ilc emi and bunch length measurements gary bower, slac nick sinev, u. oregon, speaker sean walston,...
TRANSCRIPT
ILC EMI and bunch length measurements
Gary Bower, SLAC
Nick Sinev, U. Oregon, speaker
Sean Walston, LLNL
July 20, 2006 ILC EMI & bunch length studies 2
Important Contributors• Ray Arnold, SLAC• Karl Bane, SLAC• Eric Colby, SLAC• Joe Frisch, SLAC• Doug McCormick, SLAC• Marc Ross, SLAC• Yasuhiro Sugimoto, KEK• Mike Woods, SLAC• Hitoshi Yamamoto, Tohoku University• Japan-US Cooperative Program in High Energy
Physics by JSPS
July 20, 2006 ILC EMI & bunch length studies 3
Overview
• EMI issues
• EMI measurements
• EMI and electronics
• Bunch length issues
• Bunch length measurements
July 20, 2006 ILC EMI & bunch length studies 4
EMI - Introduction
• Going back to the 70s there has been concern about beam generated EMI affecting detector electronics.
• SLD Vertex Detector electronics.
• As part of the SLAC ILC test beam studies:– Make EMI measurements with antennas.– Expose the SLD VXD electronics to beam EMI.
July 20, 2006 ILC EMI & bunch length studies 5
Beam line sources of EMI
• Accelerator beam is usually enclosed in evacuated conducting beam pipe.– The beam pipe is thick enough to contain all
wakefield radiation.
• However, to monitor beam properties (location, emittance, current, etc) “gaps” in the conducting beam pipe are needed.– The instrumentation gaps may allow leakage
of wakefield radiation into the ambient environment.
July 20, 2006 ILC EMI & bunch length studies 6
Beam line & EMI antennas
July 20, 2006 ILC EMI & bunch length studies 7
“old” ceramic gap & 100GHz horn
July 20, 2006 ILC EMI & bunch length studies 8
New ceramic gap & VXD
July 20, 2006 ILC EMI & bunch length studies 9
Antennas
• Used two AHSystems EMI antennas:– Biconical: Precision calibrated for 30-330MHz.– Log-periodic (“yagi”): Precision calibrated for
650-4000MHz.
• However, both antennas are sensitive to roughly the same, much larger range.
July 20, 2006 ILC EMI & bunch length studies 10
Antenna measurement technique
• Antennas were connected via 50ohm coax signal cable to a 2.5GHz resolution digital scope– Amplitude distorted signals observable up to
~10GHz (scope can do 20Gsamples/sec)
• This enabled measuring E field signal shape and strength.
• By orienting antennas polarization could be measured.
July 20, 2006 ILC EMI & bunch length studies 11
Far field regime
• Radiation from moving charges has a complex near field structure and a simple transverse EM wave far field structure.
• Near field( E~1/r2 ); Far field (E~1/r).• A signal 1 meter from a source is dominated by
the far field regime for a 1GHz signal (λ=0.3m).• In the far field regime, measure only E field
strength and infer B field strength using the wave equation.
July 20, 2006 ILC EMI & bunch length studies 12
Measurement results
• The following slides present the results and interpretations of measurements that were made to characterize the beam induced EMI radiation.
• All this analysis is PRELIMINARY in nature and subject to change with additional data and further analysis.
July 20, 2006 ILC EMI & bunch length studies 13
Using time delays to find sources
• Trigger on an external accelerator beam crossing signal.
• Know the length (time delay) of cables.
• Know antenna locations and compare relative signal strength at different locations.
• Can determine the location of EMI sources along the beam line.
July 20, 2006 ILC EMI & bunch length studies 14
Data: Typical waveform
full scale: 50 ns – apparent frequency ~800MHz
July 20, 2006 ILC EMI & bunch length studies 15
Waveform shape implications• The waveform was very stable under a wide
range of machine conditions (current, emittance, bunch length, etc). – This suggests the shape is determined primarily by
the beam pipe geometry.
• The waveform was stable against moving the antenna large distances from the source.
• The amplitude ~1/r when antenna is moved.– These observations support making the far field
assumption above for the signal frequencies observable with these antennas and this scope.
July 20, 2006 ILC EMI & bunch length studies 16
Data: Absolute field strength• Signal seen on scope is attenuated by
– antenna factor and cable attenuation.
• Accounting for these we find an absolute E peak to peak field strength of ~20 volts/m at ~1 meter from the gap for a current of ~1.5x10^10.
• This result is approximate since it is based on a linear extrapolation for cable attenuation that needs further checking.
July 20, 2006 ILC EMI & bunch length studies 17
FFT of sample waveform
scale in GHz
July 20, 2006 ILC EMI & bunch length studies 18
Power spectrum analysis
• FFT analysis assumes infinitely long waveform.
• Problems with finite length waveform FFTs.– Example: consider a fixed frequency signal, f,
modulated by a Gaussian. Then overlap several such pulses all with the same frequency. An FFT will not recover f.
• Try FROG or Wavelet analysis. (To do.)
July 20, 2006 ILC EMI & bunch length studies 19
Wakefield theory predicts
• E field strength ~ bunch charge.• E field strength ~ (1/bunch length)1/2.• Radiation is very forward directed, however,
gap diffraction has different effects on different wavelength components.
July 20, 2006 ILC EMI & bunch length studies 20
Data: E vs charge
• Plot shows linear relation as predicted.
July 20, 2006 ILC EMI & bunch length studies 21
Data: E vs bunch length
• No effect is seen in the ~ 1GHz range.
• There is an effect in the 100GHz range (see later slides on bunch length measurements.)
