Steve Smith

1

• Steve Smith
• for
• J. Frisch, T. Borden, H. Loos, T. Montagne, M.
  Ross, D. Schultz, J. Wu, et al
• April 20, 2006

2
Applications of Bunch Length

• Beam longitudinal profile for accelerator
  physics
• Calibrated profile needed to understand machine
• Measurement can be low rate, invasive
• Bunch length signal for feedback
• Non invasive
• Signal at full repetition rate of beam
• Only need an output which is monotonic and stable
  with respect to bunch length tuning phases

3
Bunch Length Monitor Requirements

• After BC1
• 80 to 360 microns at 1nC
• 130 600 GHz Gaussian width
• 25 to 120 microns at 0.2nC
• 400GHz 2THz Gaussian width
• After BC2
• 8 to 40 microns at 1nC
• 1.2THz to 6 THz
• 4 to 20 microns at 0.2nC
• 2.4THz to 12 THz
• Bunches not Gaussian ? frequency distribution
  will be somewhat different.
• Goal for commissioning run
• Instrument and commission BC1
• Gain operational experience
• Discussion here almost entirely for BC1

4
Measurement Options

• Temporal
• Works like a high speed oscilloscope.
• Transverse deflection Cavity (LOLA)
• Electro-optical measurement
• Spectral
• Measure power spectrum radiated by beam
• Coherent radiation
• Any kind
• Synchrotron
• Edge
• Diffraction
• Gap
• Spectral measurement does not include phase
• ?information is lost.

5
Precision Measurement

• Transverse RF deflection structure (LOLA)
• High resolution
• directly calibrated
• using known phase shifts.
• Measurement from LOLA
• TTF at DESY
• 4 micron resolution (13 fs) demonstrated ???????
• Intercepting

13 femtosecond FWHM spike!
1 picosecond

• LOLA deflection cavity installed in LCLS will be
  used as the Gold Standard bunch length
  measurement
• Beam physics experiments
• Calibration of spectral detectors
• Run LOLA at some slow rate (as needed)
• to maintain calibration of non-intercepting bunch
  length monitors

6
Coherent Radiation Detectors

• BC1 range is 100GHz to 1THz
• (BC2 to 10THz)
• Corresponds to the 100um to few mm wavelength
  range for BC1
• Two approaches
• Waveguides
• Optics
• Standard microwave waveguide techniques difficult
  above 10 GHz
• near impossible at THz
• Free space quasi-optical techniques difficult at
  longer wavelengths (mm) due to diffraction.
• Materials absorption not well known in this
  frequency range.
• Calibrated measurement difficult
• Saved by LOLA
• Use both free-space and waveguide technology at
  BC1

7
Spectral Measurements

• Detect coherent radiation two ways
• CSR or Edge radiation in a bend
• Coherent radiation from ceramic gap
• Both provide order of a microJoule of energy.
• CSR/edge radiation provides somewhat more power
  and lower divergence
• easier to collect on the detector.
• Radiation from last bend of BC1 available
• BL11 is CSR/edge radiation detector
• Easy to add ceramic gap downstream.
• BL12 is gap radiation detector
• Calibration of bunch length vs. spectral power
• difficult to do from first principles
• but we have transverse cavity (LOLA)
• As long as signal is monotonic and reproducible,
  we can do periodic calibrations
• Eliminates the most serious problems with
  spectral detection.

8
Detectors

• High performance mm-wave detectors are cryogenic.
• Used for astronomy, etc.
• Avoid cryogenics if possible
• Room temperature detectors in principle have an
  energy sensitivity of
• Ethermal kBT 10-20J.
• Real detectors much worse
• Two common technologies
• Pyroelectric
• Diodes

9
Diodes vs. Pyroelectrics

• Diodes limited to 750GHz
• Diodes have better sensitivity
• Diodes have worse dynamic range, 10,0001, but
  this is probably not a limit
• Diodes more expensive (5K at high frequencies),
• rather than 500 (including preamplifier for pyro
• Diodes are more damage sensitive.

