Design, realization, and high power test of high gradient, high repetition rate brazing-free S -band photogun

Abstract

rf photoguns find several types of applications as high brightness electron sources for free-electron lasers, energy recovery linacs, Compton and Thomson sources, and high-energy linear colliders. The high peak current and low transverse emittance of the generated beam are obtained with the combination of a high peak electric field (> 100 MV=m) at the cathode surface, a proper choice of the solenoid field around, or immediately after, the gun, and special fabrication and treatments of the cathode itself. On the other hand, to increase the average electron current, a high repetition rate (> 100 Hz) and/or a multibunch rf gun have to be developed. These types of devices are, in general, fabricated by brazing processes of copper machined parts. The brazing processes require a large vacuum furnace, are very expensive, and pose a not negligible risk of failure. A new fabrication technique for this type of structure has been recently developed and implemented at the Laboratories of Frascati of the National Institute of Nuclear Physics (INFN-LNF, Italy) and already applied to an rf gun now operating at a relatively low cathode peak field and low repetition rate [D. Alesini et al., Phys. Rev. Accel. Beams 18, 092001 (2015)]. It is based on the use of special rf-vacuum gaskets that allow a brazing-free realization process. The S-band gun of the Extreme Light Infrastructure-Nuclear Physics Gamma Beam System, under construction in Magurele (Bucharest, Romania), has been realized with this new technique and represents a further and fundamental step toward the consolidation of this technology for high gradient particle accelerator fabrication. It operates at 100 Hz with a 120 MV=m cathode peak field and 1.5-μs-long rf pulses to house the 32 bunches necessary to reach the target gamma flux. High gradient tests, performed at full power and a full repetition rate, have shown the extremely good performances of the structure in terms of the breakdown rate and conditioning time and definitively demonstrated the reliability and suitability of such fabrication process for high gradient structure realization. In this paper, we report and discuss the electromagnetic and thermomechanical design, the realization process, and all the experimental results at low and high power at a full repetition rate.

Experimental studies of a high-gradient X-band welded hard-copper split accelerating structure

“…In this approach, the following issues require careful examination: vacuum properties, cleanliness, weld shrinkage, fixturing and electron beam control. There are other approaches for heat-free bonding, such as an external vacuum vessel or clamping [29], which are difficult to apply for full length structures.…”

Development of a novel approach for construction of high gradient braze-free S-band cavities

“…This can result in more compact and lower cost facilities, expanding the range of applications of linacs.In recent years, research on high-gradient accelerators has become one of the most exciting topics for accelerator research groups. The different aspects of the research for high gradient cavities includes radio-frequency (RF) design 1-3 , materials 4,5 and construction methods [6][7][8][9] . This report will describe innovations in construction methods supported by a comprehensive design methodology.The most common methods used to join machinable single cells into the multi-cell structures needed for linacs are brazing and bonding, which are carried out in high-temperature furnaces.…”

“…A new fabrication technique based on clamping has been recently developed and used for the construction of a high gradient S-band photocathode RF gun by INFN-LNF, Italy. The RF gun was successfully tested up to a field of 120 MV/m on the cathode surface 7,12 . Another clamping method, made in combination with electron beam welding (EBW) and Tungsten Inert Gas (TIG) welding, has been used at SLAC for the construction of a high gradient X-band single cell cavity 6,11 .…”

“…In recent years, research on high-gradient accelerators has become one of the most exciting topics for accelerator research groups. The different aspects of the research for high gradient cavities includes radio-frequency (RF) design 1-3 , materials 4,5 and construction methods [6][7][8][9] . This report will describe innovations in construction methods supported by a comprehensive design methodology.…”