High Intensity Pulsed Proton Injector
Description of the Work Packages
Five Work Packages are outlined. Management and communication matters are covered in the first. Each of the other four Work Packages is focused on a specific technology/competence. For their definition, the participating laboratories have adapted their own work plans and baseline choices to benefit from the collective efforts and avoid duplication. Different solutions will be investigated in parallel, and their progress and achievements will be regularly communicated and discussed. Finally, comparative assessments will be published to provide elements for well-justified choices for the upgrades foreseen in the three laboratories.
- Work Package 1 : Management and Communication
The following tasks are treated: oversee and coordinate the work of all work packages, organise steering committee meetings, ensure proper reviewing and reporting as well as dissemination of knowledge within the JRA and the CARE project.
- Work Package 2: Normal Conducting Accelerating Structures
Normal conducting RF structures are good candidates for beam acceleration in a pulsed proton linac, up to an energy exceeding 100 MeV. This is especially true if this is the final energy, as in the case of the three foreseen upgrades, because investment in cryogenic infrastructure can be avoided. The CERN accelerator has the additional requirement to be able, with a high duty factor (14 %), to deliver a beam quality that is adequate for a cascaded high energy superconducting linac (no halo). Such differences lead to different choices of RF structures and beam dynamics, which have to be developed in parallel and experimentally compared to help optimise the designs. In the case of CERN, a classical beam dynamics is considered, and the types of structures considered are DTL (Alvarez) for the energy range from 3 to 40 MeV, Coupled Cavity Drift Tube Linac (CCDTL) for 40 to 100 MeV and probably Side Coupled Linac (SCL) above 100 MeV. Design, construction and test of prototypes are planned, to validate the technological choices and help select the economical optimum. For the GSI linac with a final energy of approximately 70 MeV, the KONUS beam dynamics is foreseen, with the use of H-mode structures over all the range of energies. Low power model cavities have to be built and measured, and a prototype 352 MHz CH cavity is proposed to be built and tested in a high power test stand at GSI or at CERN. Simultaneous development of these complementary structures in a single JRA will result in an optimum use of the existing infrastructure (high power RF test places, computer codes, etc.), an enlargement of the knowledge accessed by every individual contributor, and finally in a better justified choice of technological solutions in any future realisation.
- Work Package 3: Superconducting Accelerating Structures
Superconducting (SC) RF cavities have much larger efficiency, accelerating gradient and bore aperture than normal conducting (NC) structures. This technology is then expected to be advantageous in a linac in terms of power consumption, construction cost and beam loss. Although this conclusion is well accepted for the high energy part of the accelerator, this is not the case at low energy, mostly because of the short distance required between transverse focusing magnets, which reduces the energy gain per real estate meter. On top of that, the Lorentz-force-induced detuning, which modulates the accelerating field in phase and amplitude, becomes larger as the lower energy. This effect has to be particularly taken into account in the case of a pulsed linac, because of the dynamic nature of the induced perturbations and of the possibility of exciting mechanical modes. It is therefore of high importance to improve the knowledge on the comparative performance of SC versus NC accelerating structures to help determine the lowest energy at which low beta superconducting cavities could safely and economically operate.
The associated critical component, required for any high intensity accelerating structure, is the input power coupler. The RF peak power transferred to the beam is typically 500 kW with duty cycles of the order of 10%, resulting also in a high average power. The peak power limitations come mainly from multipactoring and outgassing and are very frequency dependent. Couplers have to be tested up to 1 MW of forward peak power for reliability issues. High power couplers are also being developed in JRA2-WP2 for the needs TESLA but at much lower duty cycle and higher frequency. Tight links between both JRAs will be established to share the construction technology.
Three types of 700 MHz elliptical cavities will be tested in existing vertical cryostats at low power, and two of them in existing horizontal cryostats at full power. In the last test, the cavities will be fully equipped, housed in a helium tank, with tuning system and power coupler. There is presently no test site in Europe equipped with a 700 MHz high power RF source in the MW range. In the frame of this JRA, we propose to realise such a test stand at Saclay, and to make it available to the partners in HIPPI and later to any other interested European teams.
