Minutes of the Linac4 Diagnostics Working Group Meeting held on 8 December 2008

Present: U.Raich, J.Tan, G.Bellodi, M.Ludwig, C.Dutriat, B.Cheymol, L.Soby, E.Bravin, F.Lenardon, T.Hermanns, K.Hanke, B.Mikulec.


1. Communications

2. Follow-up of open actions

3. First conceptual design of the 3MeV diagnostics movable bench (G.Bellodi)

4. News on the Feschenko monitor (U.Raich)

5. Summary of the APL proposal on laser profile monitors (U.Raich, K.Hanke)

6. AOB

1. Communications

  • C.Dutriat presented the new fellow, Benjamin Cheymol, who will contribute to BI Linac4 studies.

  • There will be a meeting this week Thursday and Friday in Germany on emittance measurements in the framework of CARE. R.Scrivens, A.Lombardi and U.Raich will participate and report later on discussed subjects of potential interest for Linac4.

  • Emittance meter: C.Dutriat reported that the head has been mounted and that J.Brahy is currently working on the fine tuning of the movement. The emittance meter is still located in the BI lab in building 864; software to be checked.

  • SEMgrid for new transfer line: C.Dutriat confirmed that the standard box will be used that has a length of 210 mm flange to flange (same value as already given to the drawing office).

2. Follow-up of open actions

-. Slow wire scanners for chopper line: C.Dutriat assured everybody that a solution will be found; maybe a new motor has to be bought.

3. First conceptual design of the 3MeV diagnostics movable bench

G.Bellodi presented a first conceptual design of the 3MeV diagnostics movable bench (see 3MeVMovableBench_GB.pdf). The following measurements are required for RFQ, MEBT and possibly DTL tank1 commissioning:

  • beam profiles and emittance measurement
  • transverse halo measurement
  • beam position measurements
  • beam transmission
  • average beam energy
  • longitudinal 'halo' measurements (chopping efficiency)
  • energy spread
  • bunch shape measurements and longitudinal emittance (?)

The energy spread has to be measured using a spectrometer line. For this purpose the IPHI sector dipole with bending angle of 28.5 deg and 0.17-0.34 T magnet field followed by 2 IPHI quadrupoles are used. A movable slit with 0.1-0.2 mm half-opening at the end of each structure to be commissioned defines the beam fraction to be studied; slit cooling to be investigated. In case of the RFQ 2-3 additional quadrupoles are needed to provide a parallel beam at the slit location; for the chopper structure it should be possible to use the last quads of the chopper with rematched parameters. A SEMgrid and beam stopper will end the spectrometer line. For sufficient sensitivity it seems necessary to require a SEMgrid with 0.5 mm wire spacing. To be studied by BI.

The measurement of the beam energy is foreseen to be performed using the time-of-flight method with 3 PUs. A 1 deg phase error should not be exceeded and one has to assure that the beam is not completely debunched at the PU positions.
The PUs will also allow to determine the absolute beam position as well as the absolute beam phase with respect to a RF reference.
This straight line will have to be terminated with a beam dump as well (0.5 mA resolution).

3 transformers will yield beam transmission.

The location of the Feschenko monitor in the straight line behind the sector bend should be reviewed as the beam is almost debunched there. Its aim is to measure the bunch shape.

A halo monitor before the sector bend will determine the transverse halo and will also allow to measure the residual H- ions in the chopped buckets and therefore the chopping efficiency.

For the transverse emittance it is proposed to use a movable slit (H+V) with 0.25-0.5 mm half-opening and a SEMgrid at the end of the straight line. Their distance has to be optimised to obtain the desired resolution. For the 3 MeV commissioning the beam should be measured directly after the structure; here a movable slit (H+V) at the exit and a diagnostics box (PU+SEMgrid) before the sector bend are proposed. This first slit could be the one from the spectrometer.
E.Bravin proposed to get rid of the 2nd slit and use the SEMgrid at the end of the straight line with the 1st slit to obtain larger beam sizes at its location as otherwise we are at the resolution limit of the SEMgrid. To be studied.

