Minutes of nanoBPM meeting on 27 June, 2003

We focussed on a discussion on the stabilization of 3 BPMs. Generally, there are three methods of laser beam, reference block and inertial sensor, at least. We discussed on each possibility as explained below.

After discussions, we chose the reference block system for temporary stabilization scheme because of its simplicity, less sensitive to environment and availability than the laser beam and the inertial sensor, respectively.

For the stability, X and Y positions of 3 BPMs shall be monitored by laser interferometer with a reference block together with active feedbacks. So, there must be X and Y-active movers on X,Y,X',Y' movers. This mover system is set on a girder made of granite rock with stainless steel plate on top for bolting movers etc. . The girder can be mechanically aligned.

Honda and Higashi will design the whole system until next meeting on 25 July. Yamaoka will design the reference block with "vibrartion-free" support at the same time including ANSYS calculations.

"Possibility of using laser light ray as reference", Y.Higashi

(transparencies, ;11 pages, pdf ,1.3MB) Higashi measured stability of laser beam spot for feasibility check. Laser beam (He/Ne, 1mW) was transmitted by 2m long single mode fiber (core diameter of several um with 500um diameter clad). Emitting from the fiber, the laser beam was focussed with a bin hole on a photo-diode device of QPD. Distance between the fiber end and the QPD is 50mm. All the optical components including the laser source were put on the same rigid table. The beam spot size was about 56um in diameter. The QPD has dead boundaries of 10um width in 4 divisions. From the 4 outputs(A,B,C,D) of the QPD, horizontal position was evaluated by (A+C)-(B+D). Typical beam intensity difference was estimated to be 0.2% for 10nm displacement with this configuration. The positions were measured for a few hours.@@Gradual change was observed within 10nm . The observed beam intensity is extremely sensitive to ambient parameters such as temperature (0.02K), pressure (1Pa), humidity(1%) and CO2(21 ppm) during measurement, where values in parentheses are requirements for 10nm change.

He would like to fabricate a cavity BPM in addition to one set of mover system at the KEK mechanical engineering center. However, for investigating the stability, the BPM shall not be necessary. So, the fabrication is low priority in this regard.

Designs of the BPM and electronics have been completed by a collaboration of KEK and Tohoku Gakuin university. Its fabrication will be ordered to a company in this fiscal year, anyway.

"Displacement monitor system utilizing reference block", Y.Honda

(transparencies, ;5 pages, pdf ,40KB) Honda explained a monitoring system of stability for three BPMs with a reference block. In this system, displacements of BPMs relative to the block are measured by laser-interferometer. Major issue is a stability of the 1m long block. The lowest order vibrations may not be relevant for alignment of three BPMs, while higher order modes must be appeared at sufficiently high frequencies as >100Hz. These vibrational properties can be well predicted by ANSYS calculation and stiff material must be chosen for the above requirements. The other issue is an isolation of the block from ambient vibrations (ground, beam pipe, sound, air flow..). The block can be supported by air suspension with rubber stacks.

BPM positions must be monitored and actively corrected both vertically and horizontally. Therefore, 6 laser interferometers are necessary.

"Review of inertial sensor", T.Tauchi

(transparencies, ;15 pages, pdf ,665KB, and also PAC03 paper by J.Frisch, SLACPUB9843) Tauchi briefly introduced the inertial sensor R&D at SLAC by using transparencies presented by J.Frish and E.Doyle at ISG10. The inertial sensor has been developed in the vibration stabilization R&D collaboration at SLAC.

In 2002, they demonstrated stabilization of single block , which was limited by sensor noise. They succeeded to reduce integrated RMS of motion below a few nm at frequency of less than 20Hz with feedback, while 10 times more motions were observed without feedback.

They developed an inertial sensor of 3x10-9m/s2/sqrt(Hz) noise at f>0.1Hz since required properties of its compactness and nonmagnetic sensor are not available commercially. The sensor can only measure one directional motion by a cantilever system with capacitive readout. The cantilever is 11 cm long and supported by a pre-bent spring with a frequency of 1.46Hz whose second order resonant frequency is about 96Hz (ANSYS calculation) . Thermal effects are very large in the spring with a 10-8 C change corresponding to the thermal noise at 0.1Hz. Therefore, housing and spring are gold plated and they are put in a 18cm x 11cm x 6cm stainless vacuum chamber. The sensor has been tested by comparing to STS-2. Good correlation between STS-2 and sensor was observed at frequency of more than 0.2Hz. After the sensor is verified to meet the requirement, production of sensors will begin in late 2003.