Minutes of the Booster Commissioning Working Group held on 19th of March 2009

Present: W. Weterings, J. Borburgh, C. DeAlmeida Martins, Y. Kadi, R. Rocca, O. Aberle, C. Carli, R. Scrivens, T. Hermanns, K. Hanke, B. Mikulec.
Excused: S. Maury

Agenda:

  1. Communications
  2. Follow-up of open actions
  3. LBS-line upgrade studies (T. Hermanns)
  4. AOB

1. Communications

The minutes of the last meeting have been approved.

K. Hanke is in the process of iterating the second round of the work package description.

2. Follow-up of open actions

A PSB access will be carried out this afternoon by W. Weterings and B. Mikulec to look for possible installation locations for the dumps in the injection region (head/tail dump of distributor and H0/H- dumps).

G. Bellodi will follow up the action to identify a possible location for a collimator in the Linac4 transfer line to protect the distributor from halo particles.

3. LBS-line upgrade studies

T. Hermanns presented the issues related to the potential upgrade of the LBS (spectrometer) line (see LBS_0903.pdf). The principal equipment affected by the increase of beam energy from 50 to 160 MeV are slit (material, aperture, thickness), spectrometer magnet (bending angle, radius, edge angles), SEMgrid (wire spacing, number of channels, time resolution, installation properties) and dump. In addition the distance between these elements could be optimised, but the total length of the LBS line is almost unchangeable due to the restricted space.

As starting point the current layout has been taken together with the new beam parameters. The average beam energy will be 159.4 MeV with an energy spread of 144 keV; energy painting will lead to a shift of the average beam energy by approximately +/-1 MeV. A slit defines first a representative beam sample which will be analysed on a SEMgrid after deviation by the spectrometer magnet. For an absolute energy measurement the centre of the energy distribution has to be determined. Beam transport simulation show a clear correlation between the energy and the measured position at the SEMgrid (polynomial fit 2nd order down to half height was used to determine the centre of the distribution). Without taking into account various error sources, the beam distribution should be fully contained on +/-4 mm; including energy painting this number increases to approximately +/-15 mm.

It was assumed that the sampling step of the wires at the SEMgrid should translate to half of the nominal energy spread (~70 keV). The image at the SEMgrid is mapped 1:1 to the beam slice defined by the slit; an energy deviation can be resolved if the image slices at the detector do not overlap. With the current Linac2 configuration, but taking the Linac4 beam parameters, this would yield an energy resolution of ~135 keV or ~62 keV/mm. To achieve 70 keV instead, the ratio d/r (slit width to bending radius; currently d=2.2 mm; r=1200.8 mm) has to be decreased. The SEMgrid wire spacing should be changed to 1 mm.

SEMgrid specifications:

  • effective wire spacing in central parts of the detector: 1 mm (now: 3.6 mm wire spacing, but effective resolution can be improved by turning the SEMgrid)
  • active area: min. 30 mm (to be cross-checked with error studies)
  • additional wires with larger spacing should be added at the edges for beam steering
  • time resolution: 1 us (energy modulation is done within 10 us)
  • retractable (only needed if the LBS line is used to dump the beam or parts of it and in case no collimator absorbing the beam is present upstream )
  • rotatable? (the current SEMgrid can be rotated to increase the resolution)

Slit:
In principle only a very thin foil with an aperture could be used instead of a bulky slit, but then the problem would be to extract the neutral H0 and the protons. This would lead to a complicated vertical bending magnet design (C-shaped with Y-shaped vacuum chamber?). Another possibility would be to use an 'inverse slit' in the form of a thin stripe or a thin wire (mechanical challenge; thin to limit energy straggling and for high enough measurement statistics). For both solutions it would be necessary to study the interaction of the inversely charged beam fractions after the foil/wire; it should be confirmed if this p/H- interaction can be simulated with PATH.
But in the end both proposals are not compatible with the parallel use of the LBS line as ion beam measurement line for Linac3.
Instead, a conventional slit should be adopted. The slit depth would have to be increased from currently 2 cm to about 20 cm (carbon was proposed) which will stop ~85% of the impinging H- (and the stripped protons). A new cooling system would have to be designed (at the moment only the holding rods are cooled). The very limited space of the current location of the slit might not be sufficient for the new slit, in which case the optics of the line will have to be recalculated. Also the mechanical tolerances for the construction and alignment of a slit with these dimensions will have to be checked. The slit should be movable.

Dump(s): A dump is required at the end of the line behind the SEMgrid. R. Scrivens said that currently there is a 20 mm graphite block bolted on the end flange. A detailed dump study will have to be prepared by Y. Kadi and his colleagues; the dump location (not on floor, but at certain height) and the very limited space is a challenge. Layout drawings of this region have to be found; 'easy' dump handling has to be guaranteed.
Finally the best solution seems to be the construction of a collimator-slit system plus a dump at the end of the line in case energy measurements are performed. If this is not the case, the collimator-slit will be closed and will receive the beam during Linac4 transfer line commissioning.
Y. Kadi needs clear specifications to start with the dump design. These will be provided by T. Hermanns and B. Mikulec. It was estimated to be acceptable to run at 0.1 Hz during Linac4 transfer line commissioning with a maximum pulse length of 100 us, but this still has to be confirmed by the Linac4 beam dynamics team.

Magnet upgrade: LTB.BHZ40 requires no upgrade, but a new magnet is needed to replace LBS.BVT10. With an unchanged radius, the B-field for the new magnet would be close to saturation (1.58 T). Magnet interlocks have to be implemented.

Power supply for new LBS.BVT10 magnet: To allow for parallel H- and ion operation the power supply has to be bipolar.

Scheduling: The issue has been raised that not only Linac2 cool-down is needed before the bending magnet LT.BHZ20 is turned for the Linac4 transfer line commissioning and Linac4 energy measurements. For the installation of the new LBS line also the PS requires a cool down period. One idea would be to advance the LBS line upgrade by one year if compatible with PS proton operation (Thomas should check that with the upgraded LBS line the Linac2/3 beams can still be measured). This would allow to advance the LBS line commissioning.
The scheduling issue should be taken up by the concerned workpackage owners.

Magnetic measurement: R. Scrivens pointed out that a NMR probe has to be integrated into the vertical bending magnet (10 MHz clock). The currently installed NMR probe has a precision of 1 ppm and the power supply of 100 ppm. Radiation tolerance and thermal drift of available NMR probes should be checked.

Depending on the collimator-slit position and the dump position at the end of the line, the optics will have to be adapted. Therefore no final specifications can be given yet. Once the open points are clarified, error studies have to be made and functional specifications to be defined as well as alignment requirements.

4. AOB

Linac4 quadrupole design: S. Maury has transmitted the information that the quadrupole design will be delayed. The new deadlines given by T. Zickler and L. Soby are end of September 2009 for the quadrupoles and pick-ups of Linac4 and end of October 2009 for the quadrupoles and dipoles of the transfer line. S. Maury stresses that these dates should be kept as fabrication takes approximately 2 years and the installation is planned for 2012.

-- BettinaMikulec - 20 Mar 2009

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