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

July 20, 2006ILC EMI & bunch length studies2 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, 2006ILC EMI & bunch length studies3 Overview EMI issues EMI measurements EMI and electronics Bunch length issues Bunch length measurements

July 20, 2006ILC EMI & bunch length studies4 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, 2006ILC EMI & bunch length studies5 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, 2006ILC EMI & bunch length studies6 Beam line & EMI antennas

July 20, 2006ILC EMI & bunch length studies7 “old” ceramic gap & 100GHz horn

July 20, 2006ILC EMI & bunch length studies8 New ceramic gap & VXD

July 20, 2006ILC EMI & bunch length studies9 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, 2006ILC EMI & bunch length studies10 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, 2006ILC EMI & bunch length studies11 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/r 2 ); 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, 2006ILC EMI & bunch length studies12 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, 2006ILC EMI & bunch length studies13 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, 2006ILC EMI & bunch length studies14 Data: Typical waveform full scale: 50 ns – apparent frequency ~800MHz

July 20, 2006ILC EMI & bunch length studies15 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, 2006ILC EMI & bunch length studies16 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, 2006ILC EMI & bunch length studies17 FFT of sample waveform scale in GHz

July 20, 2006ILC EMI & bunch length studies18 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, 2006ILC EMI & bunch length studies19 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, 2006ILC EMI & bunch length studies20 Data: E vs charge Plot shows linear relation as predicted.

July 20, 2006ILC EMI & bunch length studies21 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, 2006ILC EMI & bunch length studies22 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~1.7 o (radiation remains forward). –λ~30 cm (1GHz)  Δθ ~170 o (radiation is no longer forward).

July 20, 2006ILC EMI & bunch length studies23 Data: E vs θ Scope limits frequency resolution < 10GHz Observed signals ~< 1GHz No polar angle dependence observed due to diffraction effects.

July 20, 2006ILC EMI & bunch length studies24 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, 2006ILC EMI & bunch length studies25 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, 2006ILC EMI & bunch length studies26 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, 2006ILC EMI & bunch length studies27 VXD Observations The PLL signal displays an EMI-like ringing signal at beam crossing. The PLL signal sometimes drops to ns 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, 2006ILC EMI & bunch length studies28 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, 2006ILC EMI & bunch length studies29 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, 2006ILC EMI & bunch length studies30 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, 2006ILC EMI & bunch length studies31 Three Frequency Ranges So Far… X band: –WR90: Cutoff Frequency = 6.6 GHz –Low Pass Filter: 16 GHz K u band: –WR75: Cutoff Frequency = 7.9 GHz –Low Pass Filter: 23 GHz W band: –WR10: Cutoff Frequency = 59.1 GHz GHz

July 20, 2006ILC EMI & bunch length studies32 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, 2006ILC EMI & bunch length studies33 WR90 Waveguide 16 GHz Low Pass Filter 23 GHz Low Pass Filter WR90-WR75 Taper Diode WR10 Waveguide Horn

July 20, 2006ILC EMI & bunch length studies34 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, 2006ILC EMI & bunch length studies35 16 GHz 23 GHz 100 GHz Right Diode 100 GHz Left Diode Slow Diodes 100 GHz Diodes Raw Diode Signals from 5 GS/s Scope 20 ns/div 10 mV 20 ns/div 20 mV 80 ns/div 5 mV 80 ns/div 5 mV

July 20, 2006ILC EMI & bunch length studies36 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, 2006ILC EMI & bunch length studies37 Strong Correlation Between Left and Right 100 GHz Diodes

July 20, 2006ILC EMI & bunch length studies38 Bunch Compressor Voltage Compressor Voltage 100 GHz Diode Signal

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