In general, a modular approach should be adopted for the different commissioning stages. There is very little space between the end of each structure to be commissioned and the sector bend. For the moment the last PU seems not very useful (except for a 3rd position measurement) as the beam is completely debunched.

The procurement of power supplies and vacuum equipment has to be assured in addition.

Addendum (information from G.Bellodi after the meeting): G.Bellodi has moved the halo and bunch shape monitor before the sector bend and redefined the position of the PUs (at locations where beam is not completely debunched; see 3MeVMovableBench_update_GB.pdf). This assumes that the transformers fit into 15 cm (longitudinal length), the PUs and SEMgrids into 5-10 cm, the halo monitor into 45 cm and the bunch shape monitor into 25 cm. This has to be clarified with BI.

Assigned to Start date Description State Result
U.Raich 2008-12-16 Check longitudinal dimensions of BI equipment for diagnostics movable bench with equipment specialists and give feedback to G.Bellodi. Done. See minutes of January 19th 2009. edit

The beam pipe diameter is given by the IPHI magnets (66 mm); the length of the line would now yield 3.7 m/4.8 m to the respective SEMgrids (straight/bent line). For rematching after the RFQ 2 quadrupoles are sufficient; the last 4 quadrupoles in the chopper line are assumed to be used for this purpose. The numbers indicated for the quadrupole gradients are typically 30% higher than the actual values used in simulations.

These studies result in the following shopping list:

  • 3 transformers
  • 3 PUs
  • 1 movable H/V slit (0.1-0.2mm half-opening; reuse LEBT fixed width slit?)
  • 2 SEM grids: 1 H+V (emittance meter), 1H (spectrometer); minimum 0.75mm resolution
  • 1 halo monitor
  • 1 bunch shape (Feschenko) monitor
  • 1 sector dipole, α=28.5 deg, ρ=1.5 m, B=0.17-0.34 T, gap=75 mm
  • 2 quads (spectrometer line): L=150 mm, =66 mm, G<8T/m
  • 2 quads (RFQ rematching section): L=150 mm, =66mm, G<15-20T/m
  • power supplies
  • 2 beam dumps
  • vacuum equipment

A modular approach should still be considered, in particular moving to higher energies (displace last PU to obtain better resolution).

4. News on the Feschenko monitor

U.Raich reported on recent discussions he had with T.Kurtyka on the procurement of a Feschenko monitor.
A Feschenko monitor consists of in intercepting wire (at 10 kV; movement into beam with actuator) producing secondary electrons, a slit in front of deflecting plates (RF deflection unit), a corrector magnet, a bending magnet (to distinguish between stripped and secondary electrons) and an electron detector with another slit in front (see slide 3 of feschenko.pdf). The HV unit, control system and other electronics is part of the device. It can be delivered with a LabView control program.

The way to advance is including the following points:

  • CERN provides first the technical specifications to INR (institute for nuclear research in Moscow).
  • The price has to be below 200 kCHF for administrative reasons. A crude first price estimate was around 150 kCHF, which seems to be rather high.
  • If CERN supplies control equipment to INR this price will come down. INR proposes for example the wire actuator, but this part is rather complicated as explained by E.Bravin.
  • The power supplies have to be bought by CERN.
  • Need ~6 months of 'travel money'.

The timescale is already critical (~18 months after the signature of the contract). U.Raich will follow this up with high priority.

5. Summary of the APL proposal on laser profile monitors

This item on the agenda has been postponed for the next Linac4 diagnostics WG meeting.

6. AOB

Racks in Linac4 building: L.Soby has requested to move some BI racks close to the transfer line to reduce cable length. In case this will be required by other equipment specialists, information should be sent to Lars who volunteered to follow this up with C.Rossi.

-- BettinaMikulec - 08 Dec 2008

Topic attachments
I Attachment History Action Size Date Who Comment
PDFpdf 3MeVMovableBench_update_GB.pdf r1 manage 21.0 K 2008-12-16 - 12:59 BettinaMikulec Update to design of 3MeV movable bench.
PDFpdf feschenko.pdf r1 manage 311.9 K 2008-12-11 - 18:30 BettinaMikulec News from the Feschenko monitor.
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Topic revision: r7 - 2009-01-19 - BettinaMikulec
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