Damping Effect Studies for X-band Normal Conducting High Gradient Standing Wave Structures*

The Multi-TeV colliders should have the capability to accelerate low emittance beam with high rf efficiency, X-band normal conducting high gradient accelerating structure is one of the promising candidate. However, the long range transverse wake field which can cause beam emittance dilution is one of the critical issues. We examined effectiveness of dipole mode damping in three kinds of X-band, π-mode standing wave structures at 11.424GHz with no detuning considered. They represent three damping schemes: damping with cylindrical iris slot, damping with choke cavity and damping with waveguide coupler. We try to reduce external Q factor below 20 in the first two dipole bands, which usually have very high (RT/Q)T. The effect of damping on the acceleration mode is also discussed. Contributed to the Particle Accelerator Conference, PAC 09, Vancouver, Canada, May 4-8, 2009 ___________________________________________ *Work supported by the DOE under Contract DE-AC02-76SF00515. # [email protected] DAMPING EFFECT STUDIES FOR X-BAND NORMAL CONDUCTING HIGH GRADIENT STANDING WAVE STRUCTURES* S. Pei, Z. Li, S. G. Tantawi, V. A. Dolgashev, J. Wang, SLAC, CA 94025, U.S.A. Abstract The Multi-TeV colliders should have the capability to accelerate low emittance beam with high rf efficiency, Xband normal conducting high gradient accelerating structure is one of the promising candidate. However, the long range transverse wake field which can cause beam emittance dilution is one of the critical issues. We examined effectiveness of dipole mode damping in three kinds of X-band, π-mode standing wave structures at 11.424GHz with no detuning considered. They represent three damping schemes: damping with cylindrical iris slot, damping with choke cavity and damping with waveguide coupler. We try to reduce external Q factor below 20 in the first two dipole bands, which usually have very high (RT/Q)T. The effect of damping on the acceleration mode is also discussed.The Multi-TeV colliders should have the capability to accelerate low emittance beam with high rf efficiency, Xband normal conducting high gradient accelerating structure is one of the promising candidate. However, the long range transverse wake field which can cause beam emittance dilution is one of the critical issues. We examined effectiveness of dipole mode damping in three kinds of X-band, π-mode standing wave structures at 11.424GHz with no detuning considered. They represent three damping schemes: damping with cylindrical iris slot, damping with choke cavity and damping with waveguide coupler. We try to reduce external Q factor below 20 in the first two dipole bands, which usually have very high (RT/Q)T. The effect of damping on the acceleration mode is also discussed. INTRODUCTION The Multi-TeV colliders should have the capability to accelerate low emittance beam with high rf efficiency, Xband normal conducting high gradient accelerating structure is one of the promising candidate. The long range transverse wake field which can cause beam emittance dilution is one of the critical issues and need to be addressed in the design. The high gradient structures must be efficient in acceleration and effective in damping the high order dipole mode in order to maintain transverse beam stability for multi-bunch operation. We studied dipole mode damping effectiveness with Eigen mode solver Omega-3P in three a/λ=0.14, X-band, π-mode standing wave structures at 11.424GHz with no detuning considered. They represent three damping schemes: damping with cylindrical iris slot which is a new idea, damping with choke mode cavity such as those used in S and C-band choke mode structures [1, 2] and damping with waveguide coupler similar to that used in CLIC structure [3]. We try to achieve external Q factor below 20 in the first two dipole bands, which usually have very high (RT/Q)T. Here RT is the transverse shunt impedance and Q is copper Q-value due to rf losses in copper of the correspondent transverse mode. In this paper, we present comparisons on the effectiveness of dipole mode damping of these structures. We’ll also discuss the effect of the damping geometries on the acceleration mode. IRIS SLOT STRUCTURE For all the damping schemes we consider standing wave structures with π phase advance per cell. The power is fed into every one or three cells. We think this will allow us to reach high working gradients based on the single cell structure test results [4]. Here we describe damping with a slot in the middle of each iris. Figure 1 shows the 4-cell iris slot structure used in our simulation. A slot is located in the center of each iris, which splits each iris into two parts. A dipole mode load made from a cylindrical absorber is located at the outer radius of the slot, in Omega-3P it is set to be pure absorbing boundary condition. 3 mm iris thickness was chosen to incorporate the slot. Figure 1: 4-cell iris slot structure. Effect on the acceleration mode To study the effect on the acceleration mode, especially the surface field, decent meshing was applied to the iris tip. Figure 2 and Table 1 show the results for π-mode in one regular single cell. In the table, R is shunt impedance, Es is surface field, Ea is accelerating field, Q0 is quality factor and k is coupling coefficient. It was found that the slot has no big effect on the acceleration mode properties, but the cell to cell coupling increases 10%. The acceleration mode’s Qext is about 10^7 (<10^8). Figure 2: π-mode electric field distribution. Table 1. Results for π-mode in regular single cell No slot 1mm slot 1.2mm slot Frequency / MHz 11423.87 11423.7