Pulsed, High Power Microwave Processing of Field Emission in Superconducting Cavities
The phenomenon of field emission is very well known: electrons are extracted from within the solid state potential well of a metal and are emitted from the metal's surface under the presence of an accelerating potential. In many accelerators, electromagnetic energy is delivered to charged particles by means of microwave cavities excited in modes with electric field components aligned along the particles trajectory. If the mode used is of the TM type (most accelerators operate in the TM{sub 010} mode), then a surface electric field inside the cavities exists which can produce field emitted electrons when allowed by the phase of the fields. These field emitted currents can cause considerable current loading and bremsstrahlung radiation in normal conducting cavities (mostly copper), but in superconducting cavities they have the additional effect of locally heating the superconducting material above its transition temperature and causing performance degradation of the cavities and eventually quenches (transition to the normal conducting state). At present this phenomenon constitutes the limiting factor in superconducting cavity performance, and is receiving a great deal of attention. Several diagnostic methods have been developed to detect, locate and characterize the sources of field-emitted electrons. Methods have also been proposed and tested which decrease the incidence of field emission sites on metal surfaces, but the most effective method to date requires high temperature firing of the superconducting structures in an ultra high vacuum. This can be done only if the cavities are completely removed from their cryostat, a lengthy and costly process. In this paper the properties and advantages are examined of a different method for field emission processing, which does not require a cavity disassembly and which can be performed in situ. The method described makes use of short, high peak power RF pulses to reach high electric fields for a short time. At the same field levels, CW operation would lead to breakdown; during the short burst, field emission sites are destroyed by the intense current itself, leaving behind small craters or defects at the cavity surface. CW operation at field levels below the maximum pulse field can then be maintained with reduced field emission current and with enhanced performance of the accelerator.
- Research Organization:
- Thomas Jefferson National Accelerator Facility (TJNAF), Newport News, VA (United States)
- Sponsoring Organization:
- USDOE - Office of Energy Research (ER)
- DOE Contract Number:
- AC05-84ER40150
- OSTI ID:
- 882268
- Report Number(s):
- CEBAF-PR-92-60; DOE/ER/40150-3874; TRN: US200721%%776
- Resource Relation:
- Conference: 27th Microwave Power Symposium, August 3-5, 1992, Washington D.C.
- Country of Publication:
- United States
- Language:
- English
Similar Records
R&D for a Soft X-Ray Free Electron Laser Facility
Room temperature continuous-wave operation of GaInNAs long wavelength VCSELs