10
Waveguide Attenuation

• Waveguides available as small as WR-0.51
• Internal Dimensions 130um X 65um
• Frequency 1.4-2.2 THz
• Attenuation can be very high for small waveguide
• 3dB/M at 100GHz
• 17dB/M at 300GHz
• 100dB/M at 1THz
• (attenuations from empirical fit to data)
• Limits use of waveguide at high frequency

11
Waveguide vs. Free space
Compare Rayleigh length for free space with
sigma .5cm relative to length for 10dB
attenuation in Waveguide Approximate
cross-over At 400GHz
12
Coherent Synchrotron Radiation

• Narrow opening angle, large transverse size at
  end of magnet suggest use of free space optics to
  image onto detector.
• Expect order of 1uJ collected on detector.
• gt1000X Pyroelectric sensor sensitivity.
• No advantage to diodes here
• Since free space optics works well at high
  frequencies, this seems a good solution for
  frequencies gt250GHz

13
Conceptual Design
Diagnostics
Focusing
200mm
DR
10mm
Bend
ER
Mirror
Focusing f 200mm
SR
38mm
200mm
14
BL11 Bunch length monitor

• Use CS/edge radiation
• free space
• pyroelectric detector.
• Systems like this already in use
• M. Hogan at SPPS
• Retractable mirror in vacuum.
• Use flat mirror
• Off axis parabola would collect slightly more
  signal
• but has difficult alignment issues
• Slight modification of existing vacuum chamber
  and insertion design
• Hole for beam passage
• Small optical table for detector components
• Insertable wavelength filters.
• Alignment diode
• has phase space similar to mm-wave radiation

15
BL 11 Quasi-Optical / Pyroelectric Monitor

• Image coherent synchrotron edge radiation on
  pyroelectric detector

16
BC1 Radiation Distribution

• Wavelength 1mm
• 200mm downstream of BC1
• Near field integration of acceleration field
• Edge length ??²
• Mainly ER from both bend edges, 4x larger than SR
• Radiation from Entrance edge hits vacuum chamber

Horizontal Pol.
Vertical Pol.
17
Propagate Gauss-Laguerre Modes

• Use Gauss-Laguerre modes with radial mode number
  1 for field of each polarization
• Needs ?/2 transverse modes to get correct far
  field distribution

Horizontal polarization at magnet edge
? 1cm ? 500
18
CER Transmission Through Optics
For one polarization, normalized to total 2p
emission
3 cm-1
at detector
15 cm-1
3 cm-1
15 cm-1
19
Transverse Profile Through Optics
3 cm-1
15 cm-1
20
Is Interference of CER CDR a Problem?

• Get field at detector for CER and CDR
• CDR is not focused on detector
• Wave front curvature differs from CER
• Intensity at detector shows narrow fringe pattern
• Fringes much faster than changes in form factor
• Conclusion
• CDR can be ignored

21
Predicted Detector Signal vs Bunch Length
22
Pyroelectric Detectors

• Crystal which converts thermal directly to
  electrical output
• LiTaO3
• physics is fast nanosecond
• coatings can slow down the detectors.
• Integrate all input energy (DC-gamma rays)
• Very good linearity up to damage threshold.
• Act as current sources, approximately 1uC/J
• Noise limited by preamplifier.

23
Pyroelectric Detector Sensitivity

• ELTEC420m3
• 5mm diameter detector (20mm2).
• 0.3 uC/Joule sensitivity
• Detector capacitance Cd 100 pF
• A good charge preamplifier (Amptek A250F) should
  see 300 electrons RMS noise
• based on 100pf capacitance
• Corresponds to 0.15nJ detector noise.
• Parts cost
• Detector 75
• Pre-amplfier about 500.
• Threshold sensitivity 7.5pJ/mm2

24
BL11 Bunch Length MonitorDevelopment Plan

• Use of flat mirror in vacuum and existing chamber
  / mover design minimizes engineering before
  installation
• Optics and detectors on table can be modified as
  needed
• only humidity proof cover required
• Serves as a model for the BC2 bunch length
  monitor
• Short bunch length / high frequencies requires
  pyro detectors
• allows for easy use of free space optics .

