e-cloud Measurements by TE Wave Reflectometry on PEP-II Stefano De Santis (LBNL), M. Pivi (SLAC) April 5th, 2007

A Brief History of the “Microwave Transmission Method” Method developed at CERN (Caspers, Kroyer). Measurements on SPS during 2003-04. Some results still not 100% clear (attenuation with beam). Bench tests. The basics: using a BPM to excite a TE (or TM) mode in the beam pipe and picking up the signal from another BPM (as distant as possible). Then, evaluating the beam influence on the transmitted signal (amplitude, phase).

A Brief History of the Measurements on PEP-II Measurements using 2 BPM’s in IR12 (~50 m apart). July 2006 and March 2007. Our set-up is substantially simpler than what used in the SPS experiment. July 2006: measured transmission between BPM’s at various frequencies. Attenuation was -70/90 dB. With the available amplifier, our S/N < 40 dB. Beam on/off made no difference. March 2007: repeated measurement at 2.7/2.5 GHz. Beam and solenoids on/off made no difference.

Experimental Set-Up Amplifier Signal generator (up to 3 GHz) Mauro Signal generator (up to 3 GHz) Spectrum Analyzer Cables to/from BMP ~ 50 m Short solenoids Long solenoid

Experimental Results TE1,1 Wave (2.9 GHz) No change in signal amplitude with beam on/off and solenoids on/off. Phase modulation sidebands should appear at +/- 136.3 kHz from the TE wave. Our modulation index is way too small. Also the LER was operating with a very short gap in March (1.5 %). Is this enough to modulate the electron density effectively ? ? ? 136.3 kHz Beam harmonics

Theoretical Estimates (w/ M. Furman, K. Sonnad) Theoretical estimates and simulations (VORPAL) are in very good agreement. Phase modulation sidebands should appear at +/- 136.3 kHz from the TE wave. Our modulation index is way too small. Also the LER was operating with a very short gap in March (1.5 %). Is this enough to modulate the electron density effectively ? For 1012 e-/m3, 50 meters of “e-clouded” beampipe cause a phase shift of less than 1°. That means a very low modulation index and a carrier-to-sideband amplitude ratio of more than 40 dB. Compare with our 30 dB S/N !

Conclusions We did not see any amplitude modulation. The phenomenon is not yet very well understood, anyway. Should we expect to see it at all ? Would a larger gap improve things ? We did not see any phase modulation either. Theoretical analysis says we shouldn’t expect to. Much depends on the actual electron density in the ring. Pressure data while switching off solenoids seemed to indicate a fair build up of electrons. We can improve our set-up, using a more powerful amplifier (+30 dB could be adequate). We can also use a 2-BPM input (easy) and look if at higher frequencies we can obtain a better transmission between BPM (requires a new pulser). Increasing the gap could prove beneficial (higher modulation index)