Wilson, P.B.

Paper	Title	Page
MOP72	RF Breakdown in Accelerator Structures: From Plasma Spots to Surface Melting	189
	P.B. Wilson SLAC, Menlo Park, California
	Plasma spots are known to form at field emission sites in regions of high dc or rf electric field. Several mechanisms for the formation of plasma spots in an rf field have been proposed, and one such mechanism which fits experimental data is presented in this paper. However, a plasma spot by itself does not produce breakdown. A single plasma spot, with a lifetime on the order of 30 ns, extracts only a negligible amount of energy from the rf field. The evidence for its existence is a small crater, on the order of 10 microns in diameter, left behind on the surface. In this paper we present a model in which plasma spots act as a trigger to produce surface melting on a macroscopic scale (~0.1 mm2). Once surface melting occurs, a plasma that is capable of emitting several kiloamperes of electrons can form over the molten region. A key observation that must be explained by any theory of breakdown is that the probability of breakdown is independent of time within the rf pulse–breakdown is just as likely to occur at the beginning of the pulse as toward the end. In the model presented here, the conditions for breakdown develop over many pulses until a critical threshold for breakdown is reached.
	Transparencies
THP33	Progress toward NLC/GLC Prototype Accelerator Structures	675
	J. Wang, G. Bowden, V.A. Dolgashev, R.M. Jones, J. Lewandowski, C.D. Nantista, S.G. Tantawi SLAC/ARDA, Menlo Park, California C. Adolphsen, D.L. Burke, J.Q. Chan, J. Cornuelle, S. Döbert SLAC/NLC, Menlo Park, California T. Arkan, C. Boffo, H. Carter, N. Khabiboulline FNAL, Batavia, Illinois N. Baboi DESY, Hamburg D. Finley, I. Gonin, S. Mishra, G. Romanov, N. Solyak Fermilab, Batavia, Illinois Y. Higashi, T. Higo, T. Kumi, Y. Morozumi, N. Toge, K. Ueno KEK, Ibaraki Z. Li, R. Miller, C. Pearson, R.D. Ruth, P.B. Wilson, L. Xiao SLAC, Menlo Park, California
	The accelerator structure groups for NLC (Next Linear Collider) and GLC (Global Linear Colliders) have successfully collaborated on the research and development of a major series of advanced accelerator structures based on room-temperature technology at X-band frequency. The progress in design, simulation, microwave measurement and high gradient tests are summarized in this paper. The recent effort in design and fabrication of the accelerator structure prototype for the main linac is presented in detail including HOM (High Order Mode) suppression and couplers, fundamental mode couplers, optimized accelerator cavities as well as plans for future structures. We emphasize techniques to reduce the field on the surface of the copper structures (in order to achieve high accelerating gradients), limit the dipole wakefields (to relax alignment tolerance and prevent a beam break up instability) and improve shunt impedance (to reduce the RF power required).