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.
A New 180 MeV H Linac for Upgrades of ISIS
Several options have been studied to raise the beam power of the ISIS spallation neutron source to a level of 1 MW with the possibility of going to 4-5 MW in the longer term [1]. All scenarios can operate in 2 modes, where the beam power is either delivered to a spallation target or, alternatively, to a target suitable to produce muons via pion decay for a neutrino factory. A more recent upgrade option takes an intermediate step and uses a 180 MeV H linac, which is also foreseen for the 4-5 MW upgrade, as a replacement for the current 70 MeV injector. First estimates indicate that, due to the lower space charge forces, the ring would be able to carry twice as many particles as are expected in the current harmonic upgrade, thus doubling the final beam power to 0.5 MW. A further step in this scenario could be to extend the 180 MeV linac to 800 MeV, the actual ISIS output energy, and to use the ISIS synchrotron as an accumulator/compressor ring. This paper presents a first design for the 180 MeV linac, using a triple frequency jump from 234.8 to 704.4 MHz. The design benefits from the development of 704.4 MHz cavities and RF equipment within the framework of the European HIPPI collaboration. The low frequency for the front-end was chosen to ease the DTL design as well as the development of a low energy beam chopper, which will be necessary to reduce beam losses at injection into the synchrotron.
Proposed beam diagnostics instrumentation for the LANSCE refurbishment project
Presently, the Los Alamos National Laboratory is in the process of planning a refurbishment of various subsystems within its Los Alamos Neutron Science Center accelerator facility. A part of this LANSCE facility refurbishment will include some replacement of and improvement to existing older beam-diagnostics instrumentation. While plans are still being discussed, some instrumentation that is under improvement or replacement consideration are beam phase and position measurements within the 805-MHz side-coupled cavity linac, slow wire profile measurements, typically known as wire scanners, and possibly additional installation of fast ionization-chamber loss monitors. This paper will briefly describe the requirements for these beam measurements, what we have done thus far to answer these requirements, and some of the technical issues related to the implementation of the instrumentation.