The EURISOL Beta-beam http://cern.ch/beta-beam/ Steven Hancock AB Department, CERN on behalf of the Beta-beam Study Group http://cern.ch/beta-beam/

Outline Overview Beam loss management RF aspects Conclusions Dynamic vacuum Collimation and absorption ACCSIM/FLUKA simulations Activation RF aspects Stacking efficiency Beam loading Conclusions EURISOL, Pisa, 2009 S.Hancock

Introduction Beta-beam proposal by Piero Zucchelli. A novel concept for a neutrino factory: the beta-beam, Phys. Let. B, 532 (2002) 166-172. AIM: production of a pure beam of electron neutrinos (or antineutrinos) through the beta-decay of radioactive ions circulating in a high-energy storage ring. Baseline scenario. Based on known technology and machines. Makes maximum use of the existing CERN infrastructure. Annual (107 s) rate of 2.91018 antineutrinos or 1.11018 neutrinos. 9.71013 6He2+ or 7.41013 18Ne10+ ions stored at =100. EURISOL, Pisa, 2009 S.Hancock

Acceleration to final energy EURISOL Beta-beam Ion production Acceleration Neutrino source Experiment Proton Driver SPL Acceleration to final energy PS & SPS Ion production ISOL target & Ion source SPS Neutrino Source Decay Ring Beam preparation Pulsed ECR PS Ion acceleration Linac Acceleration to medium energy RCS EURISOL, Pisa, 2009 S.Hancock

RCS Operating at 10 Hz, this machine is a relatively conventional RCS (cf., ISIS). It has been designed with a modest B-field swing (0.2 – 1.1 T), which means a reasonable maximum ramp rate (< 30 T/s) and, in turn, reasonable rf requirements (100 kV). Eddy current effects require the vacuum chamber to be thin, but it does not need to be ceramic. The RCS delivers both species of beam at a magnetic rigidity of 14.5 Tm. EURISOL, Pisa, 2009 S.Hancock

PS The PS operates on its highest harmonic (h=21) and accumulates 20 bunches one by one from the RCS. Number of ions accumulated versus RCS batches. Relative intensity along the bunch train. The situation is better in the neon case due to its longer half-life and more advantageous charge-to-mass ratio, but the PS extraction kicker gap must still be established at a different position within the bunch train from batch to batch in order to even out the bunches that are ultimately stored in the decay ring. The PS delivers both species of beam at a magnetic rigidity of 86.7 Tm in a cycle time of 3.6 s. EURISOL, Pisa, 2009 S.Hancock

SPS There is no accumulation in the SPS, which is less than 10% filled, but the machine was designed for fixed-target physics and its rf is not ideally suited. The space charge bottleneck at SPS injection is addressed by adding a 1 MV, 40 MHz rf system to the existing 8 MV, 200 MHz one. This allows much longer bunches to be transferred from the PS. Then, near transition when the bunches are short enough, the standard system takes over for the bulk of the acceleration. The SPS delivers beams at γ=100, which corresponds to a magnetic rigidity of 935 Tm for helium ions and to 559 Tm for neon. The advantageous charge-to-mass ratio of neon and fixed γ at ejection result in a cycle time of only 3.6 s compared with 6.0 s for helium. EURISOL, Pisa, 2009 S.Hancock

Decay Ring The decay ring is a superconducting machine in order to minimize the length of the arcs, where any ion decays are wasted. Nevertheless, it is the same size as the SPS with one straight section occupying 36% of its circumference. In order to suppress the atmospheric background in the experiment, the ions must be concentrated into a few very short bunches. The time structure established in the PS is therefore retained. Indeed, it is exploited to permit kickerless (off-momentum) injection. Accumulation is necessary to achieve the unprecedented intensities required and is possible because the half-life of the highly relativistic stored ions is more than an order of magnitude longer than the cycling time of the injectors. It is proposed to stack the ions using asymmetric bunch pair merging, which relies on a dual-harmonic rf system to combine adjacent bunches in longitudinal phase space such that each fresh, dense bunch is embedded in the core of a much larger stored one with minimal emittance dilution. Cooling is not an option. Electron cooling is excluded because of the high electron beam energy and, in any case, the cooling time is far too long. Stochastic cooling is excluded by the high bunch intensities. EURISOL, Pisa, 2009 S.Hancock

Outline Overview Beam loss management RF aspects Conclusions Dynamic vacuum Collimation and absorption ACCSIM/FLUKA simulations Activation RF aspects Asymmetric bunch pair merging Beam loading Conclusions EURISOL, Pisa, 2009 S.Hancock

Loss Comparison Machine activation due to beta-decay losses alone is not a show-stopper but still deserves careful consideration. Scaling from the 1998 neutrino experiments CHORUS and NOMAD to nominal CNGS operation gives loss power figures comparable with a Beta-beam run. A.Fabich and M.Benedikt, EURISOL DS task12/12–05–05 EURISOL, Pisa, 2009 S.Hancock

