Proposal for Research on High-Brightness Cathodes for High-Power Free-Electron Lasers (FEL)
Abstract
It is widely recognized that the development of high-power free-electron lasers will depend critically on advances in optical resonators and electron injectors. In the last few years, there have been major improvements in optical coatings, cooled mirrors and alignment systems. Likewise, electron injectors have seen enormous improvements since the beginning of the FEL program. The so-called "normalized brightness" of an electron beam is a measure of the beam quality, the degree to which high current can be focused into a small spot with good collimation. The normalized brightness of various electron beam sources is summarized in Figure 1.1 [i]. As shown there, the peak current of various sources spans nine orders of magnitude, while the peak brightness spans more than eight orders of magnitude! At the bottom of the graph we find the dc photocathode gun used in the high-power FEL experiments at Jefferson Laboratory [ii]. Higher brightness and current are provided by rf photoelectric injectors, which are currently the injectors of choice for high-power, short-wavelength FELs. High-average-power FELs became a realistic possibility because of the development of the room-temperature rf photoinjector. The key to the high brightness and current of the rf photoelectric injector is the very high field at the cathode in an rf cavity and the fact that the emission from a photocathode can be timed to the optimum phase of the rf cycle. Emittance growth due to the time-varying rf fields has been successfully addressed by "emittance compensation." Nevertheless, as indicated in Figure 1.2 [iii], the most durable photocathodes are the least efficient, and the most efficient photocathode material available at the present time is probably too slow for use in an rf gun. In the last few years very little progress has been made on this problem. Some progress is being made in dispenser photocathodes and diamond amplifiers, but the outcome is uncertain.
Document Details
- Document Type
- Technical Report
- Publication Date
- May 09, 2013
- Accession Number
- ADA580721
Entities
People
- Charles A. Brau
Organizations
- Vanderbilt University