1. Bunch Length Monitor Development
Facility Advisory Committee
Oct
Josef Frisch

2. Bunch Length Measurement Requirements
Single shot bunch length measurement after BC1, and BC2
BC2 required later – will use the same technology
Can calibrate single-shot system measurement using multi-bunch measurements
Transverse cavity available for absolute bunch length measurements
TCAV far downstream, want to minimize use for calibrating BC1 measurement

3. Bunch Length Measurement Requirements
Parameter
1nC
BC1
BC2
0.2nC
Nominal RMS bunch Length (microns)
200 20 60 8
Coherent radiation frequency (THz)
0.25
2.5
0.8
6.0

4. Electro-optical Sampling
Non-invasive
Directly measures bunch longitudinal profile
Single Shot Measurement
Resolution down to ~100fsec
~200 fsec demonstrated
Allows direct laser vs. beam measurements
Requires high peak current
LCLS BC1 current ~10X lower than at SPPS where some experiments were done.
Best at short pulses (BC2) which have higher current
Expensive and Complex – femtosecond laser, etc.
Probably will install after BC2
David Fritz

5. Transverse Deflection Cavity
SPPS
Can measure longitudinal profile vs. energy (if spectrometer included).
Single Shot
Resolution <100fsec (15fsec demonstrated at TTF2 / FLASH DESY)
Good Dynamic range (can adjust sweep strength)
Invasive.
Pulse stealing
May require optics change for best resolution
Expensive and complex (but uses standard SLAC supported technology)
Installed before bend DL-1 and after second compressor BC-2
Primary bunch length calibration for LCLS
DESY
(bad pulse)

6. Millimeter Wave Coherent Synchrotron Radiation
Single Shot (assuming single shot spectrometer, or multiple detectors)
Non-Invasive
Measures from arbitrarily short to ~mm bunches (with appropriate filters).
Simple high rate readout – can use signal from single detector with input filter
Measures power spectrum (no phase information) – cannot reconstruct bunch shape
Variations on spectral response must be calibrated using external bunch length measurement – not practical to provide a calibrated signal
To be installed after BC1 and BC2.

7. Millimeter Wave Gap Radiation
Single Shot (assuming single shot spectrometer, or multiple detectors)
Non-Invasive
Simple high rate readout – can use signal from single detector
Very simple, low cost
Low noise readout <1% RMS demonstrated
Diode detectors work to ~300GHz -> ~200 micron bunch length
Possibly can be extended to ~ 1THz, ~70 micron bunch length
Provides only relative measure of bunch length
To be installed after BC1.

8. Optical Synchrotron Radiation Noise Measurement
P. Catravas et al, Physical Review Letters 82 (1999) 5261
1.5ps pC, 44MeV beam using a spectrometer with a resolution of 0.05nm/pixel
1.5ps
4.5ps
RMS distribution measurement does not require calibration
Non-invasive
Not single shot
Will test after BC1
Can upgrade to (near?) single shot measurement using optical spectrometer

9. Transverse Cavity - Injector
0.5 Meter long S-band transverse cavity
1.4 MV deflecting voltage (max)
Powered from 20-5d
Deflected spot (E vs. T) viewed on OTR screen in front of injector dump.
Operation initially manual, developing software for automated measurement
Screen configuration not designed for “pulse stealing”.
Could add slightly off-axis screen, and operate TCAV slightly off of 0 phase if continuous measurement is necessary.
Unfortunately, not easy to transform measured bunch lengths to BL11, BL12 bunch length monitors after bunch compressor.

10. Injector TCAV
TCAV tested successfully at full gradient in klystron lab
3MW, 120Hz, 3us pulse
2MW, 120Hz, 3us pulse was requirement

11. Transverse Cavity Sector 25/29
Cavity now operating in sector 29. Will be used there for commissioning, moved to sector 25 later.
2.44M long structure, 20MV deflecting voltage
Easily resolves 20 micron bunch (in simulation).
8 micron bunch (low charge case) more challenging
Can change optics for measurement to have larger B function in deflection cavity (?)
5 micron bunch measured at DESY / FLASH
Would like to improve optics and camera

12. Transverse Cavity results from ESA Experiment
Simulation (including longitudinal wakes)
Experiment
Note: double image probably
Due to camera problem

13. Millimeter Wave Gap Monitor
Gap, with 2 pairs of millimeter wave detectors
100GHz detectors
Very good intensity sensitivity
Initial tuning, ~1mm bunch length sensitivity
Same detectors used for End Station A run
300GHz detectors
200 micron bunch lengths for normal operation
New type of detectors
Detectors produce short (~100 picosecond) output pulse, set by dispersion in waveguide
Amplifiers
Best sensitivity with ~1KOhm input impedance amplifier
Best linearity with ~50 Ohm input impedance amplifier
Both types tested, will try both in actual operation.
Data acquisition similar to LLRF system

14. Millimeter-wave gap monitor tests in End Station A
Output of 100GHz detectors as phase (bunch length) is adjusted
M. Woods
SLAC
Comparison of 2, 100GHz detectors for a range of operating conditions
RMS difference 1.4% for 10,000 pulses

