The 2008 SPS electron cloud transmission experiment: first results F. Caspers, E. Mahner, T. Kroyer...

The 2008 SPS electron cloud transmission experiment: first The 2008 SPS electron cloud transmission experiment: first resultsresults

F. Caspers, E. Mahner, T. KroyerB. Henrist, J.M. Jimenez

Thanks to the SPSU study team membersFor helpful discussions

Outline

Layout Basics and history First results during scrubbing run

Conclusion and outlook

ILCDR 08 July 2008 Cornell

F. Caspers, E. Mahner, T. Kroyer ILCDR 08 Cornell July 2008 2

Basic layout for microwave transmission setup (SPS 2008)

Principle idea:“When electromagnetic waves are

transmittedthrough a not too dense plasma, theyexperience a phase shift and possibly a

smallattenuation”

Antenna1 Antenna3

14.4 m 33.1 m

Antenna2 (excitation)

dipolequad

F. Caspers, T. Kroyer

2008 is similar to the 2004 setup


Some basics and history (1)



The presently used setup in BA5 is rather similar, except that we have dedicated button like PU‘s and kickers with a circulator after the hybrid and rather narrow-band filters (40 MHz) with steep slopes.

The filters are centered at 2.5 GHz. We used this time as receiver a conventional spectrum analyzer and a vector spectrum analyzer.


Preliminary measurement results in 2008 - scrubbing run (1)

SPS super-cycle 17656

Display from a conventional spectrum analzyer.

The phase modulation is in the small peaks, which are +/- 44 kHz away from the center.

The beam induced signal are the two bigger peaks next to the center line.



Display from a conventional spectrum analzyer.

The phase modulation is in the small peaks, which are +/- 44 kHz away from the center.

The beam induced signal are the two bigger peaks next to the center line.

In this display one can see the true width of the carrier, the modulation sideband and the beam signals.

SPS super-cycle 17651



SPS super-cycle 17568-69

Time axis is from top to bottom, intensity is color coded. CW signal on




CW signal off-on-off-on




Shift of CW frequency




Higher harmonics of the modulation sidebands since the modulation is rather rectangular than sinewave



SPS super-cycle 17552Expanded view of one modulation sideband vs time.



SPS super-cycle 17641Intensity of the modulation sideband vs time in logarithmic scale



SPS super-cycle 17650Intensity of the modulation sideband vs time in linear scale


Preliminary measurement results in 2008 –AM contamination (1)

SPS super-cycle 21196, July 9th 2008

How about AM contamination?

Are we really sure that what we see on the green trace (upper half) is pure FM or PM

At least in this case its pure AM since the demodulation function of the VSA tells us that there is NO FM or PM signal where we would expect it.(lower trace)




Here one can see a small assymetry in height of the modulation lines in the conventional SPA display which is ALWAYS a hint for superposition of AM and FM or PMAND we also see a very small FM –PM Linie in the lower half of the display where we expect it.




Another exampleUpper half:Conventional spectrumLower halfPM demodulation

This shot was taken after acceleration at flattop where usually we observed the strongest PM signals

2 batches




Here we have in the upper half the conventional spectrum and in the lower half the PM demodulated partObviously we cannot see anything during accerlation and a quantitative evaluation from this colour coded plot is not really possible.


LHC beam in SPS, 25 ns bunch spacing, 72 bunches, 1 batch


PS-type shielded button pickup signals

Yellow traceYellow trace stripline in stainless steel chamber #1Blue traceBlue trace pickup in chamber #1 containing enamel clearing electrodeMagenta traceMagenta trace pickup in stainless steel chamber #2 w/o enamel, for referenceNote, that there is some (unexpected) low frequency ringing (EMI) in the blue and magenta traceWe will try to understand its origin and get it removed in the future

Note that depite EMI contamination one can clearly see (light blue trace) the buildup of the e-cloud related current over the duration of the batch


Conclusion and Outlook

The intermodulation problem observed in 2003 (experiments in BA2) has been considerably reduced using a proper RF setup with highly selective filters at the frontend, both, for excitation and on the pickup side. However the application of the phase demodulation function (using a vector spectrum analyzer) is strongly recommended for the elimination of residual AM contamination due to faint saturation effects [gain compression] (more than 50 dB below the carrier) caused by beam induced signals.

First electron cloud transmission experiments performed during the 2008 SPS scrubbing run in June have shown very promising results in BA5. The presence of electron clouds was identified simultaneously with button pickups.

Microwave transmission over a full LHC arc at cryogenic temperature has been already successfully demonstrated around 7 GHz. The technique discussed here can be directly applied for integral electron cloud diagnostics over a full arc in the LHC. For this purpose the installed in situ reflectometer couplers at the end of each arc could be used.

Many thanks to Christian Carli for lending us the real-time spectrum analyzer and contributing during data taking.

The 2008 SPS electron cloud transmission experiment: first results F. Caspers, E. Mahner, T. Kroyer...

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