PS Dynamic Vacuum The dynamic vacuum simulation code STRAHLSIM developed at GSI has been improved. The desorption rate due to high-energy particles at grazing incidence is modelled on the basis of machine experiments. The calculated pressure evolution is now stable for both principal ions of interest. This is despite the fact that the PS machine does not have an optimized lattice for unstable ion transport and has no collimation system. P.Spiller et al. EURISOL, Pisa, 2009 S.Hancock

Collimation and Absorption In addition to strategically placed absorbers to deal with decay products, momentum collimation is also required in the decay ring to limit the acceptance of the accumulated stack in a controlled fashion. The stored beam energy is ~9 MJ in the helium case (~20 MJ for neon, which is approaching that of one LHC beam at injection). Roughly 50% (75%) of the 0.8 MJ (1.1 MJ) delivered with each cycle of the injector chain must be removed by collimation; the rest goes into the decay products. p-collimation merging decay losses injection Straight section Arc Momentum collimation A.Fabich EURISOL, Pisa, 2009 S.Hancock

Particle Loss Modelling The tracking code ACCSIM developed at TRIUMF has been modified to cater for arbitrary ion species produced way off-momentum by particle decay. ACCSIM serves as an event generator for FLUKA. A semi-automatic link, including the necessary coordinate transformations, is made between the two codes using Mathematica. Q B Absorbers 6Li3+ 18F9+ FLUKA (single cell) model: concentric cylinders of Cu (coil) and Fe (yoke); no beam pipe. E.Wildner EURISOL, Pisa, 2009 S.Hancock

Power Deposition First results indicate that the deposited power is within cryogenic limits (10W/m) globally, but that there are local hotspots which exceed the quench limit (4.3 mW/cm3). A solution could be open midplane magnets. 6Li 18F E.Wildner EURISOL, Pisa, 2009 S.Hancock

Open Midplane Dipole By opening up the midplane by ±5º the heat deposition on the critical area of the magnet can be decreased by at least an order of magnitude and the aperture can be reduced. J.Bruér EURISOL, Pisa, 2009 S.Hancock

Activation Activation at the entrance to the arcs would be confined to dumps. There are hotspots in some elements of the injection bumps, but these are normally conducting. Activation due to the collimators is still being evaluated. The following are the worst-case residual ambient dose equivalent rates (mSv/h) at 1 m from the beamline after an irradiation time of 107 s. BUMP 2 1h 1d 1w 6He 18Ne 1st Q- 1st BN2 15 1.8 12 0.72 7.5 0.54 3rd Q - 1st BN3 2.7 3.6 1.2 1.44 0.6 1.08 2nd BN3 - 7th Q 90 0.9 60 33 0.27 2nd BN2 - 9th Q 30 1.62 18 ARCS 1h 1d 1w 6He 18Ne Arc 1 (entrance) 9 21.6 7.5 14.4 3 5.4 Arc 2 (entrance) 24 54 15 10.8 Arc (worst cell) 1.2 0.45 3.6 0.3 1.44 S.Trovati EURISOL, Pisa, 2009 S.Hancock

Outline Overview Beam loss management RF aspects Conclusions Dynamic vacuum Collimation and absorption ACCSIM/FLUKA simulations Activation RF aspects Asymmetric bunch pair merging Beam loading Conclusions EURISOL, Pisa, 2009 S.Hancock

Decay Ring Merging EURISOL, Pisa, 2009 S.Hancock

Survival Profile Starting from an empty stack, the relative increment versus the number of successive merging steps is the survival profile of the first injected bunch. ESME tracking includes momentum collimation but no beta-decay, so this profile must be corrected before integrating to obtain the limiting intensity in the stack. Of course, some particles would have decayed before reaching the collimator, but how they are lost does not matter for the summation. This yields the effective steady-state number of shots accumulated as 8.9 (cf., 10.7) in the helium case and 14.0 (cf., 17.4) for neon. The overall figure of merit for the stacking process is in excess of 80% in both cases. Cf., Stack acceptance = 15 injections Collimation Beta-decay EURISOL, Pisa, 2009 S.Hancock

Transient Beam Loading The problem with heavy beam loading in a stationary bucket is that the beam current is in quadrature with the gap voltage, which detunes the cavity. Since the decay ring is above transition, the beam current is inductive and will detune the cavity “up”. To compensate, the cavity must be tuned “down” to a resonant frequency ω0 < ω given by since The beam occupies only a small duration, TB, of the full revolution period, T, but if the phase slip, (ω-ω0)(T-TB), during the absence of beam is an integer multiple, n, of 2π, then it would be possible to drive the cavity at ω0 rather than ω during this latter period and still have the correct phase when the beam returns. Even if only approximately met, this condition could help to reduce the mismatched load otherwise seen by the final amplifier. It requires E.Jensen EURISOL, Pisa, 2009 S.Hancock

Conclusions Beta-beam baseline scenario is well established. Main technical challenges rest firmly with the decay ring, with the focus of attention on rf and collimation systems. Accelerator design is made “top-down” so that machine performance is compatible with target figures from physics, while the production side is studied within EURISOL. EURISOL, Pisa, 2009 S.Hancock