Microwave diagnostics for electron cloud detection on the SPS F. Caspers, E. Mahner, T. Kroyer B. Henrist, J.M. Jimenez Thanks to the SPSU study team members For helpful discussions Outline  Layout  Basics and history  First results during scrubbing run  Conclusion and outlook ECM 08 Nov 2008 CERN

2 Nov. 20th 2008 Microwave diagnostics in the SPS ECM 2008 Caspers, Mahner, Kroyer, Henrist, JimenezF. Caspers, E. Mahner, T. Kroyer Basic layout for microwave transmission setup (SPS 2008) Principle idea: “When electromagnetic waves are transmitted through a not too dense plasma, they experience a phase shift and possibly a small attenuation” Antenna1 Antenna m 33.1 m Antenna2 (excitation) dipole quad F. Caspers, T. Kroyer 2008 is similar to the 2004 setup

3 Nov. 20th 2008 Microwave diagnostics in the SPS ECM 2008 Caspers, Mahner, Kroyer, Henrist, Jimenez Some basics and history (1)

4 Nov. 20th 2008 Microwave diagnostics in the SPS ECM 2008 Caspers, Mahner, Kroyer, Henrist, Jimenez Some basics and history (1a) This general expression for the phase shift in a low density plasma filled waveguide could suggest that it would be ideal to work at or very close to waveguide cutoff… Well in the end we are interestested in a good signal to noise RATIO and if we have too much attenuation when working close to cutoff, it does not help a lot. Also we must keep in mind that we dont like steep slopes in the HTf (hardware transfer function) in order to avoid HTF related AM/PM conversion in other words the upper and the lower modulation sideband should pass with the same attenuation otherwise the difference between upper and lower sideband would look like a phasemodulation.Thus avoid steep slopes !

5 Nov. 20th 2008 Microwave diagnostics in the SPS ECM 2008 Caspers, Mahner, Kroyer, Henrist, Jimenez Some basics and history (2)

6 Nov. 20th 2008 Microwave diagnostics in the SPS ECM 2008 Caspers, Mahner, Kroyer, Henrist, Jimenez Some basics and history (3)

7 Nov. 20th 2008 Microwave diagnostics in the SPS ECM 2008 Caspers, Mahner, Kroyer, Henrist, Jimenez Some basics and history (4)

8 Nov. 20th 2008 Microwave diagnostics in the SPS ECM 2008 Caspers, Mahner, Kroyer, Henrist, Jimenez Some basics and history (5)

9 Nov. 20th 2008 Microwave diagnostics in the SPS ECM 2008 Caspers, Mahner, Kroyer, Henrist, Jimenez Some basics and history (6) 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.

10 Nov. 20th 2008 Microwave diagnostics in the SPS ECM 2008 Caspers, Mahner, Kroyer, Henrist, Jimenez Preliminary measurement results in scrubbing run (1) SPS super-cycle 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.

11 Nov. 20th 2008 Microwave diagnostics in the SPS ECM 2008 Caspers, Mahner, Kroyer, Henrist, Jimenez Preliminary measurement results in scrubbing run (2) 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

12 Nov. 20th 2008 Microwave diagnostics in the SPS ECM 2008 Caspers, Mahner, Kroyer, Henrist, Jimenez Preliminary measurement results in scrubbing run (3) SPS super-cycle Time axis is from top to bottom, intensity is color coded. CW signal on

13 Nov. 20th 2008 Microwave diagnostics in the SPS ECM 2008 Caspers, Mahner, Kroyer, Henrist, Jimenez Preliminary measurement results in scrubbing run (4) SPS super-cycle CW signal off-on-off-on The VSA has only 80 dB SFD (spurious free dynamic range) due to ADC limitations!

14 Nov. 20th 2008 Microwave diagnostics in the SPS ECM 2008 Caspers, Mahner, Kroyer, Henrist, Jimenez Preliminary measurement results in scrubbing run (5) SPS super-cycle Shift of CW frequency

15 Nov. 20th 2008 Microwave diagnostics in the SPS ECM 2008 Caspers, Mahner, Kroyer, Henrist, Jimenez Preliminary measurement results in scrubbing run (6) SPS super-cycle Higher harmonics of the modulation sidebands since the modulation is rather rectangular than sinewave

16 Nov. 20th 2008 Microwave diagnostics in the SPS ECM 2008 Caspers, Mahner, Kroyer, Henrist, Jimenez Preliminary measurement results in scrubbing run (7) SPS super-cycle Expanded view of one modulation sideband vs time.

