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 Spectral
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, ~10,000:1, 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 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.
>1000X Pyroelectric sensor sensitivity.
No advantage to diodes here
Since free space optics works well at high frequencies, this seems a good solution for frequencies >~250GHz

13. Conceptual Design Diagnostics Focusing 200mm DR Bend 10mm ER Mirror
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 « λγ²
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 γ/2 transverse modes to get correct far field distribution
Horizontal polarization
at magnet edge
λ = 1cm
γ = 500

18. CER Transmission Through Optics
For one polarization, normalized to total 2π 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 Monitor: Development 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 < 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 F<~130GHz
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 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