Arguments to choose the Frequency for a new 180 MeV Linac and the associated Front-End Test Stand at RAL
In case the ion source fails to deliver the required currents and duty cycles a beam funnel could help to achieve the necessary beam intensity. Up to now there have been two experiments for a beam funnel after an RFQ. One of them is well documented and was constructed as a one-leg funnel at 5 MeV with a 425 MHz strongly collimated (40 %) input beam [4], [5]. It was found that 25-40mA beams can pass the funnel structure with 100 % transmission without a major transverse or longitudinal emittance increase (within measurement precision). However, no reference was made to the used duty factor (which leads to assume that the duty factor was very low) or if the longitudinal beam properties satisfied the conditions for injection into a higher frequency RF system. Other critical points are the cooling of the funnelling device and the beam dynamics after the funnel. Any development of beam halo can only be measured after several focusing periods and was not assessed in the experiment. Since the beam will not see a constant field along the length of a bunch, different portions of the bunch will experience different deflection forces, which will yield transverse oscillations after the funnel device. More recent efforts towards funnelling which will be detailed in the following fall in two categories: a) funnelling at an energy of around 20-30 MeV (so far theoretical work), b) low energy RFQ funnels (theoretical and experimental work) For recent high-power proton linac studies (e.g. ESS [1] [2], CONCERT [3]) a merging of the beams at an energy of 20-30MeV is suggested to avoid beam blow-up through space charge. Furthermore, the space charge forces in the two chopper lines would be relaxed putting less constraints on the beam dynamics design in this area. The required frequency doubling demands that the subsequent accelerating structure is reasonably efficient at the higher frequency. For high-intensity linacs this limits the frequency after the funnel to ≈ 400 MHz if one wants to avoid a high number of single RF sources (see section 4). Neither CCDTL, SDTL nor traditional DTL structures are practical at higher frequencies for this energy. Higher energies for beam funnelling are excluded because of the required voltages for the funnelling device. The 2nd scheme is studied by IAP suggesting beam funnelling within a funnel-RFQ [6]. With this device every eventual beam loss will take place in the RFQ, at low energy. Up to now these funnels still suffer from emittance growth (≈ 60 %) but given enough time for R&D they represent an interesting option for the 352/704MHz and 400/800MHz option: in case of too low source currents one could replace the source and RFQ part with a half-frequency front-end instead of building two complete legs including source, RFQ, chopper line, and first DTL tank. Since the RFQ basically defines the longitudinal phase space, the risk of additional phase and energy jitter is much lower than for scenarios with two frequency jumps at later stages in the linac.
Low energy part of the CONCERT high-power proton linac
The 1.3 GeV linac of the CONCERT multi-users facility has to accelerate protons and H/sup -/ ions in a series of current pulses distributed over each 20 ms period. The peak current is identical for all pulses to keep the space charge forces about constant apart from a slight emittance change between H/sup +/ and H/sup -/ bunches. The beam power required for a given application is then obtained by adjusting the pulse duration with a total maximum power of 20 MW and 5 MW for the proton and H/sup -/ beams respectively. The reference design of the low-energy part of this linac is made-up of two separate proton and H/sup i/njectors up to 23 MeV, the H/sup -/ one using a funnelling system. These injectors are followed by a series of copper cavities up to the transition with superconducting cavities at about 185 MeV. The overall architecture, the technical choices and the beam dynamics in this high-power proton linac front end are presented and discussed.
ISIS megawatt upgrade plans
The ISIS spallation neutron source has been running successfully for more than 15 years and at 160 kW remains the most powerful source of its kind in the world. With machines due to operate at or near the megawatt level under construction in the United States and Japan and expected to come on line within the next decade, advances in Europe have not progressed at the same rate. A positive decision on the European Spallation Source (ESS) looks unlikely and one is led to consider the feasibility of alternative options. An ISIS upgrade is one such possibility. The current installation of a radio frequency quadrupole (RFQ) in the linac and a dual harmonic RF system in the synchrotron should lead to an increase in intensity of up to 50%, but plans are also under way to increase the beam power to 1 MW with the possibility of going to 4-5 MW in the longer term. The 1 MW option is based on an increase in energy to 3 GeV by means of a second synchrotron using ISIS as a booster. Details of the new ring and studies of the accelerating system are given in this paper. The ring also has the option of accelerating to 8 GeV at reduced frequency and this could be used as a test bed for the nanosecond bunch compression needed for the proton driver for a neutrino factory (NF). The cost of these proposals is relatively modest compared with a completely new facility. In the longer term, a combination of two such rings with a new synchrotron booster (replacing the existing ISIS) would give several options, for example: 4 MW for neutrons, or 2 MW for neutrons plus 2 MW for neutrino/muon studies, or 4 MW for a neutrino facility.