High-gradient testing of an S -band, normal-conducting low phase velocity accelerating structure

Abstract

A novel high-gradient accelerating structure with low phase velocity, v=c ¼ 0.38, has been designed, manufactured and high-power tested. The structure was designed and built using the methodology and technology developed for CLIC 100 MV=m high-gradient accelerating structures, which have speed of light phase velocity, but adapts them to a structure for nonrelativistic particles. The parameters of the structure were optimized for the compact proton therapy linac project, and specifically to 76 MeV energy protons, but the type of structure opens more generally the possibility of compact low phase velocity linacs. The structure operates in S-band, is backward traveling wave (BTW) with a phase advance of 150 degrees and has an active length of 19 cm. The main objective for designing and testing this structure was to demonstrate that low velocity particles, in particular protons, can be accelerated with high gradients. In addition, the performance of this structure compared to other type of structures provides insights into the factors that limit high gradient operation. The structure was conditioned successfully to high gradient using the same protocol as for CLIC X-band structures. However, after the high power test, data analysis realized that the structure had been installed backwards, that is, the input power had been fed into what is nominally the output end of the structure. This resulted in higher peak fields at the power feed end and a steeply decreasing field profile along the structure, rather than the intended near constant field and gradient profile. A local accelerating gradient of 81 MV=m near the input end was achieved at a pulse length of 1.2 μs and with a breakdown rate (BDR) of 7.2 × 10 −7 1=pulse=m. The reverse configuration was accidental but the operating with this field condition gave very important insights into high-gradient behaviour and a comprehensive analysis has been carried out. A particular attention was paid to the characterization of the distribution of BD positions along the structure and within a cell.

High Gradient Testing of off-Axis Coupled C-band Cu and CuAg Accelerating Structures

“…High gradient testing of accelerating structures operating at X-band is routinely conducted at SLAC National Accelerator Laboratory demonstrating peak surface electric fields greater than 350 MV/m for 1×10 −4 to 1×10 −2 BDR(1/pulse/meter) for RF pulse length of 85-300 ns [12][13][14]. This is similar with testing of X-band structures at CERN/KEK [22] and S-band work form CERN [23] with both showing a maximum achievable surface electric field of 225 MV/m for a BDR of less than 1×10 −7 (1/pulse/meter) for the 250 ns pulse length at X-band and surface field of 220 MV/m for a BDR of less than 1×10 −6 (1/pulse/meter) for the 1.2 µs pulse length at S-band. Those same X-band (11.424 GHz) experiments showed that the breakdown rate correlates with peak pulse surface heating.…”

High-Power Test of Two Prototype X-Band Accelerating Structures Based on SwissFEL Fabrication Technology

“…Fig. 2: Cross-section of a rendering of a constituent disk showing the channels for the brazing alloy (1) and the integrated cooling channels (2). One half of an RF cell is present on each side of the central portion of disk (shown in the close-up).…”

“…To date, over twenty prototype X-band (12 GHz) accelerating structures have reached unloaded accelerating gradients of 100 MV/m during testing [2][3][4][5] and recently, high-gradient X-band technology has also found use in other applications including free electron lasers (FELs) [6,7], radiation therapy [8,9], and inverse Compton scattering (ICS) sources [10]. Typically, individual precision machined disks are stacked and diffusion bonded to form the multi-cell accelerating sections on the CLIC structures [11].…”

Design Criteria for High-Gradient Radio-Frequency Linacs

“…Advances in understanding limitations on accelerating gradients go beyond linear colliders. Accelerators are used in applications such as inverse Compton scattering gamma ray sources [5,6], compact free-electron lasers (FELs) [34][35][36], and compact medical linacs for hadron therapy [37][38][39][40][41][42].…”