High Current Beam Dynamics in an Ess Sc Linac

Three alternative designs of the European Spallation Source (ESS) high energy linac are described. The most promising ones are either a normalconducting (nc) coupled cavity linac (CCL) up to final energy or a change at 407 MeV to only one group of 6 cell superconducting (sc) elliptical cavities. Fully 3d Monte Carlo simulations are presented for both options, optimized for reduced halo formation at the linac end. For the error free matched case, especially the halo formation in the longitudinal plane is more pronounced for the hybrid solution with its superconducting cavities, caused by the unavoidable phase slippage, but still quite well acceptable for loss free ring injection. Simulations however for a 30% mismatched dense core, surrounded in addition by 1.5% halo particles are showing few particles with very large amplitudes even in real space. This case represents halo formation in front to end simulations, caused by current fluctuations, filamented RFQ output distribution and enhanced by accumulated field errors. 1 OPTIONS FOR THE HIGH ENERGY PART OF THE 6% D.C. ESS LINAC The current reference design of the ESS contains a 1.334 GeVH linac with a 5% duty cycle, a 50 Hz repetition rate and a peak current of 114 mA. The beam current is chopped with a 70% duty cycle [1]. The radio frequency is 280MHz for the two front end RFQs and DTLs. After funneling at 20 MeV final acceleration to 1.334 GeV is accomplished in a nc CCDTL and CCL operating at 560 MHz. A sc version of the high energy linac is also being studied. The 1 msec long linac pulse is injected into 2 compressor rings, to produce a final beam pulse length of 1 sec. Any design of the ESS high energy linac must ensure loss free ring injection. This demands an unfilamented 6d phase space distribution for the linac beam. Table 1, lists 3 high energy, 6% duty cycle, linac design options for a 107 mA, 60% chopped beam using 700 MHz structures. These were the linac parameters from the ESS study [2]. The 700 MHz linac frequency is also the same as considered for the CONCERT [3] multi-user facility. The following conclusions are valid for both 560 and 700 MHz frequencies, but they are limited to linacs with about 6% duty cycle. For all three options, a doublet focusing system with warm quadrupoles is assumed either after 2 nc cavities [2] or after ( 2, 3, 4 ) 6-cell elliptical sc cavities. The accelerating gradient is kept constant at EoT = 2.8 MV/m for the nc cavities resp. Eo = ( 5 MV/m, 8.50 MV/m, 13.7 MV/m Table 1: Options for the high energy part of the ESS 6% d.c., 64 mA pulse current, 700 MHz linac Normal conducting (nc) linac Superconducting (sc) linac Hybrid solution Energy range 105 1334 MeV: CCL 120-1334 MeV: =0.52, 0.65, 0.8 105-407 MeV: CCL >407 MeV: = 0:8, sc Total length 631 m 493 m 148 m (nc) + 267 m (sc) = 415 m # of cavities 232 212 62 (nc) + 116 (sc) = 178 # of klystrons 116 212 31 (nc) + 116 (sc) = 147 Peak RF power per klystron 2 MW 0.4 MW, 0.75 MW 2 MW (nc), 0.75 MW (sc) Total peak RF Power 232 MW 101 MW 137 MW # of circulators NONE 212 116 Cryogenic power NONE 4 MW 3 MW ) for the 3 sc cavities. Two power coupler/ cavity are assumed for the = 0:8 sc cavities. The synchronous phase is kept constant at 25o in the nc cells, whereas only the sc cavity midphase can be kept constant at 25o as a consequence of phase slippage. All sc cavities are assumed to be made out of 6 identical cells. The relative dependence of the transit time factor is the same as for the SNS 805 MHz sc high = 0:76 6-cell cavities [4], obtained from superfish calculations where end field effects are included. The average transit time factor is smaller by at least 10% than the =4 = 0:79 value of = 1 sc elliptical cavites. The average synchronous phase per cavity is smaller than 37o at beginning resp. end of each sc section. It is obvious from table 1, that a pure nc ESS linac version from 105 MeV on is the cheapest in capital cost, but not in operating cost. A sc ESS linac from 120 MeV on requires much less peak RF power, but it is in capital cost quite expensive and substantial R & D is necessary for the 50 Hz pulsed mode behaviour of = 0:52 elliptical sc cavities [5, 6], including the ESS 2.7 msec long pulse option XX International Linac Conference, Monterey, California