Minutes of Second meeting, 28 May, 2003

"Cavity BPM mover requirements", H. Hayano

(transparencies, 8 pages, pdf ,214KB ) Hayano comprehensively talked on (1)resolutions, (2)3 BPM method, (3) role of BPM mover and (4)specification of BPM mover.

Signals of cavity BPM come from beam displacement(x), beam tilt(x'), common mode and thermal noise. The signals are proportional to total charge of beam-bunch. The tilt and common mode are out of phase in 90 degree while the displacement is in phase. In general, the tilt should be removed by phase detection, or precise cavity alignment and beam orbit tuning are necessary. The common mode should be cancelled out by 180 degree combiner with two opposite signals. The thermal noise should be minimized by low noise circuit.

The common mode signal can be estimated by an error in a pill-box cavity with outer wall coupling, that is 0.305 lamda_110/ Q_110, where lamda_110 and Q_110 are wave length and Q-value of TM_110 mode. ATF-BPM has an error of 1.65um with lamda_110=0.046 m and Q_110=8500. The 180 deg. combiner has 20~40dB (1/10~1/100) reduction. The phase detection would have additional 20~40dB. Therefore, the error would be 16.5 ~ 0.165nm.

The title signal is proportional to tilt angle and power of bunch length. The ATF beam has an angle jitter of 4 urad, which produces 43nm signal with "scalar" detection. The phase, i.e. vector, detection will provide 20~40dB reduction. Therefore, a tilt mover must be necessary to calibrate the phase detection for proper phase position,

Ultimate resolution must be limited only by thermal noise. However, actual resolution comes from electronics circuit unless noise figure (NF) is zero. A good example is the FFTB cavity BPM called as Shintake BPM. The displacement signal is produced at 25uV/nm/nC accompanied by 9 uV thermal noise with 100MHz band width (BD). Since the electronics has 30dB total gain with 4dB NF, the signal would be 790 uV/nm/nC with noise of 451uV ( 9 x 104/20 x 10 30/20 ). So, the "electronics" error (resolution) would be 0.57nm, while the measured one was 20~30nm. Since a dynamic range of the electronics was 2 V, a measurable range was estimated to be 2.5 um.

Usually the resolution can be estimated by three BPMs aligned linearly. If they are equally spaced with the same resolution (sigma), the resolution of middle BPM is obtained by SQRT(3)/2 sigma, where sigma is projected one.

BPM mover has a role of;

Specification of the mover is;

"KEK support system", Y. Higashi

(transparencies, x pages, pdf ,xxxKB ) Higashi talked on a preliminary fabrication idea. First, he introduced high technologies relevant to the nanoBPM project, which KEK mechanical engineering center has. Fabrication technology of the BPM is very similar to that of x-band accelerating structures. Typical accuracy is less than 1 um everywhere; the center of the cavity can be aligned within 1 um relative to the center of cylindrical outer surface which could be coated by hard material; this accuracy can be maintained in a case of bonding. Very precise horizontal angle positioning method utilizing cylindrical torsion was also shown, which can be applied to tilt mover. Alignment of three BPMs can be realized to be within 2 um which satisfies the rough mover condition, since a V-block has straightness of less than 1um over 500mm length. The V-block is made of granite or Fe/Cr alloy.

There was a discussion on single and three V-blocks for supporting BPM's. Since the specification requires independent movement of the three BPMs, the three V-blocks must be used.

"nanometer measurement by laser", Y.Honda

(transparencies, x pages, pdf ,xxxKB ) Honda briefly talked on precise position measurement by laser technology. As presented at the nanoBPM workshop in March 2003, a cavity-type interferometer has a sub-angstrom resolution ( ~wavelength/2/1000 !) in principle. For our purpose, a Michelson interferometer might be enough. So he setup such interferometer of 50mm path length/arm using laser wire technology. He observed 1.6V for 130nm displacement with photo-diode laser of 500nm wave length, whose gain would be 10mV/nm. Since a noise level is about 2mV, sub-nanometer resolution can be easily obtained. He also measured a vibration of support table on which the laser system was set. Typical vibration was 10nm at 10Hz.

"Livermore Support tube system", H. Yamaoka

(transparencies, 7 pages, pdf ,282KB ) Yamaoka reviewed the Livermore support tube system. The system has dual tube structure, where the inner and outer tube are BPM alignment frame and metrology frame, respectively. Dynamic analysis of the system has been executed based on finite element method by Livermore group. Major resonant frequencies are 155, 183, 231 and 233 Hz. We found asymmetrical positioning of three BPMs along a beam line. He will compile questions on this system. Finally, he showed an ANSYS calculation on a simplified tube structure of 12.6 thick x 450 diameter x 586 length mm3 with 4 supporting legs. He obtained 1st and 2nd resonance frequencies at 53 and 58Hz , respectively. These results are far from the Livermore's ones, anyway.

Future plan

We will have meetings in next weeks during SLAC people works at ATF. Higashi and Honda will make a first version of support system for three BPMs. Yamaoka will compile questions on the Livermore support tube system and he will also update the ANSYS analysis adding more detailed information.