1. ILC EMI and bunch length measurements
Gary Bower, SLAC
Nick Sinev, U. Oregon, speaker
Sean Walston, LLNL

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. ILC EMI & bunch length studies
Overview
EMI issues
EMI measurements
EMI and electronics
Bunch length issues
Bunch length measurements
July 20, 2006
ILC EMI & bunch length studies

4. ILC EMI & bunch length studies
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. ILC EMI & bunch length studies
New ceramic gap & VXD
July 20, 2006
ILC EMI & bunch length studies

9. ILC EMI & bunch length studies
Antennas
Used two AHSystems EMI antennas:
Biconical: Precision calibrated for MHz.
Log-periodic (“yagi”): Precision calibrated for MHz.
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. ILC EMI & bunch length studies
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. ILC EMI & bunch length studies
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. ILC EMI & bunch length studies
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. ILC EMI & bunch length studies
Data: E vs charge
Plot shows linear relation as predicted.
July 20, 2006
ILC EMI & bunch length studies

21. ILC EMI & bunch length studies
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. ILC EMI & bunch length studies
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. ILC EMI & bunch length studies
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 ~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. ILC EMI & bunch length studies
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. ILC EMI & bunch length studies
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
120
July 20, 2006
ILC EMI & bunch length studies
GHz

32. RF Bunch Length Monitor for ESA
To 16 GHz and 23 GHz Diodes
WR90 Waveguide
(0.9 x 0.4 inches)
WR90 Waveguide
To 100 GHz Diode
Ceramic Gap
Beam Pipe
Ceramic Gap
~8 cm
WR10 Waveguide
(0.1 x 0.05 inches)
To 100 GHz Diode
Initially, there was too much signal in the 100 GHz diodes, so the horns were removed and the waveguides retracted ~8 cm.
WR10 Waveguide
July 20, 2006
ILC EMI & bunch length studies

33. ILC EMI & bunch length studies
23 GHz Low Pass Filter
16 GHz Low Pass Filter
Diode
Diode
WR90-WR75 Taper
WR90 Waveguide
Diode
Horn
WR10 Waveguide
July 20, 2006
ILC EMI & bunch length studies

34. Measurement Electronics
Gated 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. Raw Diode Signals from 5 GS/s Scope
16 GHz
80 ns/div
5 mV
23 GHz
80 ns/div
5 mV
Slow Diodes
100 GHz
Left Diode
100 GHz
Right Diode
20 ns/div
10 mV
20 ns/div
20 mV
100 GHz Diodes
July 20, 2006
ILC EMI & bunch length studies

36. ILC EMI & bunch length studies
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
100 GHz Diode Signal
Compressor Voltage
July 20, 2006
ILC EMI & bunch length studies

39. ILC EMI & bunch length studies
Damping Ring Phase
100 GHz Diode Signal
Phase Ramp
July 20, 2006
ILC EMI & bunch length studies