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.
COMBINING CAVITY FOR RF POWER SOURCES: COMPUTER SIMULATION AND LOW POWER MODEL
A combining cavity for RF power sources has been investigated as a way of saving space, in comparison to waveguides, and as a way of combining power with graceful degradation if one or more component were to fail. The cavity has been investigated as the maximum power output of an Inductive Output Tube (IOT) for CW is 80KW at 500MHz and a proposed output of 20KW at 1.3GHz and most RF systems for particle accelerators require much more than this. Although 1.3GHz klystrons do exist they are vastly more expensive to purchase and maintain. Also the down time could be minimised to minutes in the even of a single IOT failure where as a klystron has a minimum downtime of several days in the event of a failure. Initially the cavity and its inputs were simulated in CSTs’ Microwave studio. After optimising the cavity to ensure the minimum reflection at the input ports and maximum transmission at the output port, a low power model was then created from aluminium. Signal generators were used to power the model and a network analyser was used to check the output. The model was used to compare the results gained from the computer simulation and to obtain results from asymmetric positioning of the ports, which was not possible in the simulation.
SPLinac : Computer Simulations of SC Linac RF Systems with Beam
The beam in a proton linac is very sensitive to field perturbations in the cavities. Therefore a simulation program was written modelling longitudinal beam dynamics in a realistic composite linac RF system. Fast RF vector sum feedback loops control several cavities with β-dependent transit time factors driven by one transmitter. Modelling of feedback loops covers limited transmitter power and bandwidth and possible loop-delay. Vector sum calibration errors, power splitting errors and scatter in the coupling strength to the cavities are optional as well as beam loading of the pulsing beam. Different modes of mechanical cavity perturbations including Lorentz force detuning can be chosen. A multitude of phase-space representation of bunches as well as RF quantity plots are available, most of them can be assembled as a movie, showing the system dy namics in ‘real time’.