Döbert, S.

Paper	Title	Page
WE101	Gradient Limitations for High-Frequency Accelerators	513
	S. Döbert SLAC, Stanford
	While the physics of gradient limitations in high frequency rf accelerators still lacks a full theoretical understanding, a fairly complete empirical picture has emerged from the experimental work done in the past few years to characterize this phenomenon.Experimental results obtained mostly in the framework of the NLC/GLC project at 11 GHz and from the CLIC study at 30 GHz will be used to illustrate the important trends.The dependence of achievable gradient on pulse length, operating frequency and fabrication materials will be described. Also, the performance results most relevant to linear colliders will be presented in some detail. Specifically, these relate to the requirements that the structures sustain a certain gradient without incurring damaged, and that more importantly, they run reliably at this gradient, with breakdown rates less one in a million pulses. Finally interesting observations concerning the dynamics of breakdowns like spatial and temporal correlations and dark currents will be covered briefly, including the insights they provide into the breakdown mechanism.
	Transparencies
THP34	A High-Power Test of an X-Band Molybdenum-Iris Structure	678
	W. Wuensch, A. Grudiev, T. Heikkinen, I. Syratchev, T. Taborelli, I. Wilson CERN, Geneva C. Adolphsen SLAC/NLC, Menlo Park, California S. Döbert SLAC, Stanford
	In order to achieve accelerating gradients above 150 MV/m, alternative materials to copper are being investigated by the CLIC study. The potential of refractory metals has already been demonstrated in tests in which a tungsten-iris and a molybdenum-iris structure reached 150 and 193 MV/m respectively (30 GHz and a pulse length of 15 ns). In order to extend the investigation to the pulse lengths required for a linear collider, a molybdenum-iris structure scaled to X-band was tested at the NLCTA. The structure conditioned to only 65 MV/m (100 ns pulse length) in the available testing time and much more slowly than is typical of a copper structure. However the structure showed no sign of saturation and a microscopic inspection of the rf surfaces corroborated that the structure was still at an early stage of conditioning. The X-band and 30 GHz results are compared and what has been learned about material quality, surface preparation and conditioning strategy is discussed.
	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).