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[acfa-ir 89] Minutes of 51st FFIR/BDSIM meeting on 4/23/2003

Subject:   [acfa-ir 89] Minutes of 51st FFIR/BDSIM meeting on 4/23/2003
From:   田内 利明 <toshiaki.tauchi@xxxxxx>
Date:   Wed, 07 May 2003 14:45:19 +0900

Dear Colleagues,

We will have a next meeting at 13:30-15:30 on 21 (Wed) May 2003.  The place
is  room#425 at the 4th floor of 3-gokan.  It will be TV(ch#31) conference.
In this meeting, Hayano will talk on "Review of Hardware Crab Cavity of
NLC-type" and other status report.

Can you also find an appended minutes of 51st FFIR meeting?
Please also see a web site of http://acfahep.kek.jp/subg/ir/minutes.html .
It may take some time to uodate the web site. So if you do not see the
minute, please wait for about 30 min. .

Comments and questions are welcome.

Best regards,
T.Tauchi  (KEK)

Minutes of 51st FFIR/BDSIM meeting on 4/23/2003
The meeting was held in a room of 325 at KEK, 13:30-15:00, 4/23/2003. We
discussed on  crab cavity, electron beam scattering at LINAC, BDSIM, FEATHER
and others. 

(1) NLC Crab Cavity Review (H. Hayano, KEK)
(transparencies, 15 pages, pdf ,829KB )   Hayano reviewed the NLC crab
cavity studies based on "NLC-ZDR chapter 11.6", SLAC Report 17(1963),
transparencies of J.Frisch and M.Ross 'Crab cavity phase
stabilization'(1998) and J.Frisch'Crab cavity RF control'(nanobeam 2002). In
the ZDR, following three types have been considered for the crab cavity ;
(a) s-band LOLA-III, 3m long, 0.8~1.7MW power for beam energies of
250~750GeV, (b) x-band LOLA-X, 37.5cm long, 0.20~0.32MW and (c) x-band
Lolita-X, 5.23cm long, 4.8~7.4MW.  The Lolita-X is 6 cells of LOLA-X.  So,
the Lolita-X is the most preferable for shorter structure and smaller wake
field although it must be driven with high power.  The ZDR identified that
the most difficult tolerance is "pulse-to-pulse phase difference jitter
between cavities" to be less than 0.2 degree x-band phase within 1/5 seconds
for less than 2% luminosity loss.  For more than 1/5 sec, a steering
corrector compensation will work effectively.    Scaling to s-band, the
phase difference would be 0.05 degree.  Since the best SLAC Klystrons have
about 0.1 degree jitter, this requirement must not ridiculous although it is

J.Frisch and M.Ross proposed a system consisting of one klystron to drive
both cavities, slow phase adjust with wave guide temperature control and
fast phase adjust by coupling lower power RF source (i.e. an additional
Klystron) to one of the cavities.  The fast phases can be measured by mixers
and the differences can be obtained by their comparison.

In the mixer phase measurement, typical signal is measured with 1mV/degree
sensitivity for s-band. It corresponds to 50uV detection for 0.05 degree
phase difference.  With 1MHz band-width detection, S/N ratio between 50uV
signal and thermal noise floor will be 35dB. Phase detector circuit noise
can be smaller than 35dB easily. So that, the phase detection can be
achievable and the fast feedback also can be achievable.

Hayono concluded that their estimation was very reasonable. So, the NLC crab
cavity must be feasible.

