Inivestigation of the possibility of high efficiency l-band srf cavity for medium-beta heavy ion multi-charge-state beams

The possibility of L-band SRF elliptical cavity in order to accelerate heavy ion multi-charge-state beams is being investigated for accelerating energy higher than 200 MeV/u. A first simple analytic study was performed and the result showed that the longitudinal acceptance of 1288 MHz is sufficient for heavy-ion multi-charge-state (5 charge states) medium-beta linac. The cryogenic heat load is calculated for this linac with taken into consideration cavity doping technology. In this paper, a summary of the beam dynamics and cryogenic heat load calculations for 1288 MHz linac for heavy-ion multi-charge-state (5 charge states) medium-beta beams. INTRODUCTION High Superconducting Radio Frequency (SRF) structures are attractive for a variety of reasons. They make the accelerator very compact since the cross-section of the cavity is proportional to the inverse of the frequency squared, i.e. the higher frequency RF the smaller cavity becomes. Although, the SRF cavity surface resistance is proportional to frequency squared, it can be minimized via doping technology. That reduces cavity cryogenics’ heat load significantly. In addition, cavity-doping technology has shown the trend that its reduction in cavity surface resistance is more pronounced in higher frequency cavities [1, 2]. That reduces cavity cryogenics’ RF heat load significantly at LBand frequency. HEAVY ION LINAC CRITERIA Longitudinal acceptance must be sufficient. ContinuesWave (CW) operation, high Q0 is desired. Low current linac (Uranium < 1mA), i.e. HOMs are not a serious concern. Multi-species and multi-charge state acceleration i.e. accelerates protons to uranium. Velocity acceptance must be sufficient; number of cavity-cells must be optimized. Optimized energy upgrade to boost U-238 from 200 MeV/u to 400 MeV/u. 1288 MHz LINAC PARAMETERS The proposed 1288 MHz (L-band) linac layout consists of eleven cryomodules; each includes nine 6-cell 1288 MHz SRF cavities and 22 room temperature magnetic quadrupoles for beam focusing. Table 1 summarizes the 1288 MHz linac parameters. The conceptual cryomodule layout is shown in Fig. 1. Figure 1: 1288 MHz conceptual cryomodule layout. Table 1: Summarizes the 1288 MHz Linac Parameters Parameter Unit Value Number of Cryomodules 11 Number of Cavities 99 Number of Cells per Cavity 6 Number of Cavities per Cryomodule 9 Number of Quadrupoles 22 Operating Frequency [MHz] 1288 Beta Geometry βg 0.61 Cryomodule Length [cm] 633.0 Bellow Length [cm] 7.1 Linac Total Length [cm] 7933.0 Number of Cryomodules 11 Number of Cavities 99 SUMMARY OF 1288 MHz CAVITY PARAMETERS The number of cells of the cavity was chosen to be six, such that it accelerates a multi-charge-state uranium-238 beam from an initial energy of 200 MeV/u to ≥ 400 MeV/u and maintain a sufficient velocity acceptance for lighter ions and protons. The larger cell number than six also investigated but it does not meet the upgrade requirement. The length of the accelerating cell is β λ/2 and the total length (flange-to-flange) of the cavity is 58.59 cm including beam pipes at both ends with β = 0.61 and a bore radius of 3.0 cm. The choice of the cavity bore radius to be 3.0 cm is advantageous due to the smaller cavity radius the uniform accelerating fields the ions will experience, i.e. smaller transverse kicks when ions pass through cavities, which minimizes the beam centroid oscillations in the space-phase plane. Specially, there were not beam correctors utilized in the 1288 MHz linac structure. In addition, a smaller radius cavity lowers the Epk /Eacc that is due to the fact that smaller radius adds more electric volume in the high electric field region of the cavity, i.e. cavity ends. The same thing is for Hpk/Eacc ratio. ____________________________________________ *This work was supported in part by the U.S. National Science Foundation, under Grant PHY-1102511 and by the U.S. Department of Energy Office of Science under Cooperative Agreement DE-SC0000661. 19th Int. Conf. on RF Superconductivity SRF2019, Dresden, Germany JACoW Publishing ISBN: 978-3-95450-211-0 doi:10.18429/JACoW-SRF2019-THP063 Facilities New Proposals compact accelerators THP063 1035 Co nt en tf ro m th is w or k m ay be us ed un de rt he te rm so ft he CC BY 3. 0 lic en ce (© 20 19 ). A ny di str ib ut io n of th is w or k m us tm ai nt ai n at tri bu tio n to th e au th or (s ), tit le of th e w or k, pu bl ish er ,a nd D O I.