Minutes of KEK-SLAC collaboration, 4 June, 2003

General agenda is listed below;

"Cavity BPM mover requirements", H. Hayano

(transparencies, 8 pages, pdf ,214KB ) Hayano reviewed requirements for cavity BPM mover by using the same transparencies as the previous KEK meeting.

Requirements for the mover are summarized below;

  1. Dynamic range: capture beam within 2.5um (rough mover)
  2. Position calibration; need 1nm resolution for 2.5 um range (precise mover)
  3. Tilt tuning and phase tuning: need < 1urad resolution for > 100urad range, assuming initial tilt < 100 urad.
There was a discussion on above item (2) as follows. If a linearity of the mover is assumed to be very good, the resolution could be relaxed to about 10~100nm from 1nm. So we could know a slope of signal/position with enough accuracy if the mover can always set positions with good repeatability. We must be aware that position mover may produce a tilt which will give non-linearities. Also, 2.5um of item (1) is a peak-to-peak value, which is tighter requirement than that of RMS.

"Some results from March beam measurements and electronics tests",M.Ross

One student of LBL is investigating signal responses and fitting by using minuit/root. Although fit results look very good, chi-square value is large. So, resolution of ADC digitization was apparently poor. Good signals of the cavity-BPM can be extracted for fitting after the initial transient time of about 60nsec, where about 70% of total energy excited in the cavity was discarded. In this March run, an observed signal is 1.12mV/nm/1010 . Sensitivity was 0.04 counts/nm with 12 bit ADC, where the amplifier gain was 33.5dB. Position resolution was observed to be about 150nm .

Major improvements in this June run are (1) detailed knowledge/experiences of beam tuning, (2) 14 bit ADC from 12bit, (3) 53.5dB amplifier gain with reduced noise figure of 3dB-NF from 33.5dB, 9dB-NF and (4) higher beam current.

"Cavity BPM with Dipole-mode Selective Coupler",S.Smith

(transparencies, 17 pages,pdf,915KB, also, Z.Li et al., PAC2003 paper(PDF, 3 pages,252KB)) S.Smith explained cavity BPM with selective coupler based on the NLC Q-BPM collaboration ( Z.Li, R.Johnson, S.Smith, T.Naito, J.Rifkin ). In general, there are two "hardware" methods to be "free" from the common mode signal (TM01 as monopole mode), One is two port coupler with a hybrid and the second is a selective coupler. The selective coupler is electrically isolated from a cavity and it couples magnetically to TM11. So the two cases have ideally no common mode signal. However, an actual rejection rate against the common mode signal would be 20~40dB. They calculated signal and thermal noise ratio (S/N) with various dimensions (cavity gap, pipe radius etc.) of cavity BPM with a dipole frequency of 11.424GHz. S/N exceeds 10 and intrinsic resolution limit is 0.1nm at room temperature for 1nC beam. They also evaluated tolerances of machining errors in coupling slots. Typical error is 100um. So, the cavity BPM with selective coupler can have enough resolution with "loose" tolerance. Since the BINP BPM (ATF) is one of this type cavity with one port in each direction, it may reach nanometer resolution.

At KEK there is a consideration that only one port may not be enough to reject the common mode signal. In such a case, an addition of the hybrid method with two ports can improve the rejection by 20dB at least. Therefore, KEK and Gakuin university will fabricate a cavity BPM with two port selective coupler.

"Questions on the Livermore support tube system", H. Yamaoka

Yamaoka compiled questions on the Livermore support tube system for ANSYS calculation belows;
  1. Materials, dimensions for calculation of Young's modulus etc. .
  2. Detailed configurations for assembling, mechanical structures
  3. Purpose of Metrology frame, how to be supported.
Items of (1) and (2) are already asked to the Livermore group by Marc. Since engineers will come to the ISG from Livermore, we can discussed on these issues at SLAC.

"Beam stability (discussion)"

Present beam jitter was observed to be 2~3um in RMS, while tolerable range is 2.5um in peak-to-peak as Hayano discussed. Therefore, we should need improvement on the stability. First, we shall identify where the instability comes from. For this purpose, all the BPMs at the ring and extraction line should be used for orbit corrections. So the first priority might be establishment of such orbit correction method.

Other

We discussed on a possible collaboration on "nano technology" for nanometer controlled support system and sub nanometer monitors etc. .

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