15. BL12 Gap Monitor

17. 300 GHz diode detector

18. Diode Amplifiers / Electronics
Diodes act as ~1K Ohm output impedance devices, ~ 1 V/mw sensitivity
Maximum linear output ~50mV
Very high output bandwidth ~1GHz
Have tested 1KOhm input amplifier (100MHz BW), and fast 50Ohm amplifiers
Choice will depend on observed signal level.
Ideally would use fast gated integrator, ~200ps gate width
Will start with ~30MHz bandwidth limit amplifiers
Bandwidth limit to match to 100Ms/s digitizers
In principal loose ~x5 in sensitivity
From End station A tests, think we are OK
Can add commercial (SRS 255) fast gated integrator if needed

19. Data Acquisition Change w to provide Pulse integral rather Than I/Q
Use LLRF type digitizers
Operate at 102Ms/S, 16 bit
500 samples can be processed in real time
16K samples available for diagnostics
LLRF software algorithm requires only slight modification for diode signal
Change w to provide
Pulse integral rather
Than I/Q

20. Gap Monitor with Pyroelectric Detectors
Tested at End Station A
Just put a pyro detector with RF horn next to the ceramic gap.
Same detector as used for BL12 (discussed next)
Sensitivity too low for single shot measurements, but should extend measurement range to <100 microns
Limit not yet tested
Lower performance than the mm-wave CSR monitor, but simple to commission
M. Woods
Pyro detector looking at gap in end station A during RF phase change

21. BL12 Gap Monitor Overall Status
Expect system to be ready at turn-on
Detector distance from gap must be adjusted
Damage threshold ~100X max normal operating range, but signal level is difficult to calculate.
Only see signals when bunch is fairly short (few mm)
Can find shortest bunch length without calibration, but for length measurement, Need calibration – use TCAV at end of linac.
1Km of beam line away: this is probably our most serious problem
Usual commissioning problems – if no signal:
Long bunch? Bad alignment? Dead diode? Bad cables? Bad timing? Ceramic doesn’t transmit mm-waves?

22. BL11 Millimeter-wave CSR bunch length monitor
Mirror with hole after bend to collect synchrotron radiation stripe
Reflective optics (off-axis parabolas) to collect and transport light
Beam splitting filter for high pass / low pass to 2 mm-wave detectors
Different filters available
Compare power on detectors for (uncalibrated) bunch length measurement
Similar in concept to gap monitor, but bend and collecting optics give larger (>X10) signal, at cost of increased complexity
Need higher signal for short bunch measurements where diode detector do not work

24. Minor modification of existing design used for screens.
Vacuum chamber
Minor modification of existing design used for screens.
Flat mirror with hole for beam, directs mm (and optical) radiation vertically through Z-axis quartz window.
No particular technical issues
Chamber is in fabrication, but will not be ready for installation before turn-on
Will install spool piece until ready.

27. Optical Path Off-axis parabolas for transport / focusing
Millimeter-wave radiation awkward
Too small for waveguide
Too big for free space optics
Alignment not critical – mm, not micron wavelength.
Will be aligned with visible source
Filters not yet designed
In principal easy – pattern of squares, or wire grid on PC board to give low pass or high pass
Commercial solutions too sophisticated, very expensive (need multiple units for flexibility)
Difficult to test – millimeter wave test equipment expensive.
Do not need exact performance, only stability (calibrate system using TCAV)
Can be quickly installed
Most significant outstanding technical issue
Horns used to concentrate signal on detectors
Chamber purged with dry air / nitrogen
Choice is safety paperwork vs. performance

30. Detectors
Pyro-electric detectors used as good compromise on performance / cost
Wanted to avoid cryogenic detectors
Large area detectors needed to collect long wavelength signals.
Large area -> large capacitance ~700pf. Complicates amplifier design
Approx. requirement, comparison
Of 9mm and 2mm detector energy
collection

31. Detector Amplifiers
Need good charge sensitivity at high capacitance (~700pf)
Similar to physics detector amplifiers
Have simple op-amp based amplifier with ~3000 electrons RMS noise
Expect ~1000 electrons noise with external FET front end
Expect ~ 1uJ X 1uC/J = 1pC charge on detector max ~ 107 electrons.
Easiest to make low charge noise amplifier with slow output ~ microseconds

32. Will use same digitizer as LLRF
Data Acquisition
Will use same digitizer as LLRF
May need to run digitizer at ~10Ms/s to record long waveforms (up to 50usec)
Have synchronous divider for clock
Low frequency bandwidth of digitizer too low (~1MHz)
Use detector signal to modulate RF tone
Ugly, but simple, and avoids need for new hardware or software.

33. BL11 Millimeter Wave CSR monitor, overall status
Vacuum chamber slightly delayed, will probably not affect commissioning
Electronics / data acquisition not finished, but no significant problems foreseen
Filters need to be designed!
Similar issues to gap monitor for calibration – need downstream TCAV.
Largest operational issue

34. Optical Synchrotron Radiation Correlation Measurement
Plans to add optical detector with narrow band filter to BL11 bunch length monitor
Optionally can add single shot optical spectrometer
New plan – no real schedule yet, but might be possible to install quickly
Would provide calibrated measurement of RMS bunch length (but not shape).

35. BC1 vs. BC2 systems
BC2 will have TCAV available for calibration, or for continuous measurement in pulse stealing mode
Assuming BC1 mm-wave monitors are successful, will use nearly same design for BC2
Gap monitor probably not useful for short bunches, but may still be a useful tuning tool
CSR monitor easier for short bunches since shorter wavelengths are easier to focus.
Hope to have operational electro-optical bunch length monitor at BC2
If possible might then install similar system after BC1 for calibrated measurement
Optical spectrum fluctuation measurement may be useful in both locations.