July 20, 2006 ILC EMI & bunch length studies 22
Diffraction effects• Wakefield radiation at gap is very forward.• However, diffraction occurs at the gap.• Coherent radiation from a source obeys
(gap width*divergence) ΔxΔθ ≈ λ/2.• For gap width ~5 cm:
– λ~0.3cm (100GHz) Δθ~1.7o (radiation remains forward).
– λ~30 cm (1GHz) Δθ ~170o (radiation is no longer forward).
July 20, 2006 ILC EMI & bunch length studies 23
Data: E vs θ
• Scope limits frequency resolution < 10GHz
• Observed signals ~< 1GHz
• No polar angle dependence observed due to diffraction effects.
July 20, 2006 ILC EMI & bunch length studies 24
EMI measurements summary
• Typical waveform rings @ ~1GHz for about 50ns.
• Absolute peak to peak E field strength ~20 V/m at 1m at ~1.5x10^10 current.
• E is linear with current.
• E shows no dependence on bunch length or polar angle in the ~1GHz range.
July 20, 2006 ILC EMI & bunch length studies 25
EMI and VXD electronics
• An SLD VXD electronics module was placed near the new ceramic gap.
• EMI antennas were placed at the same location.
• The phase-lock loop signal circuit was monitored. – When working properly it asserts a DC
voltage. When it fails it asserts 0 voltage.
July 20, 2006 ILC EMI & bunch length studies 26
VXD phase lock loop drops
Top trace: VXD board phase-lock loop signal
Other traces: the two EMI antennas.
Time offsets are due to cable length differences.
July 20, 2006 ILC EMI & bunch length studies 27
VXD Observations
• The PLL signal displays an EMI-like ringing signal at beam crossing.
• The PLL signal sometimes drops to 0.
• 20-40ns after the EMI waveform appears the DC signal drops to 0 in < few ns.
• It always drops at the bottom of a wave cycle in the waveform.
July 20, 2006 ILC EMI & bunch length studies 28
Cause of VXD failure
• By various combinations of shielding cables and the VXD module, it is determined that:– The failure is not due to ground currents
induced by beam image charges.– The failure is not due to amplifier overload.– The failure is not due to EMI radiation on the
cables.– The failure is due to EMI radiation on VXD
module.
July 20, 2006 ILC EMI & bunch length studies 29
VXD failure rate vs EMI strength
• The VXD module phase lock loop lost lock on about 85% of beam crossing when the module was exposed to ~20 V/m of EMI.
• The VXD module lost lock about 5% when exposed to ~1 V/m of EMI.
July 20, 2006 ILC EMI & bunch length studies 30
Bunch length measurement• The power spectrum radiated by a bunch is related in a
non-trivial way to the length of the bunch:– Shorter bunch = more power, especially at shorter wavelengths– Longer bunch = less power overall, and what’s there resides at
longer wavelengths
• A ceramic gap is used to get signals out of the beam pipe• With the 300 micron bunch at ESA, we ideally want to
look at millimeter and sub-millimeter wavelengths • By begging, borrowing, and stealing, parts were
scrounged from around SLAC and an RF bunch length monitor was built at a ceramic gap in End Station A– Many thanks to Doug McCormick, Eric Colby, Joe Frisch, and
Marc Ross for parts and technical assistance
July 20, 2006 ILC EMI & bunch length studies 31
Three Frequency Ranges So Far…
• X band:– WR90: Cutoff Frequency = 6.6 GHz– Low Pass Filter: 16 GHz
• Ku band:– WR75: Cutoff Frequency = 7.9 GHz– Low Pass Filter: 23 GHz
• W band:– WR10: Cutoff Frequency = 59.1 GHz
10 20 30 40 50 60 70 80 90 100 120GHz
July 20, 2006 ILC EMI & bunch length studies 32
WR10 Waveguide(0.1 x 0.05 inches)
WR90 Waveguide(0.9 x 0.4 inches)
Ceramic Gap
To 16 GHz and 23 GHz Diodes
To 100 GHz Diode
Beam Pipe
~8 cm
WR90 Waveguide
WR10 Waveguide
Ceramic Gap
Initially, there was too much signal in the 100 GHz diodes, so the horns were removed and the waveguides retracted ~8 cm.
RF Bunch Length Monitor for ESA
To 100 GHz Diode
July 20, 2006 ILC EMI & bunch length studies 33
WR90 Waveguide
16 GHz Low Pass Filter
23 GHz Low Pass Filter
WR90-WR75 Taper
DiodeDiode
Diode
WR10 Waveguide
Horn
July 20, 2006 ILC EMI & bunch length studies 34
Measurement ElectronicsGated integrator installed to allow ~2 ns gates for expected signals from diodes(Actual signals ~20 ns, so standard GADC being used for July run)
DC output from gated integrators read by SLC control system SAM
July 20, 2006 ILC EMI & bunch length studies 35
16 GHz 23 GHz
100 GHzRight Diode
100 GHzLeft Diode
Slo
w D
iod
es10
0 G
Hz
Dio
des
Raw Diode Signals from 5 GS/s Scope
20 ns/div10 mV
20 ns/div20 mV
80 ns/div5 mV
80 ns/div5 mV
July 20, 2006 ILC EMI & bunch length studies 36
A Few Observations
• Slow diodes correlate well with toroid signal (bunch charge)
• Left and right fast diodes highly correlated with each other
• Fast diodes vary with damping ring phase and bunch compressor voltage suggesting they may actually be measuring something proportional to the bunch length
July 20, 2006 ILC EMI & bunch length studies 37
Strong Correlation Between Left and Right 100 GHz Diodes
July 20, 2006 ILC EMI & bunch length studies 38
Bunch Compressor Voltage
Compressor Voltage
100
GH
z D
iode
Sig
nal
July 20, 2006 ILC EMI & bunch length studies 39
100
GH
z D
iode
Sig
nal
Phase Ramp
Damping Ring Phase