25
Radiation from Gap

• 1 nC, 200micron bunch, 1cm gap gives about 2 uJ
  total energy
• Calculations fro Juhao Wu
• Radiation is distributed over a wide area
  difficult to collect.
• Corresponds to about 1.6nJ/mm2 for a 2cm radius
  gap
• Pyroelectric detectors (7.5pJ/mm2) marginal
  (especially for a 0.2nC bunch).
• Diode detectors (.03 to 0.4 pJ/mm2 depending on
  wavelength) look OK.
• RF horn will gain 10-20dB in diode detectors
• but probably lose 10dB in waveguide at high
  frequencies
• Looks reasonable, limited by waveguide, and diode
  frequency response to frequencies below about
  500GHz.

26
BL12 Bunch Length Monitor

• Located just after the BL11 monitor
• Uses gap and diode detectors
• Only vacuum component is conventional ceramic gap
• Initially instrument with 100GHz diodes
• Add higher frequency diodes as needed
• Diodes used in pairs to reduce effect of beam
  motion
• 20cm waveguide used to disperse pulse (1ns),
  keep peak power reasonable on diodes.
• 20dB gain horns on diodes

27
BL12 Waveguide / Diode Monitor
28
RF Diode Detectors

• Very fast diode to rectify the input signal
• Vout ?? Pin
• for input voltages lt diode drop
• Typically modest output impedance ( few KOhm).
• Linear output range limited to 100mV.
• Use waveguide dispersion to stretch mm-wave pulse
  to keep diode in linear range.
• Very high sensitivity 1V/W, or 1mC/J .
• Typically connected to waveguide
• Many vendors for Flt130GHz
• Few (only Virginia diodes found so far) for
  higher frequencies up to 800GHz.

29
RF Diode 100 GHz

• Millitech DXP-10
• WR10 input waveguide
• Active area 3mm2.
• 20dB gain horn available.
• Approx 2KOhm output impedance
• Output charge 0.15mC/J.
• Capacitance small 1pf.
• Assume A250F charge amplifier
• expect 100 electrons noise
• corresponds to 0.1pJ detector noise
• Maximum linear signal 500pJ
• Cost 1K, preamplifier 500
• Threshold sensitivity 0.03 pJ/mm2 (energy
  density)

30
Microwave Diode at 300GHz

• Virginia Diodes WR-2.2ZBD
• WR-2.2 input waveguide
• Active area 0.16mm2
• 20dB gain horn avaialble
• Output impedance 3KOhm
• Output charge 1mC/J
• Capacitance small 1pf.
• With A250F charge amplifier, expect 100
  electrons noise, corresponds to 0.016 pJ detector
  noise.
• Max linear signal 0.1nJ
• Cost 5K, preamplifier 500
• Sensitivity 0.1pJ/mm2
• Note, for 750GHz diode get threshold sensitivity
  0.4pJ / mm2

31
BL12 Development Plan

• Similar diodes operating at 100GHz tested in End
  Station A.
• Additional test in end station A in April 2006
• using same electronics as LCLS
• Will use pair of diodes to check measurement
  noise.
• Initial test in LCLS will be done with a pair of
  100GHz detectors.
• As shorter bunch length measurements are
  required, additional diodes and waveguide can be
  added
• Use of optical breadboard makes installation of
  new diodes (on optical clamp mount)
  straightforward.
• Will try using a pyroelectric detector mounted
  next to the gap.
• Should be able to measure total mm-wave power
• compare with toroid current measurement to get
  bunch length signal
• Very simple and inexpensive system if it works.
• In principal extends to very short bunch lengths
• Must be calibrated with LOLA

32
Controls Interface

• Pyroelectric detectors, and diodes will use very
  similar nuclear physics type charge sensitive
  preamplifiers
• Signals can be read with a conventional GADC
  (gated ADC).
• Initially will use SLC CAMAC ADC
• Existing software for control and histories
• Can provide slow feedback to main LCLS EPICS
  control system for feedback tests.
• For high bandwidth feedback convert to EPICS GADC
  in VME.
• Other controls interface is straightforward
• pneumatic actuators
• temperature monitoring

33
Summary

• Two coherent radiation bunch-length monitors for
  BC1
• Measure bunch length every pulse
• non-intercepting
• BL11
• Quasi-optical
• Pyroelectric readout
• BL12
• Waveguide
• Diode readout
• Both are calibrated by transverse deflecting
  cavity