Minutes of the joint meeting Booster Commissioning and PSB Beam Dynamics with Linac4 Working Groups held on 17th of September 2009

Present: A. Lombardi, C. Carli, W. Weterings, J. Tan, B. Goddard, T. Hermanns, K. Hanke, B. Mikulec.

Brainstorming on PSB Commissioning with Linac4

A brainstorming discussion on PSB Commissioning with Linac4 was initiated, based on slides prepared by C. Carli. The commissioning work package document has been used as basis. The aim of this series of meetings is to define the different commissioning stages and what is required for each stage in terms of beam properties, hardware, software or instrumentation, identify potential problems and their workarounds, define procedures and to estimate in more detail the time needed for PSB commissioning with Linac4.

Due to time constraints only the first 4 slides have been discussed, corresponding to commissioning to nominal performance.

1. Hardware Commissioning New equipment should be tested by the hardware specialists in collaboration with the operations team.
The Booster has to be prepared for the higher injection energy. Procedures used currently shortly after injection should be reviewed if still applicable. One important part will be the setup of the new magnetic cycle (value of the programmed magnetic field before injection and after extraction, cycle with and without ramp, optimisation of the Bdot for optimised acceptance and to avoid cavity problems).

It has been pointed out that it might take a while to reach nominal Linac4 beam current. Therefore minimum Linac4 specifications for PSB commissioning should be elaborated. A. Lombardi mentioned that a minimum of 30 mA are needed to set up the chopper line. This value seems acceptable for PSB commissioning, but it should be checked if the signal of 30 mA is sufficient for an energy measurement in the LBS line.

Assigned to	Start date	Description	State	Result
T.Hermanns	2009-09-21	Check if 30 mA of beam current are sufficient to obtain and energy measurement with the required precision in the LBS line.	openclosed	Yes, 30 mA are sufficient (see sLHC note on LBS line).	edit

2. Beam Characterisation in the LBE/LBS measurement lines

It has to be made sure that the measurement accuracy in terms of emittance, Twiss parameters, absolute energy, energy spread and energy fluctuations can be reached. If necessary the line upstream LBE/LBS will have to be rematched. It should be clear which beam properties are required for PSB commissioning. Final transverse emittance is not that critical, but matching should be reasonable close to expectation meaning that rematching can be done within reasonable time.

3. Generation of appropriate time structure of Linac4 beam and transfer to PSB injection region

The 3 MeV chopping has to be set up including synchronisation with the injection equipment (mainly the distributer). To obtain correct synchronisation, beam instrumentation will be required. BLMs in the injection region will be one necessary tool. In addition the signal on the H0/H- dump should be measured varying the distributer timing. The signal of ring-specific BLMs installed close to the injection point can be integrated and minimised. The sensitivity of BLMs at critical positions has to be clarified beforehand with BI. J. Tan mentioned that it should be possible to use the 'middle-fast' transformer installed in 8L1 as it has sufficient bandwidth. B. Mikulec has already asked L. Soby to inquire if this transformer could be integrated into an extended version of the injection watchdog.

Assigned to	Start date	Description	State	Result
J.Tan	2009-09-21	Follow up with L. Soby and P. Odier if it would be possible to use the 'middle-fast' transformer installed in PSB section 8L1 for injection efficiency measurements of the Linac4 beam.	openclosed	It is possible to use the intermediate frequency BCTs for this purpose. The electronics has to be upgraded.	edit

Assigned to	Start date	Description	State	Result
B.Dehning	2009-09-21	Clarify sensitivity of Linac4 BLMs and the flexibility of sensitivity selection.	openclosed	See minutes of the Linac4 diagnostics working group from 14/12/2009.	edit

The required beam structure and intensities have to be specified. Both must be adapted to the beam instrumentation, and the intensity has to be just sufficient to be detected in order to limit activation.

4. Transfer low intensity beam through injection region and first turns

• Proper steering is needed; for this the beam can be set up first on the H0/H- dump and then on the injection screen (remark W. Weterings: the injection screen mechanism will be independent of the foil wheel, upstream at a short distance. The screen for the circulating beam will be separate as well as due to stripping the injection screen cannot be in the beam at the same time. There is some space available between BS1/BS2 and BS3/BS4).
• Inject ~half a turn (using chopper) to create a beam structure visible by PSB pick-ups.
• It has been agreed to start injection on a flat bottom.
• Multi-turn acquisitions are required to measure the bump closure. The current pick-up electronics do not permit such a measurement (1 ms sampling). At least two pick-ups with appropriate betatron phases must be upgraded for turn-by-turn acquistions to measure the bump closure (multiturn acquisitions with more than two or even all pick-ups would of course improve the measurement accuracy). It should be investigated if the new PS electronics could also be used for the Booster. The question was raised if it could happen that the bump closure was so bad that the beam couldn't make a few turns. B. Goddard suspects that this could indeed be the case and that adjustments of the BS magnets would be needed. Therefore it would be easier to have individual power supplies per BS magnet instead of serial powering. 3D models of the BS magnets should be made and each magnet calibrated. It might also be possible that the fringe fields of the main bending magnets could affect the BS magnetic field.

