User Guide for the P61 Pb Ion Beam


The P0 complex was designed and built to serve the experiments in the high-internsity underground cavern ECN3 with primary proton and heavy ion beams. The proton beams can be derived from the T4 or T6 primary targets and serve either the K12 (beam mode P42+K12) or the H10 branch (beam modes P41 and P61). Heavy ions are typically only provided to the H10 branch, used for the moment by the NA50 and NA60 experiments, via the T6 primary target.
In this user guide we describe only the operational aspects of the heavy ion operation of the P0 complex for NA50 and NA60. The beam mode is thus P61, user index <245>.

1. Preparations at the start of the heavy ion run

At the start of the heavy ion run, a number of operations have to be performed (by SL-EA) to allow a correct and safe operation of the P61 beam in heavy ion mode:

Normally the analog chamber MWPC 1+2 is left in place for the start of the ion period, but removed immediately after the initial tuning.

As soon as the beam from the SPS onto T6 is stable and correct (check the profiles and divergence at T6 with EA/BEAM/TARGET/SCAN, respectively EA/BEAM/TARGET/ANGLE), you are ready to tune the beam. Sometimes the horizontal spot is much too large (RMS 0.7 mm or larger) with a flat plateau in the middle. In that case the PCR may be able to improve by re-tuning the sextupoles for better extraction.
The complete tuning procedure is described separately for the P61 and H10 sections of the beam. Please note that the H10 part of the beam offers the choice of 3 different versions of the optics:

  1. The so-called Convergent Optics (as for NA10 and NA38) with magnification 0.25 in the horizontal plane, 0.8 in the vertical plane,
  2. The so-called Parallel Optics (a wrong and confusing name!) with magnification 0.8 in both planes,
  3. The Intermediate Optics with magnification 0.5 in both planes, i.e. nicely in between the two extremes mentioned above. The standard optics for heavy ion beams is the Intermediate optics (sometimes called "Semi-convergent"). It tends to provide the best performance for physics, but the maximal steering possibilities at the NA50 target are limited to ±3 mm. For special studies where larger displacements are required (e.g. ZDC calibrations, alignment studies) the parallel optics is used. This allows displacements of up to a centimetre in each plane. Fine steering of the end of the beam in parallel optics mode is described in the H10 User Guide.

    2. Observation of the performance of the beam

    3. How to find back good conditions?

    After a change of beam conditions (re-steering, spill optimisation, RF changes, gradual drifts, etcetera), it may be necessary to re-optimise the beam somewhat. Normally this can be achieved in three successive steps:

    1. Find back the beam flux through the first 4 collimators.

      Normally Collimators 1 to 4 should have rather small apertures, centred around zero (typically of the order of ±3 mm). Leave these apertures unchanged and do the following four scans (resp. double scans):
      • Center at COLL-1 with a single scan of TRIM2 on EXPT-1, 10 steps of 2 Amps at 400 GeV/c.
      • Center at COLL3, leaving the steering at COLL-1 unchanged, with a double scan of TRIM2 - 3.18 TRIM3 via 
             TUNE/DBL-SCAN/EXPT/1/2/TRIM/2/TRIM/3/5/2/-3.18
      • Center at COLL-2 with a single scan of TRIM1 on EXPT-1, 10 steps of 2 Amps at 400 GeV/c.
      • Center at COLL4, leaving the steering at COLL-2 unchanged, with a double scan of TRIM1 - 5.0 TRIM4 via 
             TUNE/DBL-SCAN/EXPT/1/2/TRIM/1/TRIM/4/5/2/-5.0
      If necessary adjust the flux by opening or closing COLL-1 and COLL-3.
      In the 2000 run, COLL-2 has some difficulties to set around position 0. Try to avoid using this collimator!
      Never use the SLIT option. Always use the JAWS option.

      For small adjustments the program TUNE/SPECIAL/ADJUST-COLL allows to slightly increase or decrease the gap without making big steps in between (to correct for mechanical play). Note, however, that this program does not update BIM.0 and that it has no error trapping. If it does not give the expected result, please use TUNE/SET/COLL instead!

    2. Center the beam at T8.

      At the old T8 location, just downstream of Fiscs 5 and 6, the NA60 collaboration has installed their beamscope detector, size 1x1 mm2. It is important to hit this detector with a beam that is even smaller (RMS about 0.35 mm in each plane!). This is achieved by the following "Finger scans":

      TUNE/SCAN/FINGER/Fisc #/Position/2/magnet/ etcetera):

      		Fisc
      Position      		 Reference
      position      		 Steering
      element

      		Fisc-6 (H)0.8TRIM-10

      		Fisc-5 (V)-1.4BEND-10
      Then check with the users that the beam is indeed centred on their detector. If necessary, fine adjustments should be made.
      To correct for the resulting change of angle at T8, scan BEND-11 through COLL-11 = ±1 mm onto EXPT-1 and set the best value.

    3. Steering to the main experiment


      The steering at the experiment is best achieved by Finger scans onto Fiscs 9 and 10:

      		Fisc
      Position      		 Reference
      position      		 Steering
      element

      		Fisc-10 (H)-0.2BEND-12

      		Fisc-9 (V)1.3TRIM-13
      Please verify and document the beam shape with Fisc profiles of Fiscs 9 and 10 (with 0.2 mm steps).
      If necessary, a more precise scan be made onto the halo counter, which has a 3 mm diameter hole. Normally it is plugged into EXPT-10. Before the scans, the experiment should remove their target, otherwise the halo rate would be dominated by backsplash from the target.
      In both planes it is preferred to perform double scans: BEND12 + 3.7 TRIM12 for the horizontal, TRIM13 + 2.12 TRIM11 for the vertical position. In both cases it is recommended to take ±5 steps of 4 Amps. E.g.: 
           TUNE / SCAN / EXPT / 10 / 2 / BEND / 12 / TRIM / 12 / 5 / 4 / 3.7
      Select the values corresponding to the center of the hole. After this operation it is important to check that the flux and halo rates are correct again.

      Sometimes the experiment wants to scan the trigger rate (EXPT-4) versus X and Y position over a wider range. The trigger rate increases dramatically when the beam hits the edges of the Beryllium Oxyde absorber (diameter 7 mm). For this the range of the double scans is insufficient and single scans of BEND-12 and TRIM-13 onto EXPT-4 (range ±60 Amps for BEND-12, ±80 Amps for TRIM13).

    Last updated : 10 October 2000 by Lau Gatignon