Transcript BC1 Bunch Length Monitor Design and Plans

BC1 Bunch Length Monitor Designs
and Plans
LCLS Injector Commissioning
Workshop Oct 9-11 2006
Josef Frisch
October 10, 2006
Injector Commissioning Workshop
1
J. Frisch
[email protected]
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
October 10, 2006
Injector Commissioning Workshop
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J. Frisch
[email protected]
Bunch Length Measurement Requirements
Parameter
Nominal RMS bunch
Length (microns)
Coherent radiation
frequency (THz)
October 10, 2006
Injector Commissioning Workshop
1nC
BC1
200
1nC
BC2
20
0.2nC 0.2nC
BC1 BC2
60
8
0.25
2.5
0.8
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J. Frisch
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6.0
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 – may be useful to
experiments
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
October 10, 2006
Injector Commissioning Workshop
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J. Frisch
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David Fritz
Transverse Deflection Cavity
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
DESY
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
October 10, 2006
Injector Commissioning Workshop
SPPS
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J. Frisch
[email protected]
(bad pulse)
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
S. Benson et al
JLAB 2004
To be installed after BC1 and BC2.
October 10, 2006
Injector Commissioning Workshop
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J. Frisch
[email protected]
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.
October 10, 2006
Injector Commissioning Workshop
M. Woods
SLAC
7
J. Frisch
[email protected]
Optical Synchrotron Radiation Noise
Measurement
1.5ps
RMS distribution measurement does not require
calibration
Non-invasive
Not single shot
4.5ps
Will test after BC1
Can upgrade to (near?) single shot measurement
using optical spectrometer
1.5ps. 200-500pC, 44MeV beam using a spectrometer
with a resolution of 0.05nm/pixel
P. Catravas et al, Physical Review Letters 82 (1999) 5261
October 10, 2006
Injector Commissioning Workshop
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J. Frisch
[email protected]
What is the Plan (for Injector Commissioning)?
Expect the 2 MM-wave systems (synchrotron and gap
radiation) to be installed and operational
Expect both transverse cavities (before BC1 and after BC2)
to be operational.
Hope to have optical synchrotron spectral fluctuation
measurement after BC1, but do not rely on this.
Measurements:
Bunch length before DL1: TCAV
Bunch length after BC1: MM-wave measurements
calibrated by taking the beam all the way down to the BC2 TCAV.
Expect gap measurement to give relative measurement for fairly long
bunches (>~200 microns).
MM-wave synchrotron measurement for shorter bunches.
October 10, 2006
Injector Commissioning Workshop
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J. Frisch
[email protected]
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.
October 10, 2006
Injector Commissioning Workshop
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J. Frisch
[email protected]
Injector TCAV
TCAV tested successfully at full gradient in klystron lab
3MW, 120Hz, 3us pulse
2MW, 120Hz, 3us pulse was requirement
October 10, 2006
Injector Commissioning Workshop
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J. Frisch
[email protected]
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 (?)
Not a problem for injector commissioning.
Would like to improve optics and camera
October 10, 2006
Injector Commissioning Workshop
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J. Frisch
[email protected]
TCAV results from ESA Experiment
Simulation
Experiment
Note: double image probably
Due to camera problem
October 10, 2006
Injector Commissioning Workshop
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J. Frisch
[email protected]
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
October 10, 2006
Injector Commissioning Workshop
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J. Frisch
[email protected]
BL12 Gap Monitor
October 10, 2006
Injector Commissioning Workshop
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J. Frisch
[email protected]
October 10, 2006
Injector Commissioning Workshop
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J. Frisch
[email protected]
300 GHz diode detector
October 10, 2006
Injector Commissioning Workshop
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J. Frisch
[email protected]
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
October 10, 2006
Injector Commissioning Workshop
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J. Frisch
[email protected]
Data Acquisition
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
S   d n wn
October 10, 2006
Injector Commissioning Workshop
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J. Frisch
[email protected]
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
October 10, 2006
Injector Commissioning Workshop
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J. Frisch
[email protected]
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?
October 10, 2006
Injector Commissioning Workshop
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J. Frisch
[email protected]
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
October 10, 2006
Injector Commissioning Workshop
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J. Frisch
[email protected]
October 10, 2006
Injector Commissioning Workshop
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J. Frisch
[email protected]
Vacuum chamber
Minor modification of existing design used
for screens.
Flat mirror with hole for beam, directs mm
(and optical) radiation vertically through Zaxis 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.
October 10, 2006
Injector Commissioning Workshop
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J. Frisch
[email protected]
October 10, 2006
Injector Commissioning Workshop
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J. Frisch
[email protected]
October 10, 2006
Injector Commissioning Workshop
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J. Frisch
[email protected]
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
flexability)
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
October 10, 2006
Injector Commissioning Workshop
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J. Frisch
[email protected]
October 10, 2006
Injector Commissioning Workshop
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J. Frisch
[email protected]
October 10, 2006
Injector Commissioning Workshop
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J. Frisch
[email protected]
Detectors
Pyro-electric detectors used
as good compromise on
performance / cost
Approx. requirement, comparison
Of 9mm and 2mm detector energy
collection
Wanted to avoid cryogenic
detectors
12
Large area detectors
needed to collect long
wavelength signals.
Large area -> large
capacitance ~700pf.
Complicates amplifier
design
Transmission (%)
10
8
6
4
2
0
0
20
40
60
80
-1
Wavenumber (cm )
October 10, 2006
Injector Commissioning Workshop
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J. Frisch
[email protected]
100
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 ~10-100 microseconds
October 10, 2006
Injector Commissioning Workshop
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J. Frisch
[email protected]
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.
October 10, 2006
Injector Commissioning Workshop
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J. Frisch
[email protected]
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
October 10, 2006
Injector Commissioning Workshop
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J. Frisch
[email protected]
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).
October 10, 2006
Injector Commissioning Workshop
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J. Frisch
[email protected]
Overall Plan
Gun bunch shape diagnosed by injector TCAV.
Energy vs. time display should provide good diagnostic
After bend and BC1, have 2 mm-wave bunch length
measurements
Provide fast relative measurements for tuning / feedback.
Controls interface looks like LLRF system.
No serious technical issues expected
However – this is a new system, must expect some commissioning
time.
Calibration of mm-wave bunch length measurements most
difficult issue
Take beam all the way to sector 29 TCAV
Possibly use optical synchrotron measurement
Consider installing electro-optical measurement in 2008
October 10, 2006
Injector Commissioning Workshop
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J. Frisch
[email protected]