In parallel, two alternative cavity designs at 352 MHz will be analysed. Testing of two spoke-type cavities (two-gaps and multigap) is foreseen at low power, and possibly at high power, depending on the availability of a suitable infrastructure. A prototype of the tuning system for a CH resonator will built and tested.
- Work Package 4: Beam Chopping
The next generation of high energy, high power proton accelerators must be designed for very low uncontrolled beam loss. In many cases, the beam from a linac is injected into a synchrotron, an accumulator or a compressor ring, and subsequently extracted. Unless suitable measures are taken to control the dynamics of the beam, both processes can lead to considerable particle loss. Loss-free injection and longitudinal capture can be achieved if the linac bunches are precisely injected inside the synchrotron buckets and no particles end-up outside. Beam loss at extraction may be minimised by ensuring that no circulating beam coincides with the field rise-time of the extraction magnet. These demands may be met by selective elimination of sets of bunches in the low energy stages of the linac by using a fast deflector or beam chopper. The field should rise and fall between the beam bunch interval, so that no partially chopped bunches remain in the machine, and this usually has to be within a period of the order of 2 ns.
Specifications for these key components are technically challenging, and programmes have been implemented at CERN and CCLRC-RAL based on the development of slow wave (E-field) transmission line structures and high-voltage, fast-transition time pulse generators. Differences in the programmes ensure that a range of ideas will be investigated and there will be benefits from developing each in parallel over similar timescales. Both approaches rely on two successive sets of meander-line structures, but differ in the functioning of the meander lines, in the performance requirements of the driver amplifiers and in the design of the beam dumps. Prototypes of the slow-wave structures, drivers and the beam dumps will be designed, built and tested with beam.
- Work Package 5: Beam Dynamics
Recent studies with high intensity beams have shown that phenomena associated with space charge and beam loss have significant impact on the design of high power proton accelerators. In linacs, beam loss is associated largely with the appearance of a beam halo, which needs to be modelled theoretically and by simulation; it requires appropriate diagnostics and must be collimated to protect the equipment and avoid activation beyond tolerated limits. The joint activity proposed in this work package, combines resources available at the participating accelerator laboratories and universities for the analysis of the following issues:
1. Validation and Benchmarking of Simulation Codes. The development of adequate 3D computer codes and the proper modelling of self-interaction by space charge is a crucial issue. Codes must be fast enough to allow large ensembles of particles in order to resolve very small loss fractions. Including the effect of errors jointly with space charge requires a significant enhancement of simulation capabilities. Benchmarking of computer simulation codes against each other and against analytical models will increase the level of confidence in their results. Strategies to minimize halo formation should result from these efforts.
2. Experiments on Beam Halo and Emittance Growth. So far, no conclusive experiments on beam halo in high intensity accelerators exist; hence this issue has highest priority. Beam experiments are proposed at CERN, CCLRC-RAL and GSI, where operational conditions, available intensities and diagnostics allow relevant measurements. Their interpretation by means of simulation programs should reveal the adequacy of theoretical and simulation approaches and the methods proposed to minimize beam halo and loss.
3. Diagnostics. The acceleration and transport of high power beams also present new challenges for beam diagnostics. While conventional methods continue to be needed, operating conditions at high intensity require modification. New measurement techniques are needed to diagnose the small fractional beam losses, which could cause serious damage to components and produce unacceptable levels of activation. Monitors for direct beam halo measurements must be designed, constructed and tested in existing machines.
4. Beam Collimation. Scrapers are needed to localize beam losses in areas designed for that purpose. Injection into rings, where much less aperture is available than in linacs (in particular superconducting linacs), requires highly effective collimators prior to injection. Such schemes should be designed based on simulation data and tested in experiments.