(2) Electron Beam Scattering at Linear Collider (T.Yamamura, M2, University
of Tokyo) 
(transparencies, 13 pages, pdf ,610KB )  Yamamura is a  graduate student(M2)
at university of Tokyo.   He studied electron beam scattering at the main
linac.  Major parameters are as follows; beam energy=500GeV, length of
LINAC=13.51km, beam intensity = 0.75 x 10 10 /bunch, initial beam
energy=8GeV, vacuum pressure=10n Torr and temperature = 300K in beam pipe.
Residual gas is assumed to be Nitrogen and its density is 5.32 x 10 14 /cm
2.  There are three kinds of scattering processes  to be estimated, which
are (1) elastic scattering, (2) bremmstrahlung with the residual gas, and
(3) Compton scattering with thermal photons.    He calculated that 3.75 x 10
5electrons have elastic scattering at the scattering angle of more than 20 n
rad, while typical angular divergence of beam is O(10 u rad).  A few tens
electrons have bremmstrahlung process with the energy loss of more than 1%,
where typical momentum acceptance is +/- 1 % at final focus system.?  Since
the bremmstrahlung process has wider angular distribution than the elastic
scattering,  it must be carefully investigated for background in detectors.
Finally, he estimated that only a few electrons would interact with thermal
photons  with Compton scattering.   He will study these processes in terms
of background in detectors by GEANT4 simulation?for the time being.  Since
the processes are major sources to make a beam halo, they are expected to be
integrated into SAD too.

(3) FEATHER: Design of Movable Kicker (N.Delerue, KEK)
(transparencies, 10 pages, pdf ,4.2MB )   Nicolas begun a talk from a
question of "Why do we want a movable electrode?"  He shortly answered;
kicker with a smaller gap can produce a higher field and thus give a bigger
kick, however, if the gap is too small it may obstruct the beam.  So,
movable electrodes are necessary.  In addition, since the FEATHER system
will be installed in middle of ATF extraction line, the electrodes of kicker
must be widely opened at no FEATHER experiment.  He has investigated
impedance performance of a general configuration consisting of a pair of
shield and electrode with total six variables, which are  length and
thickness of shield, a gap between shield and electrode, length, thickness
and position of electrode.  He found a best set of variables; length and
thickness of shield are 5 and 1mm, respectively, the gap is 8mm; electrode
thickness should be as thin as possible (say 0.5mm which may be too thin to
maintain the straightness );  electrode position should be as long as
possible; electrode length is 6mm.  However, acceptable working range ( 40
to 75 Ohm) is only 1 ~2mm  for the electrode length.

To improve this small working range, he came up with two ideas which are (1)
splitting the electrodes and (2) removing shields. For (1);  if the
electrode is split in 3 parts, the 2 outer parts can be applied with
different potential when the electrode is moved for matching the impedance.
So, we should be able to adjust the impedance within the acceptable range.
For (2); removing the shields, we would have two working ranges in the
electrode position, that is  near to the beam and far from the beam.

(4) FEATHER: Status Report (N.Delerue, KEK)
(transparencies, 7 pages, pdf ,3.4MB )   Nicolas also reported recent beam
test using a cavity BPM.  He observed the cavity BPM signals with offset
beams of 2~5 bunches.   The offset values were 190, 200, 210, 220 and 230 um
vertically.  He also clearly observed an effect of kicker with 200 um offset
beam.  So, a feedback using cavity BPM seems to be possible, assuming
position resolution of cavity BPM is nano meter in future.   He implemented
a GPIB communication over IP.  Communication with the ATF_Control software
is under study. 

(5) BDSIM update (Tanabe, M2, University of Tokyo)
Tanabe showed that extraction line up to beam dump has been installed in the
BDSIM based on GEANT4.  So, he is ready to simulate background along the
extraction line and the beam dump itself.  Input data can be prepared by
CAIN, which consist of disrupted beams and beamstrahlung photons.

CAIN informations are listed here;
Input file of JLC is jlcee.ecm500.roadmap.i .
Example file of disrupted beam is jlcee.ecm500.RM_beamstrahlung.dat .
Example file of beamstrahlung photons is
jlcee.ecm500.RM_beamstrah.photons.dat .
Example file of incoherent pairs is jlcee.ecm500.RM_icp.dat .
linux version of cain is cain21e.tar.gz , which is version 2.1e while the
latest one is 2.35, although no linux version is available.

The next meeting will be on 21May (Wed.), 2003,13:30 - 15:00 am at 3 gokan,
425 .