Assigned to	Start date	Description	State	Result
J.Tan	2009-09-21	Follow up with U. Raich if at least 2 PSB ring pick-ups could be equipped with PS-like turn-by-turn acquisition electronics.	openclosed		edit
B.Goddard	2009-09-21	3D modeling of the BS magnets.	openclosed		edit

• Matching including dispersion: Matching and the dispersion of the arriving beam must be determined with sufficient precision in the measurement lines. For dispersion measurements, the Linac4 output energy will be varied using the last two PIMS modules and the debuncher phase (Remark: it was decided to start first with the 0 dispersion injection conditions.). There is a discussion ongoing whether SEMgrids are required (working in ppm mode) and where they would have to be installed in order to measure injection mismatch. In principle mismatch w.r.t. the real machine including beta-beating can be measured well with multi-turn acquisitions of SEMgrids installed at any location, but one has to make sure that the circulating beam cannot damage the monitor, for example by stopping the beam with the help of the ejection kickers firing after a few turns. On the other hand, with the new injection principles, proper matching is less critical than with the present conventional multi-turn injection.
• For emittance measurements and empirical optimisations of the injection process one counts on the correct functioning of the wire scanners, also at low energies.

5. Establish injection and circulating beam

There are no real problems expected at this stage as circulating beam should be achieved rather automatically (provided the PSB lattice is OK) once the injection trajectories and orbits are properly adjusted. One should start with the injection of coasting beams (no RF voltage). The bump heights might have to be optimised.

6. Beam capture, acceleration and first extractions

No particular problems are expected for adiabatic capture as currently done (starting with low intensities and on flat bottom), acceleration and extraction. Possibly the Linac4 energy spread should be adjustable to fill the waiting bucket more homogeneously, but larger than the one required for longitudinal painting. A. Lombardi said that it might not be possible to change the energy spread pulse-to-pulse with the debuncher, but one could think of changing the phase by 180 degrees. The question came up if 2 cavities could be installed instead of 1. It was also mentioned that the energy spread will vary with the beam current (as the energy spread is dominated by uncompensated space charge at the Linac4 exit).

Assigned to	Start date	Description	State	Result
A.Lombardi	2009-09-21	Discuss with F.Gerigk the potential of the debuncher to increase the energy spread and assess the usefulness of 2 cavities.	openclosed	It has been decided to install only 1 cavity at a new position. The original position of the cavity will be kept as backup in case the amplifier would not be compatible with the parameters at the new position.	edit

7. Increase the intensity

• The length of the Linac4 beam should slowly be increased. It might be necessary to cut holes out of the pulse with the chopper to avoid beam loading of the Linac4 cavities.
• The working point has to be tuned accordingly to keep losses at an acceptable level. It has to be evaluated which intensities can be achieved with the injection of a coasting beam followed by capture.
• Add transverse painting starting with a certain number of injected turns (still to be determined): play with the injection parameters to paint the final emittance and minimise losses.

For this stage it is critical to have the beam loss measurement, interlocks, foil, wire scanners, tune measurement and injection efficiency measurement using transformers operating reliably.

AOB

B. Mikulec asked if one should think of installing an internal dump (x4) in the PSB. Several currently implemented software interlock conditions simply switch off the RF leading to uncontrolled beam losses around the ring. Moreover it could be very useful during commissioning to concentrate the beam losses. C. Carli mentioned that the activation at 160 MeV is roughly a factor of 10 more important compared to 50 MeV. K. Hanke said that he had seen the new design of the PS internal dump and that this design might be adaptable to the PSB, although it might be necessary to investigate on the mechanism to insert it due to the 4 superimposed PSB rings.

Next Meeting

To continue this discussion and extend it to the commissioning steps to ultimate performance, a new meeting has been organised for Thursday October 1st, 9am in 865 2-C19.

-- BettinaMikulec - 2009-09-18

• BrainstormingCommissioning09_09_17.pdf: Brainstorming on PSB Commissioning with Linac4%META:FILEATTACHMENT{name="BrainstormingCommissioning09_09_17.pdf" attachment="BrainstormingCommissioning09_09_17.pdf" attr="" comment="Brainstorming on PSB Commissioning with Linac4" date="1253257304" path="BrainstormingCommissioning09_09_17.pdf" size="99009" stream="BrainstormingCommissioning09_09_17.pdf" tmpFilename="/usr/tmp/CGItemp17445" user="bettina_2emikulec_40cern_2ech" version="1"}%