trajectory that is not parallel to the cavity
  axis also introduces a quadrature signal (in
  phase with tilt signal)
• Projected dipole sensitivity is increased by
  sz/cavity length
• 50

Relative normalized precision Beam position/beam
traj angle
s y res/sy 5 s y res/sy 10x
d
ATF sz 8mm gives expected tilt resolution
0.1mrad
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Tiltmeter plans (Dec 02)

• All offsets / angles must be zero in order to
  have maximum sensitivity
• control to correct yaw
• beam must be parallel to axis to minimize
  quadrature phase signal
• installation of beam tilter
• cavity drive power synchronization (not
  totally necessary)
• roll - yokoyure / ou ten
• pitch - tateyure
• yaw katayure (?)

Dec 2 6, 2002
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Parasitic bunch

• In May 02 we saw a parasitic satellite bunch
  one RF bucket (1/714 MHz) later than primary
  bunch
• Because of the large spacing, the tiltmeter will
  measure the angle between the two bunches
• We are making a parasitic bunch detector that
  uses synchrotron radiation
• Single photon counter
• (parasitic bunch may be very small with new gun

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Tiltmeter Tests 12.02

• Generate tilt using deflecting cavity
• Cavity is unlocked so deflection will be random
  pulse to pulse
• Some bunches will be tilted, some simply kicked
• Use downstream cavity BPM (MM4X) with I/Q
  detection circuit
• Almost digital downconversion
• Calibrate position response using movers
• Measure beam jitter/cavity resolution combination
• Tilt jitter?
• Angle response

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MM4X Cavity BPM position/angle controls
Top to bottom (6 movers for 4 degrees of
freedom) x stage y stage z stage for
orthogonalizing pitch x stage for orthogonalizing
yaw Rotary table (yaw) Pitch tilter
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C-band deflection cavity
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C-band RF at ATF!

• CW TWT amplifier (use pulse only)
• 600 W nom 1kW measured
• On loan from C-band group

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Pill box cavity design

• Rectangular pillbox standing wave cavity with
  off-axis beam pipe
• Estimated kick about 5 kV
• Measured kick about 10KV peak/peak with 600W
  input power
• Installed 700 mm downstream of QD7X

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(No Transcript)
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Typical cavity signal
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Calibration using mover
Typical response 30 mV/micron Measured circuit
noise 300 uV Estimated resolution 10 nm
To be tested using 3 BPMs in 03.03
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Deflection cavity on

• I/Q cavity response with deflection cavity at
  full voltage
• Axes show directions of pure displacement (black)
  and pure angle (bluish) (green is 90 from pure
  displacement)
• Tilter motion is not quite orthogonal
• Ellipticity is the ellipse aspect ratio (jiyouou)
• This plot shows equivalent angle trajectory

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Comparison 3.5 and .4 mA

• Effective beam tilt scale full width dipole
  projection is 0.9 of displacement for 8 mm bunch
  (scales with bunch length)
• See 29 um peak to peak kick at full I and 20 um
  projected dipole at monitor
• Good vertical streak of 7 um beam!
• Tilt angle 20um/8mm 2.5 mrad

Preliminary result
ellipticity
3.5mA
0.4mA
25um
29um
21um dipole
14um dipole
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Estimate of bunch length from ellipticity

• Ellipse min/max vs bunch length (mm) for C-band
• Only length scale used is RF wavelength

ATF bunch length range
Ellipticity (da-en)
mm
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Summary of bunch length measurements

• First bunch length measurement made entirely
  using RF cavities
• Beam/monitor jitter 1 um (very stable over
  hours!)
• Beam/monitor tilt jitter 1 um ? surprisingly
  large

Preliminary result
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1) Nano-BPM test ATF extraction line

• Mechanically connect several BPMs (4 5?)
• Must control cavity position and angle
• Electronics similar to tiltmeter optimized for
  best possible resolution
• ATF ext line (BINP) 250 nm
• FFTB (Shintake) 25 nm
• Joe Frisch, Steve Smith, T. Shintake

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C-band BPM limiting resolution (Vogel/BINP)

• Cavity properties
• For 1010 Electrons, single bunch (assumed short
  compared to C-band).
• Assume cavity time constant of 100 nanoseconds
  (1.6MHz bandwidth) (guess)
• Assume beta gtgt 1 for cavity. (All power is
  coupled out).
• Thermal noise energy is kT or 4x10-21
• Thermal noise position (ideal) 0.4nm
• Note that deposited energy goes as offset2 and
  as beam charge 2.

