Operation of a 350 GeV proton beam for neutrino physics

SPS protons originate in the PS proton linac (LINAC II) and are accelerated first in the PS Booster (PSB) and then in the PS itself. The PS and the SPS operate on a cycle which is a multiple of 1.2 s, given by the repetition time of the PSB. The PSB also sets the maximum intensity of about protons that can be accelerated in each PS cycle. Presently the SPS supercycle length is 14.4 s, corresponding to 12 PSB cycles of which two (16.7 %) are injected from the PS into the SPS at the beginning of the SPS supercycle. Due to beam emittance constraints, the SPS can accept only protons per injection of which about are accelerated to 450 GeV, for a total of protons per 14.4 s supercycle.

A number of modifications to the PSB and to the PS are foreseen to be implemented before the start-up of the LHC. Among the upgrades planned is the injection of the protons from the PSB to the PS at an energy of 1.4 GeV instead of the present 1.0 GeV. This would result in a reduction of beam losses owing to the lower emittance. The corresponding intensity of each injection from the PS into the SPS could therefore come closer to the PSB limit of protons. It is thus conceivable that in these improved conditions up to protons could be accelerated by the SPS per supercycle already before the year 2000.

A modified scheme of operation of the SPS has been worked out together with the CERN SL Division aiming at a realistic optimisation of the operation of the WANF and taking into account all the presently known operational constraints. Its guiding principles were

• In order to reduce the prompt background, the energy of the protons impinging onto the neutrino target should be lowered to about 350 GeV.
• The proposed neutrino experiment should use the maximum possible proton intensity, by increasing the SPS repetition rate.

The following considerations were taken into account :

• Since the envisaged neutrino experiment is foreseen to be operational only after the decommissioning of LEP II, the lepton cycles (of total duration 4.8 s) will no longer be required. For the same reason a larger fraction of the PSB cycles could become available for injection into the SPS. The SPS repetition rate can be increased.
• A SPS cycle acceleration to 450 GeV will continue to be necessary for SPS experiments and test beams.
• These SPS experiments require a long flat-top (i.e. a slow extraction) presently 2.4 s
• The average power dissipation over the entire SPS supercycle must stay below the 33 MW limit. Since the SPS power dissipation is very high for the duration of the flat-top at 450 GeV enough time must be included at a lower power dissipation.
• A significant fraction of the total SPS proton intensity would be directed to the test beams and experiments on the 450 GeV flat-top. This will be more so the case after the start-up of COMPASS. The total number of protons on the targets T1, T2, T4, and T6 is foreseen to rise from the present to about per SPS supercycle once COMPASS [49] is also running.
• Sufficient time should be foreseen in the supercycle to allow for PS and SPS machine development cycles.

The proposed scheme, which satisfies all the above requirements is illustrated in Fig. 2.

  
Figure 2: Proposed SPS Supercycle.

The proposed SPS supercycle duration is 19.2 s long, i.e. 16 PSB cycles, and consists of two sub-cycles. The first one, lasting 13.2 s with a 3.2 s flat-top at 450 GeV, is similar to the present one and will serve all targets. A second 6.0 s long sub-cycle will be introduced with just a bare acceleration cycle at 350 GeV, with no flat top, serving only the neutrino target T9, and power consumption level low enough to compensate for the longer flat top at 450 GeV. The duty cycle for the 450 GeV flat-top (3.2 s every 19.2 s) is very similar to the present one.

Two fast-slow extractions for neutrino physics (FS/1 and FS/2) would take place at about 350 GeV on the ramp-up of the first sub-cycle. These two spills are separated by about 100 ms, i.e. about 10 GeV energy difference. Another two fast-slow extractions (FS/3 and FS/4) would take place in the dedicated neutrino sub-cycle at energies identical to FS/1 and FS/2. Such a SPS supercycle also accommodates the necessary spare time to allow for PS and SPS machine development cycles.

The neutrino target could be served with about out of accelerated protons in the first subcycle. It will certainly be able to withstand at least protons in the second subcycle. The total is then protons per supercycle. The overall gain with respect to the present WANF configuration amounts to about a factor of two in terms of protons per unit time. However, we plan to investigate if the present target, or possibly a new target made of carbon, could withstand in the second sub-cycle more or all of the available intensity of protons.

Currently, the focusing elements (horn and reflector) fire twice within a SPS supercycle with a separation of . In the scheme presented above, the horn and the reflector need to refire with a short 100 ms time separation. The first tests of such conditions have proven very successful. Four fast-slow extractions for neutrino physics within the 19.2 s SPS supercycle will therefore be possible.