Back-bombardment compensation in microwave thermionic electron guns

Abstract

The development of capable, reliable, and cost-effective compact electron beam sources remains a long-standing objective of the efforts to develop the accelerator systems needed for on-site research and industrial applications ranging from electron beam welding to high performance x-ray and gamma ray light sources for element-resolved microanalysis and national security. The need in these applications for simplicity, reliability, and low cost has emphasized solutions compatible with the use of the long established and commercially available pulsed microwave rf sources and L-, S-or X-band linear accelerators. Thermionic microwave electron guns have proven to be one successful approach to the development of the electron sources for these systems providing high macropulse average current beams with picosecond pulse lengths and good emittance out to macropulse lengths of 4-5 microseconds. But longer macropulse lengths are now needed for use in inverse-Compton x-ray sources and other emerging applications. We describe in this paper our approach to extending the usable macropulse current and pulse length of these guns through the use of thermal diffusion to compensate for the increase in cathode surface temperature due to back-bombardment.

Free-electron laser inverse-Compton interaction x-ray source

“…3 for timing structure of the pulse in this mode). We studied the bunch compression and limitations of the macropulse length due to back-bombardment heating previously [19]. The gun delivers electron bunches at 2856 MHz repetition rates during the macropulse, but due to the maximum macropulse repetition rate of 100 Hz, the average repetition rate is only 1.43 MHz.…”

“…Thus if the macropulse length limitation is removed, a factor of 2 or more increase in stored cavity energy and x-ray flux would be possible. Our technique to counteract back-bombardment (BB), already developed and tested, uses a laser prepulse incident on the cathode surface before the rf macropulse to cool the cathode surface as laser heat diffuses into the bulk [19]. The diffusive cooling counteracts the BB heating, drastically reducing the temperature rise and extending the macropulse length and ultimately increasing the x-ray flux from these sources.…”

Influence of the Cathode Material Properties in Reducing the Back-Bombardment Effect in Thermionic RF Gun

“…Another is to mitigate the BB effect by applying an external magnetic field in front of the cathode and modify the cathode shape [7], [11], [12], [13], [14]. More recently, a compensation system for the temperature change in the cathode using an external laser system has been proposed [15]. However, few papers have looked at the influence of the cathode material's properties in reducing the BB effect [16], [17], [18].…”

Theory and simulation of backbombardment in single-cell thermionic-cathode electron guns

“…This backbombardment process is undesirable and has adverse effects on machine performance, including cathode damage and a runaway condition in which back-bombardment leads to more cathode heating, leading to more emission, which leads to more back-bombardment [4][5][6][7]. While numerous practical techniques exist to mitigate the consequences of back-bombardment [8][9][10][11][12][13], the physical understanding of how gun design features affect the process is limited to simulation results for specific machines [5,6].…”