2/23/2006 LCLS Bunch Length Monitor Marc Ross - SLAC
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LCLS Bunch Length Monitor Conceptual Design Review
Instrument Design ConsiderationsFebruary 23, 2006
2/23/2006 LCLS Bunch Length Monitor Marc Ross - SLAC
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LCLS Bunch length monitor system:
• Two subsystems:– Deflecting structure ‘LOLA’
• Accurate, calibrated, complex• Destructive – ‘pulse stealing mode ok’• Expensive• Tested
– Radiation monitors• Dipole radiation• Gap radiation• Simple sensors• Cheap• Tested
Complementary devices
2/23/2006 LCLS Bunch Length Monitor Marc Ross - SLAC
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Bunch length monitor system
LOLAGap radiation
Dipole radiation
Single signal device
Video signal
Pyro-electric detectorHigh frequency diode
WGmm waveoptics
2/23/2006 LCLS Bunch Length Monitor Marc Ross - SLAC
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Two coherent radiation monitors for BC1• Simple ceramic gap surrounded by mm-wave
diode detectors (paired)– Total radiated energy is 2 uJ
• For 1nC, 200 micron bunch. • (energy scales as Q^2/(bunch length)
– Tested at SLC and ESA (actually many years…)
• Annular reflector directs dipole radiation onto mm-wave ‘optics’ – pyroelectric detector– similar total radiated energy– Tested at FFTB/SPPS
Bunch charge distribution • Simple indicator: central frequency of radiated
energy
2/23/2006 LCLS Bunch Length Monitor Marc Ross - SLAC
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Coherent radiation detection strategy:
• Each individual detector has ~ factor 2 range
• Long bunches: use diode sequence– (100, 200, 400, 1000 GHz)– Down to 100 um rms
• Short bunches: use reflector and pyro-electric detectors– Below 150 um rms
• RD required to match – see 2007 testing plan
CERN Bunch length RF gap
2/23/2006 LCLS Bunch Length Monitor Marc Ross - SLAC
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ESA 100 GHz Gap and Detector
gap
ESA gap monitor and detector
• Gap / horn / WG-10 closeup
gap
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400 GHz diode
ESA 100 GHz System – Jan 8, 2006Sig_z_min ~300 um
gap
2/23/2006 LCLS Bunch Length Monitor Marc Ross - SLAC
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Multi-frequency diode ‘xylophone’
Frequency (GHz)
Waveguide Attenuation (20 cm)
Energy in (J) Energy out
100 WR-10 0.8 dB 5 e-9 5e-9 200 WR-4.3 3 dB 1 e-9 500 e-12 400 WR-2.2 8 dB 200 e-12 30 e-12 1000 WR -1.0 22 dB 50 e-12 300 e-15
• 1nC / 200 um example– (Total radiated energy 2uJ)
• “Energy in” assumes catalog item waveguide horn• Detector sensitivity is 2.3e-15J into 50 Ohm out.• Good S/N for 100, 200, 400… 1000GHz ?
gap
2/23/2006 LCLS Bunch Length Monitor Marc Ross - SLAC
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‘Beam view’ of multi-diode / waveguide
assembly
gap
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Reflector• (in use at FFTB for plasma wake exp. – Hogan)
– Thin Ti foil ‘in the beam’ – polished.– Si window– Simple, direct, detector optics– Typically shorter than LCLS BC1 (not BC2)
• LCLS annular reflector will be 30mm diameter with 14 mm aperture– Capture 50% ~ 1 uJ – at best
• Low frequency performance reported to be poor (<300 GHz)
e-
VariablePositionMirror
∆z
Interferometer Pyro Detector
12.5 µm MylarBeam Splitters
RT≈0.17
12.5 µm Mylar1mm HDPE
Vacuum Window(3/4” dia)
Reference Pyro Detector
Alignment Laser1 µm Titanium Foil at 45º
• Interference signal normalized to the reference signal
CTR MICHELSON INTERFEROMETER
• Motion resolution ∆zmin=1 µm or ≈14 fs (round trip)
x=60 µm, y=170 µm N≈1.91010 e-
• Mylar: R≈22%, T≈78%, RT≈0.17 reflector
CTR Energy Correlates withBunch Length
Rel
ativ
e E
nerg
y @
end
of l
inac
reflector
0
5000
1 104
1.5 104
2 104
2.5 104
3 104
3.5 104
-2.5 -2 -1.5 -1 -0.5 0 0.5 1
XCompressionfrom544X OverOpitimumUnder
Am
plitu
de (a
.u.)
