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

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


Overview

• EMI issues

• EMI measurements

• EMI and electronics

• Bunch length issues

• Bunch length measurements


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.


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.


Beam line & EMI antennas


“old” ceramic gap & 100GHz horn


New ceramic gap & VXD


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.


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.


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.


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.


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.


Data: Typical waveform

full scale: 50 ns – apparent frequency ~800MHz


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.


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.


FFT of sample waveform

scale in GHz


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.)


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.


Data: E vs charge

• Plot shows linear relation as predicted.


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.)


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).


Data: E vs θ

• Scope limits frequency resolution < 10GHz

• Observed signals ~< 1GHz

• No polar angle dependence observed due to diffraction effects.


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.


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.


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.


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.


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.


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.


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


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


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


WR90 Waveguide

16 GHz Low Pass Filter

23 GHz Low Pass Filter

WR90-WR75 Taper

DiodeDiode

Diode

WR10 Waveguide

Horn


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


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


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


Strong Correlation Between Left and Right 100 GHz Diodes


Bunch Compressor Voltage

Compressor Voltage

100

GH

z D

iode

Sig

nal


100

GH

z D

iode

Sig

nal

Phase Ramp

Damping Ring Phase

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

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