Is an RFQ a good candidate for a next- generation underground accelerator? Underground Accelerator for Nuclear Astrophysics Workshop October 27-28, 2003 Tucson, AZ Tom Wangler LANL

In any accelerator design there will be tradeoffs and choices must be made. How much physics remains to be studied by low-voltage(<1 MV), low-current DC accelerators by the time this facility will be built? Will higher current accelerators be needed? Although  E/E=10 -4 is most desirable, would an accelerator that produced  E/E=10 -3 be useful if it delivered more current? Is size an issue? What are practical space requirements? Is AC power an issue? What are practical power requirements? How much beam current is desirable, and how much can you tolerate in the target and detectors?

The RFQ The RFQ provides rf longitudinal electric field for acceleration and transverse rf electric quadrupole field for focusing -ideal for acceleration of low-velocity high-current ion beams.

Status of RFQ Technology Normal-conducting RFQ technology is very mature. LEDA RFQ at Los Alamos has operated at 100-mA CW, accelerating proton beam from 75 keV to 6.7 MeV. Superconducting RFQs have been built and will soon operate at the Legnaro heavy-ion facility. Should follow their progress. A normal-conducting 57.5-MHz 4-m-long RFQ is the front-end accelerator for RIA design, accelerating ions up to uranium from 12 keV/u to 200 keV/u. An RFQ can probably provide  E/E=10 -3, which may be acceptable for this application. Need a design study to confirm this. Energy variability has not been a requirement for most RFQs built so far. This topic was discussed at this workshop and a good concept was proposed.

What about energy variability in an RFQ? It should be straightforward to provide variable output energy in discrete steps. Separate the RFQ into independent sections, each with independent amplitude adjustment. Each RFQ section delivers beam at fixed design value when vane voltage is above threshold to form longitudinal bucket. Acceleration in downstream RFQ sections can be turned off by lowering their vane voltages below longitudinal bucket threshold. These sections still provide focusing.

Suggestion at this workshop: Continuous energy variability can be provided by installing the sectioned RFQ on a DC HV platform. By providing a variable HV platform voltage with a maximum value that exceeds the voltage gain of the individual RFQ sections, it should be possible to dial up any output energy by:  turning off appropriate number of downsteam RFQ sections  adjusting the platform HV.

Example Consider 6 RFQ sections for acceleration of q/A ≥1/2 ions from 30 keV/u to 330 keV/u, e.g. H +, D +, 3 He ++, 4 He ++ beams. Energy gain per section = 50 keV/u. 4 He ++ would be accelerated from 120 keV to 1.32 MeV with energy gain per section of 200 keV. Continuous output energies would require a HV platform voltage specification of ≥ 100 kV with q=2 for 4 He ++.

Conclusions The RFQ may be a very good candidate for achieving higher currents with 10s of mA possible for a next generation underground accelerator. A concept for providing continuous output-energy variability consists of independent RFQ sections installed on a HV platform. An RFQ design study should be carried out to answer questions such as current limits, energy spread, energy variability, size, and AC power for normal-conducting and superconducting options.