Relative Energy (MeV)
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
400 450 500 550 600 650 700 750 800
544X(Pyro)wwoImage Event with ImageEvent without Image
BP
MS
DR
13 5
44 X
CTR Pyro Amplitude (a.u.)
#61I7
#348I39
#124I14
X-ray
Pyro Is Not The Whole StoryNeed to Look at Details of the Spectra
• Pyro amplitude is ambiguous• Energy spectra are not• They are complimentary diagnostics• Clear correlation between energy spectrum and E-164X outcome
Example: Jitter from North Damping Ring:
Relative Energy [GeV]
reflector
reflectorpyro jitter
distribution – SLC NRTL
stability
Spectra vs pyro-electric signalreflector
reflectorOne pyro vs another
• meets 5 to 10% resolution goal
reflectorPyro for one band vs another
• Pyro response as a function of linac ‘chirp’ (phase - offset)
reflectorpyro response has position correlations:
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MM Wave detector Sensitivity
• Pyroelectric: Basically a charge source, approximately 1.5uC/Joule.• Capacitance is 120pf for 3mm detector. • Charge amplifier (AmpTek A250, external FET), has noise 300
electrons RMS. – Corresponds to 30pJ of mm wave energy
• Typically pyro detectors are supplied with included amplifier, performance tends to be worse.
• Detector is a thermal integrator. Dynamic range is limited by the dynamic range of the amplifier.
– For the AmpTek A250, this is approximately 60,000:1. • Commercial pyroelectric detectors (Scientech PHF02) have noise
level of 3nJ, approx 100uJ maximum signal. • Note, sensitivity is 100X worse than theoretical, dynamic range is
30,000:1
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MM wave diode detectors
• Sensitivity of ~2V/W (into 50 Ohms) with ~100GHz bandwidth (at 300GHz).
• For a 100ps pulse, Bandwidth ~5GHz.– Noise is 2.3e-15J.
• Diodes typically linear to ~30mV output.– 1.5pJ. Dynamic range 700:1
• Expect realistic amplifier (10dB noise figure) to limit dynamic range to 250:1
• Waveguide for 300GHz is WR-2.8, 0.7X0.35mm. – expected attenuation 0.2dB/cm.
• Waveguide for 900MHz is WR-1.0, 0.25X0.13mm. – Expected attenuation 1.1dB/cm
• Need something like 20cm of waveguide for dispersion – The initial pulse is very short, with extremely high peak power. Waveguide
dispersion spreads the pulse in time, while keeping the original frequency content.
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Comparison of pyro and diode detectors
• Pyro detectors have much larger dynamic range (>30,000:1, vs ~200:1).
• Noise energy diodes is 10^4 lower than for pyro detectors• Pyro area (for sample detector) is ~10 mm^2. • Diode (waveguide)
– At 300GHz 0.2mm^2– At 900GHz 0.03mm^2
• Diode Sensitivity / Area is 250x at 300GHz, 30X at 900GHz• Not clear how much gain available from horn antenna. (~10dB?)• Diodes more sensitive than pyros at 300GHz. • At 900GHz, diodes probably slightly less sensitive. • Dynamic range of pyro detectors is better• Diode alignment of waveguide is much easier.
2/23/2006 LCLS Bunch Length Monitor Marc Ross - SLAC
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Controls and data acquisition
• These systems are ‘single signal’ systems, i.e. only a simple gated digitizer is needed*– (Some concerns over gating precision and noise – to be tested)
• BC1 Feedback will require beam intensity normalization and (possibly) steering correction / feedback
• integration with LOLA improves the systematics greatly we strongly recommend an aggressive approach to LOLA data acq./integration.
• Pyro/diode systematics will be different and may require different procedures.
• 2007 testing plan
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Testing plan
• ESA – Minimum bunch length ~200 um ?– Multi-channel resolution test– No independent high accuracy reference– April and July 2006
• LCLS injector– Minimum bunch length ~ 50 um– High accuracy reference (29-4 LOLA Transverse cavity