17 Nov. 20th 2008 Microwave diagnostics in the SPS ECM 2008 Caspers, Mahner, Kroyer, Henrist, Jimenez Preliminary measurement results in scrubbing run (8) SPS super-cycle Intensity of the modulation sideband vs time in logarithmic scale

18 Nov. 20th 2008 Microwave diagnostics in the SPS ECM 2008 Caspers, Mahner, Kroyer, Henrist, Jimenez Preliminary measurement results in scrubbing run (9) SPS super-cycle Intensity of the modulation sideband vs time in linear scale

19 Nov. 20th 2008 Microwave diagnostics in the SPS ECM 2008 Caspers, Mahner, Kroyer, Henrist, Jimenez 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)

20 Nov. 20th 2008 Microwave diagnostics in the SPS ECM 2008 Caspers, Mahner, Kroyer, Henrist, Jimenez Preliminary measurement results in 2008 –AM contamination (2) SPS super-cycle 21200, July 9th 2008 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 PM AND we also see a very small FM –PM Linie in the lower half of the display where we expect it.

21 Nov. 20th 2008 Microwave diagnostics in the SPS ECM 2008 Caspers, Mahner, Kroyer, Henrist, Jimenez Preliminary measurement results in 2008 –AM contamination (3) SPS super-cycle 17024, July 7th 2008 Another example Upper half: Conventional spectrum Lower half PM demodulation This shot was taken after acceleration at flattop where usually we observed the strongest PM signals 2 batches

22 Nov. 20th 2008 Microwave diagnostics in the SPS ECM 2008 Caspers, Mahner, Kroyer, Henrist, Jimenez Preliminary measurement results in 2008 –AM contamination (4) SPS super-cycle 16972, July 7th 2008 Here we have in the upper half the conventional spectrum and in the lower half the PM demodulated part Obviously we cannot see anything during accerlation and a quantitative evaluation from this colour coded plot is not really possible.

23 Nov. 20th 2008 Microwave diagnostics in the SPS ECM 2008 Caspers, Mahner, Kroyer, Henrist, Jimenez LHC beam in SPS, 25 ns bunch spacing, 72 bunches, 1 batch Preliminary measurement results in scrubbing run (10) PS-type shielded button pickup signals Yellow trace Yellow trace  stripline in stainless steel chamber #1 Blue trace Blue trace  pickup in chamber #1 containing enamel clearing electrode Magenta trace Magenta trace  pickup in stainless steel chamber #2 w/o enamel, for reference Note, that there is some (unexpected) low frequency ringing (EMI) in the blue and magenta trace We 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

24 Nov. 20th 2008 Microwave diagnostics in the SPS ECM 2008 Caspers, Mahner, Kroyer, Henrist, Jimenez 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.

25 Nov. 20th 2008 Microwave diagnostics in the SPS ECM 2008 Caspers, Mahner, Kroyer, Henrist, Jimenez Conclusion and Outlook (additional comments July 14th) As for potential AM/PM conversion due to steep slopes in the HTF (hardware transfer function) one can test the hardware transmission with some know AM level on the generator (and of course known spurious PM) and check at the downstream end of the transmission for changes (in particular additional PM or FM). For the present case we see from the data shown (well, not all shown this morning) that we had up to 20 dB high pure AM sidebands and no PM indication. For further reduction of intermodulation risk via beam induced signals the passive front end filter BW (both for the driver and pickup amplifiers) can be reduced to say 2x f rev. (here we had 40 MHz with f rev in the SPS = 43 kHz). If additional dynamic range is required the carrier can be suppressed with a high Q passive element notch filter before the received signal is going into the first PU amplifier. Instead of using the VSA (vector spectrum analzyer) one could also apply a VNA (vector network analyzer) which has a suffiently large IF bandwidth (at least 100 KHz for the SPS) and operate it in the CW FFT mode (may require the use of macros).There might be a higher instantaneous dynamic range with the VNA as compared to the VSA (80 dB limited by the ADCs)

26 Nov. 20th 2008 Microwave diagnostics in the SPS ECM 2008 Caspers, Mahner, Kroyer, Henrist, Jimenez Related recent results from other labs (1)

27 Nov. 20th 2008 Microwave diagnostics in the SPS ECM 2008 Caspers, Mahner, Kroyer, Henrist, Jimenez Related recent results from other labs (2) [ILCDR July2008]