Minutes of 77th FFIR/BDSIM meeting on 9/8/2004

The meeting was held in a room of 425 at KEK, 10:00-11:00, 9/8/2004. We discussed on the TESLA-BDIR, LINAC simulation and beamstrahlung monitor.

(1) Brief review of TESLA-BDIR (T. Tauchi)

(transparencies, 13 pages, pdf, 5.2MB)

Tauchi briefly reviewed the TESLA-BDIR(focused on the extraction line) and the GLC-IR by referring the TESLA-TDR, talks by R.Appleby, K.Buesser and T.Aso at LCWS2004. Major features of the TESLA-BDIR are head-on collision and vertical separation of extracted beam. One problem is too narrow aperture of the septum (55m from the IP) for "beamstrahlung" photons, which is 0.2mrad. Although the radiation could be small in the nominal beam conditions, there must be no tolerance in realistic beams. Therefore, small vertical crossing (0.3mrad) and small(±1mrad)/large(±10mrad) horizontal crossing schemes have been proposed. Also, quadruplet has been proposed instead of doublet for the final quadrupole magnets to reduce over-focus of low energy tail particles. Novel FF system has been designed with L*=4 and 5m with quadruplet. The TESLA group recognized the nice features of the small horizontal crossing scheme which we have investigated in details at JLC/GLC. Since there is no multi-bunch kink instability for the long bunch spacing, the angle can be less than ours. They found that ±1mrad angle was optimized in terms of backgrounds at the TESLA extraction line.

With L* of greater than 4m, the TDR mask system has been updated with flat-faced forward calorimeters as the JLC/GLC ones, where the calorimeters are surrounded in the endcap calorimeter. Their preliminary result shows that the large angle scheme has 2 times more backgrounds at the vertex detector than the small angle one. Improvements, such as graphite mask in front of the Beam-Cal, are expected in the mask system.

Finally, present GLC mask system, which is similar to the updated TESLA one, was introduced together with background estimations. The backgrounds have been simulated with small angle, 7mrad, and large angle, 20mrad, horizontal crossing schemes. There is no difference in the background-hits of the vertex detector between those schemes, while 4T detector solenoid field clearly helps to reduce the background hit in the both schemes comparing with that of 3T. In the GLC system, the minimum aperture of forward calorimeter is larger than the Beam-Cal in the TESLA mask system. The minimum aperture shall be discussed at ILC workshop.

(2) LINAC simulation (T. Yamamura)

(transparencies, 54 pages, pdf, 34.4MB)

Yamamura has investigated the "problem" in the &phii-&phif distribution of the MAFIA-field, which was presented at the last meeting. Especially, the electron's acceleration has been intensively studied in the MAFIA-field, where initial conditions are zi=0.0, PZi=0.0, Eo=65MV/m and tracking time of ttrack=1ns. First, the acceleration of relativistic electrons (300GeV) is very similar to the case of sine-wave. Next, in the MAFIA-field, electrons with negative &phii were found to have very "irregular" paths while those with positive &phii have smooth or regular paths. In the sine-wave field, electrons in all &phii region have the regular paths. The irregularity comes from z dependence of longitudinal acceleration field (both magnitude of Ez and phase of &phiEz). The peak position of the &phii-&phif distribution is shifted by +0.2rad in the MAFIA-field, since the &phii and &phiEz are different. As the phase difference is actually a function of z. the peak position is also a function of z.

The capture threshold has been calculated both in the sine-wave and MAFIA-field with the same simulation method. The sine-wave case has the threshold of 60.5MV/m as the analytic calculation. The MAFIA-field case has the threshold of 55.5MV/m, where zi=0.0 and the tracking time=30ns. So, the real accelerating structure may have less threshold.

(3) Beamstrahlung monitor (D. Nicolas)

(transparencies, arXiv:physics/0408132, pdf)

Nicolas submitted a paper on "Beamstrahlung monitoring of the beam beam effects at the Linear Collider" to electrical publication of PR ST AB (Accelerators and Beams). Also the paper was arXiv'ed as physics/0408132 . The idea was already discussed at the previous meeting. In the paper, various techniques are proposed for monitoring the beamstrahlung profile and flux. One major consideration is a discrimination from synchrotron radiations which overlap in space while the energy is different. The proposed monitors are sensitive to visible lights or X-rays or high energy photons, respectively.

The next meeting will be on 22 September, 2004,10:00 - at 3 gokan, 325 .