53rd ATF2 meeting

1. Generator for ground vibration at ATF2 and update on trajectory feedback at IP, Yves Renier (LAL)

A simple GM generator has been developed for dynamical beam tuning studies. Finite (or discrete) Fourier transformation (FFT) was made on GM data which have been measured by R.Sugahara. The Fourier results were fitted by smooth functions and the coherency data were fitted too.

Also, PLACET simulation results with the GM-model B were transformed in FFT to be compared with the measured ones. The FFT results were very different from our expectations. Since the results can not be understood, Y. Renier (P.Bambade) will ask D. Schulte ( the author ) for PLACET.

The GM generation has following 4 steps at each lattice element, which are (1) to generate white noise in time domain, (2) to compute the Fourier transform, (3) the FFT result to be scaled by the fitted function and (4) to compute the inverse Fourier transform for displacement as a function of time at the each element. Since all the displacements are independent in the above calculations, coherency between any two elements was realized by their linear combinations with the fitted coherence functions. More information will soon be available at http://flc-mdi.lal.im2p3.fr/spip.php?article=59 .

This simple GM generator could reproduce the measurement, while it underestimate the coherence for nearby points far way from IP. It would be good for first study on feedback simulation. The 2D FFT spectrum will be investigated for more suitable technique. One suggestion (after this meeting) : i.e. an example of 2D FFT-GM was a study of LINAC simulation by K.Kubo ( here ).

Results of feedback simulation were shown by using this generator. First, growths of beam size and (vertical) displacement were calculated at IP as a function of time for 100 seconds, where the beam size varied from 35.5nm to 36.5nm and the displacement changes for ±300nm. Next, the simplest feedback with proportional correction (P-feedback) was studied in y and y' directions. The y and y' correctors are set at 29.5cm downstream of QD0 and 57.3cm upstream of QD2A, respectively, and two BPMs are set at 10cm and 20cm upstream of IP with relative precision of 5 nanometers. Results of position (y) feedback was shown for 100 sec . Also, simultaneous y and y' feedback results were shown. Finally the PID feedback results were shown.

Conclusions are (1) the a few % beam size growth can be preserved with the position(y) feedback where the corrector is set at downstream of QD0, (2) the angle (y') feedback seems to increase the beam size which could be due to off-set in the sextupoles (SD0, SF1), (3) the PID feedback scheme must be tuned for better performance than the P one, and (4) the displacement (RMS) can be set down to 35nm while 15nm correlation had been seen for 100sec.

• Q : What kind of feedback ? pulse by pulse or bunch by bunch
• A : pulse by pulse at single bunch operation.
• Q : What is the observation of flip-flop behavior after 3 sec in the results?
• A : The period seems to be 2 times of repetetion rate . Since the PID-coefficience has not been tuned, they need more tuning.
• Q : PLACET and MATLAB-LIAR has been in very good agreement. So, the PLACET results could not be understood.
• A : We neither understand for very similar methods have been taken. maybe inproper use of the generator in PLACET, we will ask Daniel Schulte.
• Q : In the PLACET FFT result, there is very small number of harmonics. It may be hint.
• A : Yes, it was the first step to evaluate the results, then we will discuss with Daniel on the usage.
• Q : In the feedback, is the orbit at sextupole to be constant ?
• A : it is difficult with only one feedback in angle, may move the sextupole position ? x y, xy', correction with a single corrector ?
• Q : Is it a vertical motion in GM ?
• A : Yes, it it only vertical.

2. Variable parameters for ATF2 commissioning and optimisation, Marie Thorey (LAL)

Optics has been investigated with &beta^*_y from 20 to 800μm, where the nominal &beta^*_y is 100 μm, strengths of QM12 - QM16 were fitted for wanted &beta^*_y, &alpha_x=&alpha_y=0, &eta_x=0 and strengths of 5 sextupoles were fitted to cancel T₁₂₂, T₁₂₆,T₁₆₆,T₃₄₂, T₃₄₆. The fitted strengths were listed together with vertical beam size ( &sigma^*_y ) at IP, i.e. &sigma^*_y was shown as a function of &beta^*_y. The minimum &sigma^*_y was calculated to be 20nm at &beta^*_y=30μm, where &sigma^*_y increases at smaller &beta^*_y . These behaviors could be due to the matching method with only the first quadrupoles but not the entire line and the second order contributions.

Next, necessity of bipolar quadrupoles has been investigated. First attempt was to force fixed polarities, i.e. no change in the matching process, since QM15 and QM16 have changed polarity in the matching process. It was found to be even more difficult to get smaller &beta^*_y . It needs more study.

Optical bandwidth, tolerance of magnet displacement were also calculated with variable &beta^*_y from 100 to 800μm.

• Q/C : You can study the bipolar performance by changing quadrupoles one by one.
• A : Yes, it should be easy.

3. C-band Reference Cavity Status, Yosuke Honda (KEK)

The reference cavity has 100mm length of flange-to-flange, The monopole resonance frequency is 6.426GHz with tunable range of about 2 MHz. It has 20dBm with 1 x 10¹⁰/bunch and the decay time of 230ns. 4 cavities have been ordered for fabrication in JFY2007.

There are 39 QBPMs which are 11 and 28 in 2006 and 2007, respectively. The first production QBPMs (2006 model) have less frequency by 1MHz. In addition, there are 3 QBPMs which have been fabricated with short beam pipes as prototype ( 2005 model). The 2005 model has 4MHz less frequency. Thermal coefficient of frequency was measured to be 0.14MHz/degree .

Finally, a modification in the Q-BPM/reference cavity layout ( v3.7 device list) was suggested for more equal separation between reference cavities, replacing one QBPM with a stripline BPM at downstream of BDMP etc. .

• Q : Some of QBPMs in the matching section have been changed to QBPMs in v3.7. Is it OK for FONT ?
• A : FONT group discussed with Mark, we will find the solution.
• Q : How many are QBPMs in total?
• A : They are 33, for just 1 is removed at the last one.
• Q : Is 250ns decay time OK for the ILC like bunch-structure?
• A : Yes, in principle, for we can remove the overlapping signal.
• Q : What kind BPMs in FD ?
• A : They are S-band BPMs.

5. KEK site meeting ( in Japanese )

There may be suggestions to use the FFTB movers instead of correctors at the extraction line and to change positions of correctors near BH1X and BH2X for the long beam pipes etc. . So, Okugi will summarize suggestions at next meeting. 8th August.

Kuroda reminded that the extraction septums will be re-aligned during the re-configuration of extraction line in next summer. We would like to verify no effect on the ATF2 beam line by the re-alignment. We asked him to summarize the situation.

The next ATF2 meeting will be held on 8th August 2007, 2:30pm -, 3-gokan, 425, KEK, TV-ID#=31110 with the KEK Gatekeeper and ILC Webex.