Minutes of 69th FFIR/BDSIM meeting on 3/3/2004

The meeting was held in a room of 425 at KEK, 10:00-11:00, 3/3/2004. We discussed on final focus system, FEATHER and dark current at the LINAC.

(1) Final focus system (S. Kuroda)

(transparencies, 2 pages, pdf, 82KB)

Kuroda has been investigating the chromaticity corrections at the optics with L*=3.5m (Roadmap report, effh1.1) and L*=4.3m (;NLC2001, effh0) as well as the ATF2(L*=2m) for the local chromaticity correction. First, X and Y chromaticity was shown at each magnet for the effh1.1 and effh0 optics. As expected, major chromaticity is generated at the final doublet(QD0 and QF1) which is "cancelled" by those at the sextupoles ( SD0 and SF1, respectively). The local chromaticity was defined by sum of downstream chromaticity from QF3.2 in the similar way as the previous study on the ATF2. Resultant chromaticity is listed in the following table together with those at the ATF2 which were presented at the last meeting, where SF1, QF1, SD0 and QD0 at the LC correspond to SF2F, QC2F, SD2F and QC3F, respectively, at the ATF2.

Local;from QF3.23,190.5-11,016.04,149.9-6,429.1
ATF2(a) ; L*=2m(b) ; L*=2m
Local;from QB3F-1718.0-258.0-115.7 -10854.0

For the LC results with the Roadmap and NLC2001 optics, there is a numerical difference in the local &xiy which are 11,016.0 and -6,429.1, respectively. The larger &xiy of the Roadmap is considered to be a source of the large radiation effect estimated at the beam energies greater than 300GeV. Sugahara-san pointed out that the remained "local" &xiy must be very carefully corrected by the upstream magnets because of Δ&sigma*=ξδ&sigma*. So, the aberration correction scheme must be established. Kuroda has identified that the most relevant error was jitter at the bending magnet for dispersion, i.e. jitter in the power supply should be less than 10-5, for the ATF2. Tolerances of possible errors shall be estimated at these FF systems.

Comparing with the chromaticity of the LC and the ATF2 in the above table, the ATF2 has very similar chromaticity correction to the LC. Therefore, the ATF2 will demonstrate the local chromaticity correction indeed.

(2) Dark current in LINAC (T.Yamamura)

(transparencies, 3 pages, pdf, 879KB)

Yamamura successfully calculated an electromagnetic field in a single cell of x-band accelerating structure by MAFIA, while the structure of 60cm length consists of 53 cells. The cell design parameters were provided by Higo-san. A periodic condition is 5/6 &pi per cell. He got a resonant frequency of 10.87467GHz which should be 11.4GHz. Tracking strategy is as follows; (1) electrons start at zero momenta, (2) motion of the electrons is confined in r-z plane and (3) the electromagnetic components of Ez, Er and B&phi are only considered. He will investigate the initial conditions for "long-survived" electrons. There was a comment for the dark current study. In principle, the dark current could generate wakefield which may affect the emittance of the beam, in addition to the background issue. Such effect shall be estimated because of no evaluation at present. Dark current measurement must be very important at the GLCTA.

(3) Beamstrahlung monitor and FEATHER (N.Delerue)

(transparencies, beamstrahlung monitor, 5 pages, pdf, 746KB and FEATHER, 7 pages, pdf, 459KB)

First, Nicolas talked on the beamstrahlung monitor. The beamstrahlung photons have been generated with vertical offsets (0, 0.5, 1, 10, 40 and 100 &sigma*y) between two beams at IP, where the beam energy and crossing angle were 500GeV and 3.5mrad, respectively. He has presented the first study at the previous meeting, where Yokoya pointed out that there must be huge synchrotron photons in the same direction. So, T. Ohgaki calculated the synchrotron photons generated at the Roadmap beam delivery system at the beam energy of 250GeV and the crossing angle of 3mrad, where the beam profile consists of a Gaussian with a 0.1% flat-halo distributions. ( Sorry for the mismatched beam parameters since Tauchi asked Ohgaki without these informations. Please note these differences for detailed comparison, while major characteristics remains "correct".)

The synchrotron radiation is dominated by low energy photons (E<1MeV) with one peak and a broad pedestal(halo). Comparing thebeamstrahlung with synchrotron photons, At low energy, synchrotron radiation photons are more intense by 1 order of magnitude and have a wider divergence: Low energy Beamstrahlung is hidden by synchrotron radiation. Therefore, one must study high energy beamstrahlung photons.

Beamstrahlung pattern can be monitored on-line at high energy. A calorimeter could be used to catch part of these photons, but this calorimeter must resist to high energy flux. Another solution could be to send a low energy electron beam on the photon flux and study the scattering pattern (methods close from laser wire beam monitor).

Next, Nicolas reported results of the latest beam test. A detector ( consisting of diode and capacitor) has been put in the circuit in order to detect the multi-bunch signals by removing the negative parts, i.e. envelope detector. The test bench signals seems to be integrated with slow time-constant. At the beam test, negative parts of the BPM signals were observed to be removed without envelope detection. While the both phenomena can not be understood, he transfered these "detected" signals to the kicker in order to see any effect on the BPM signals and the kicker signals. Apparently, very careful circuit works must be necessary for high frequency components, anyway. He set following goals; (1) Confirm the feedback observed previously, (2 ) Define a 0 axis and adjust the normalization of the signal so that the beam converges to that position, and (3) Try to combine the output of the 2 electrodes. He also mentioned that one independent button BPM installed near the movable electrode BPM could help to monitor the performances of the system. A cavity-BPM downstream of the kicker would be used for such witness measurement.

The next meeting will be on 17 March, 2004,10:00 -noon at 3 gokan, 425 .