Minutes of nanoBPM meeting on 16 October, 2003

"Position Monitors", Y.Honda

(transparencies, 9 pages, pdf, 273KB )

The prototype of movers will be received on end of November from a company. So, Honda briefly reported on investigations of position monitors. There would be two types of monitors for active feedback and mover according to necessary resolution (dynamic range) of sub-nm (a few um) and um (200um), respectively. The sub-nm resolution is provided by laser interferometer while the um one is provided by a system consisting of a laser beam and QPD (Quadrant Photo Diode). Performances of these monitors have been studied in test stands on an optical plate (Aluminum, 10mm thick) placed on a granite table.

The QPD system consists of He/Ne laser (red light, 633nm), coupler, 1.5m long single mode fiber(4um diameter), focus lens and a OPD mounted on a vertical mover. The laser was injected into the fiber with the coupler for good profile. Exiting from the fiber, the laser was focused at the QPD with diameter of about 100um. (Spot size was measured to be 50um (diameter) for the best QPD resolution by Higashi, where the QPD has 4 segments separated by 10um gaps.) The system was calibrated by the mover with 1um or 20um step. After turning on the laser, a settling time was observed to be several hours although signal drift was less than 1um. Stability was also measured for 3 hours. Within a range of 1um , ripples were observed, which were thought to be caused by air conditioning in a room. He also tested a commercially available fiber couple laser diode(653nm) instead of the He/Ne laser and coupler. The performance was also fine. The two systems have enough spatial resolution of less than 1 um.

The Michelson interferometer system was also setup on the optical plate which corresponds to the reference bar. Since the orthogonal two arms were placed on the same plate, stability of this system could be measured, where phase variations of interference fringes were observed. For 30 minutes, the variation was observed to be 1.7nm in rms including ripples caused by the air conditioning. For 11 hours, an increasing drift of about 30 nm was observed while fluctuation around the drift was 1.7nm in rms too. The measurements were executed inside of a vinyl house where temperature was 28~29 degree C. . Without the vinyl house, the stability worsened three times more. He also observed a clear effect of the air conditioning by FFT analysis on the fringe outputs.

Higashi and Honda will make a dummy cavity-BPM with flat surface on the top for a mirror of the interferometer system.

"Development of High resolution Cavity BPM", Y. Inoue

(transparencies, 16 pages, pdf, 995KB )

Inoue, who is a graduate student D1 of Tohoku Gakuin university, briefly explained present status of cavity-BPM R&D in collaboration with KEK. Describing the structure of the BPM, preliminary results of low power test was presented.

The BPM consists of 4 segments which are an end plate, a sensor cavity, coupling wave guide and a bottom flange. Overall size is 75cm diameter of outer cylindrical surface and 75cm long with the flange of 11cm diameter, 1cm thick. Since the working accuracy was so precise that the mechanical center can be obtained from the outer surface as a center of this cylinder. Parameters of the cavity are as follows; dipole frequency(TM11) of 6.54GHz (4.58cm wave length), inner diameter of 16mm for the beam pipe, cavity diameter of 53.7mm and cavity depth of 12mm. The wave guide has a resonant cavity structure with a magnetic coupling to TM11, while it has no direct coupling to TM01 (monopole, usual accelerating mode).

For a low power test, an antenna (cylinder) was made, whose head is a needle of 0.9mm diameter, 9.8mm length while the total length of needle is 12.3cm, in order to excite a TM11-mode around the center of cavity. The needle is hold at the center of 1cm diameter, 10cm long cylinder by Macor plates at both ends. Positions of the antenna can be controlled with accuracy of 0.1 um at least. Also, tilt of the antenna can be controlled with respect to axis of the cavity by a Gonio stage. The BPM was mounted on a horizontal stage. For measurement of mechanical centers, both the BPM and the antenna were also mounted on a rotation stage, where the centers were adjusted with respect to the center of the rotation stage with accuracy of 2um by using a LED/CCD optical micrometer. Detailed configuration can be seen in the transparencies.

Changing the position of antenna (needle) from the mechanical center, electrical center of the BPM was searched for by using a network analyzer. @High frequency pulses were generated with sweeping frequencies around 6.54Gz in an interval of 0.1GHz by the analyzer, and the pulses with amplitude of Vi were injected into the antenna. Then, amplitudes of output signals from the BPM (Vo) were measured by the analyzer. Ratio of Vo/Vi was plotted as a function of antenna positions. For good position resolution, a sharp V shape is expected, where the tip of V must be the electrical center. V-shape of horizontal scan was fair, while V-shape of vertical scan was shallow. Preliminary electrical centers were shifted to +15um and +46.6um from the mechanical center in horizontal and vertical directions, respectively. The vertical displacements seem to excite additional modes. Resolving these issues, an actual BPM will be designed in this November. In next February or March, at least one BPM is expected to be received for high power test.

Electronic circuit for high power test has been designed and ordered to a company. The circuit will be received in this December. At a beam test, a reference cavity (TM01 mode) must accompany for timing and beam intensity measurements, which has been designed to be fabricated soon.

Discussion on the mini-workshop, GM measurements and future plan

Mini-workshop on NanoBPM

Discussing on the agenda, we proposed following two to Marc;
  1. We would like to move Hayano's talk (half of his talk in 11/4) of "Plan for long bunch train operation(feedforward)" to next day, i.e. in front of FEATHER talk, since Nicolas can not attend the morning session of 11/4 (Nicolas likes to hear Hayano'talk) and these talks are very related.
  2. We would like ask Josef Frisch to attend the meeting in order to present the latest study of vibration isolation system at SLAC and to discuss with our experts. Especially, Sugahara-san strongly requested his participation in the meeting. Also, Josef must be a key person of the nanometer stabilization as well as the nano-BPM readout at SLAC.

GM measurements at ATF

In response to arguments at the video conference with SLAC in this morning, we discussed on possible measurements at ATF. Major issues were the 78sec-noise, "spike" noises at higher frequency and ground motions at f< 1Hz. Since the 78sec noise may be generated in the electronic system, the accelerometers will be digitized at different place from ATF. In order to discriminate the spike noises, which we speculated as electronic ones, from ground motions, the GM measurement should be performed on a vibration isolation table at ATF. There is the table at Oho hall for tests by Sugahara-san' s group. After the mini-workshop, we will set up it. There is also possibility to install a water-level system. For third issue, we need a wide-band seismometer such as STS-2 etc. to be looked for.

Near future plan

Since we recently got additional fund from the Japan-US cooperation program, we could construct some part of our system in this fiscal year. After discussions, the first candidate was a girder and the reference system. For the mover system including active feedback, we would like to ask the KEK Mechanical Engineering Center as much as possible. Therefore, Yamaoka will design the girder and the reference system with help of Honda and Higashi.

Next meeting will be held in 30 October, at 1:30pm-.