Discussion of pro/cons of:
The group started discussion of the following questions:
In brief summary, here are conclusions from these discussion.
Cost savings, due to elimination of two full power tune-up dumps at the entry to Beam Delivery, need to be studied in details. The mentioned cost numbers spanned one order of magnitude.
Discussion of impact on ILC availability has started with the Availability
group. Clearly, there will be some impact on availability if the tune-up mode
will be eliminated, i.e. if an access to IR would require switching off the beam
in the linac. The impact needs to be quantified.
It is also clear that reduced power tune-up dumps may mitigate the availability issue.
The power and the time structure of the "keep-alive" beam are important. Marc Ross is suggesting to consider 0.1Hz full train operation and rate the tune-up dump to 0.5MW. Another possibility is 5Hz, reduced number of bunches. These assumptions are to be further discussed.
Technical feasibility to build a beam dump to accept beam coming from either
side was discussed by Dieter Walz. Conclusions are the following:
Such beam dump should be feasible.
The dump will be about 50ft long (vs 20ft for one-side dump).
With proper design such beam dump may also accept both beams simultaneously, with 40MW total power in continuous regime.
If one of the beams come to the dump with an angle of 10-20mrad this is not a problem.
If the tune-up dump power rating would need to be reduced, the 0.5MW rating seems as a reasonable value which should provide cost savings.
Detailed design would need to be considered further.
In terms of optics feasibility, the conclusion is that transport of the beam
from BDS entry to main beam dump should be possible.
One of the possible difficult areas in the TDR design was the region where the emergency extraction beamline was bent in both horizontal and vertical plane, to meet the beam dump, located 2.2m below main beamline. These bend areas would be prone to particle losses.
Since in present BDS baseline the main beam-dumps are offset from the main beamline in horizontal plane by 4-8m, there is no need for large bends in tune-up beamline in the vicinity of the dump.
The energy bandpass of tune-up/emergency-extraction line would be defined by the bend/kicker region in the beginning of BDS. The 20% bandpass (which was possible for shorter beamline) need to be evaluated. (The note 2001-13 argues that -5% to +0.5% is reasonable range for energy errors).
The energy feedback in the tune-up/emergency-extraction beamline may be possible. This feedback would adjust the field of kickers or fast magnets after the first bunches. It could further facilitate extraction and reduce losses.
From conventional facilities point of view, merging the
tune-up/emergency-extraction beamline into the main beamdump should be possible.
Details should be carefully considered.
Depending on the offset of the beam-dump with respect to the main beam (range 4-8m) and location of the main-beam in the tunnel (close to one or another wall) the tunnel may need to be widened by several meters, along several hundreds meters approaching to the main dump.
For widened tunnel, the cost increase may be roughly estimated as being proportional to the tunnel cross-section.
In terms of possibility to dump "keep-alive" beam intensity to the main dump and allowing people to access detector. Lew Keller performed simulations that show that for any interesting beam intensity this is probably not feasible. The dose is about 0.7-1.4Rem/hr for the full beam power. In order to be decrease this dose to the level acceptable for people, the beam power need to be decreased 1E4 times. (Calculations were done for 250GeV/beam. With 500GeV/beam the situation will be worse).
The overall conclusion is that technically the elimination of two full power tune-up dumps should be possible, there will be impact on availability which may be partly mitigated by reduced power tune-up dumps, the cost saving need to be further evaluated, detailed design need to be made.
Andrei Seryi, 11/30/05