• Signal
• A 1nm offset deposits 2.4x10-20 J in the cavity.
• Output power for 1 nm offset is 2.4x10-13 Watt.
• Output power for 1 cm offset (cavity aperture?)
  is 25 Watts (maximum single bunch)
• Output power for 10 bunch train can be 2500
  Watts! (Need to terminate for multi-bunch
  operation)

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Electronics, Noise and Dynamic range

• Thermal noise -168dBm/Hz.
• Assume signal loss before amplifier 3dB.
  1.2x10-13 Watt -99dBm
• Signal power after amplifier for 1nm offset -79
  dBm
• Dynamic range at amplifier output 91 dB, or
  35,0001 position, or 25 microns.
• Assume maximum signal into mixer (13dBm LO)
  8dBm. (Joe thinks this should be 0 dBm)
• Full range (1 dB compression) 8 dBm into mixer
• (Note for good linearity, probably want 20 dBm
  into mixer, or 1 micron range)
• Mixer conversion loss 8dB.
• Maximum output 0 dBm

• Front end broad band amplifier 20 dB
• Assume noise figure 3dB (better available)
  -165dBm/Hz input noise.
• Front end amplifier bandwidth 10GHz -gt -65dBm
  noise input.
• Front end gain 20dB -gt -45 dBm noise power
  output (OK).
• IF amplifier 30 dB
• Final bandwidth 1.6 MHz. Noise in band power
  after amplifier -83 dBm

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Equipment

• Cavities - assume existing C band BPMs
• Filters Approximately Q10 to help limiter. May
  not need if fast limiter is available.
• Limiters Available from Advanced Control. 100W
  peak input, Limit to about 15dBm output. Try
  ACLM-4700F feedback limiter 0.8dB loss, 100W max
  pulse input, 13.5dBm max output. Unknown speed.
  Also see ACLM-47000 0.7dB loss, 100W input, 20dBm
  max output.
• Amplifier Available from Hittite with 3dB noise
  figure. Stage to get 30dB Gain. NOTE need to
  find an amplifier which can survive the 15dBm
  output from the limiter. Hittite parts seem to
  only handle 5dBm. (Maybe OK pulsed?) Amplifiers
  available from Miteq (?) with 0.8dB noise
  figure and 20dBm allowable input power
  JS2-02000800-08-0A (for future upgrade).
• Mixer Use Hittite GaAs parts - likely to be
  radiation resistant.
• IF amplifier. 30dB gain, 0.1-50 MHz. Various
  options from Mini Circuits or Analog modules.
  Need Noise Figure ltlt 10dB. gt 2V p-p swing.

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Support electronic equipment / software

• Digitizer Use spare SIS (VME) units from 8-pack
  LLRF system. Each is 100Ms/s, 12 bit, /-1 V (?)
  input signal.
• C-band source Use existing ATF synthesizer. Does
  not need to be locked.
• C-band distribution amplifier. Need to drive 8 x
  13dBm references. Approximately 25dBm output
  (including losses). Use existing ZVE-8G amplifier
  (purchased for tiltmeter
• work).
• C-band distribution splitter 18 splitter.
  Probably exists, otherwise mini-circuits.
• Control System Use existing 68040 controller and
  existing crate. Linda thinks it is easy to
  interface this to Matlab on a PC for data
  analysis.
• Matlab software Use modified version of
  tiltmeter software. Digitizes decaying signal
  from cavity BPMs and reference signal, with
  stripline BPM as time reference. Does not require
  phase locked reference, or good frequency match
  between cavities.

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Mechanical

• X/Y Tilt Stage Check Newport U400 mirror mounts
  (300 each). May be strong enough to move
  cavity.
• X/Y Tilt stage drive Use picomotors
  (450/channel motor 300/channel for driver),
  or steppers (??/channel). Steppers would provide
  position read back.
• X/Y translation stage Use existing stages.

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Mechanics

• Mechanical Issues
• The ATF beam has a position jitter of 1 micron.
  In order to demonstrate 1nm bpm resolution, we
  need to do line fits between 3 (or more) bpms.
  This requires a position stability for these bpms
  of lt1nm for several pulses (10-30 seconds).
• For bpms spaced by meters, the ground motion and
  vibration will be substantially larger than this,
  probably hundreds of nanometers. For the final
  measurement the bpms must be mounted from a
  common reference block. That reference block must
  be on supports which are sufficiently soft to not
  transmit vibrations which can excite internal
  modes in the block, probably a 10Hz mechanical
  support frequency is reasonable.
• Thermal expansion of metals is 2x10-5/C. For a
  30cm scale length, this requires temperature
  stability of 2x10-4C in a measurement time (1
  minute). With insulation and in the controlled
  ATF environment this may be possible. Use if
  Invar or a similar low expansion mounting frame
  may provide a factor of 10 relaxation in this
  requirement. (Not more, since it is impractical
  to make to cavities or cavity mounts out of
  Invar.
• If the temperature requirements cannot be met, an
  interferometer (or "queensgate" style capacitive
  system) will need to be used relative to an Invar
  or Zerodur reference block.

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Plan

• 2) Study beam stability pulse to pulse and long
  term using nanoBPM
• Assume that the ATF beam is not stable at the
  nanometer level and cannot be made stable
• 3) Use the FONT feedback on a long multi-bunch
  train (coll with UK group)
• requires
• increasing the bandwidth of the nano BPM to 20
  MHz (from 1.5 MHz)
• Extra long trains lengthen the train by
  extracting 3 trains that were injected in a
  sequence
• Installation of the FONT feedback kicker and
  sampler
• Can we stabilize the back section to the
  nanometer level?