We describe a simple method to measure the back-bombardment heating temperature rise as a function of time in pulsed microwave thermionic guns using a fast rise-time InGaAs detector and optical pyrometer. Gaining knowledge of the nature of that temperature rise and the corresponding current out of the gun are the first steps in devising a scheme to counteract the back-bombardment heating which lengthens the micropulses, limits the macropulse length, and increases the energy spread of the emitted electron beam. We measured a temperature rise of 59 K in our LaB6 cathode which delivered a peak of 600 mA over a 5 μs RF pulse in our 0.33 MV/cm peak field, 2.856 GHz thermionic electron gun.

The emissivity of the thermionic electron gun cathode material lanthanum hexaboride (LaB 6 ) at an operating temperature of 1622 K has been measured for wavelengths from 550 nm to 2400 nm. The emissivity is calculated from scanning monochromator spectral measurements calibrated with a tungsten lamp and the published emissivity of 0.82 at 1600 K for LaB 6 at 650 nm. The results show a higher emissivity at shorter wavelengths (up to 0.86 at 729 nm) and a lower emissivity in the near-IR to mid-IR (down to 0.41 at 2146 nm). These measurements were motivated by the need to make fast, accurate optical measurements of the temperature of the LaB 6 cathode in our thermionic microwave gun, and to select the wavelength most readily absorbed by the cathode for manipulation of its surface temperature as a means of counteracting back-bombardment heating.

No abstract

Free-electron lasers (FEL) and synchrotron sources of high brilliance x-rays have proven to be of tremendous value in basic and applied research. Inverse-Compton sources (ICS) can achieve brilliance matching the requirements of many applications pioneered at those FEL and synchrotron facilitiesincluding phase contrast imaging, macromolecular x-ray crystallography, and x-ray microscopy-but with size, cost, and complexity compatible with a small laboratory. The free-electron laser inverse-Compton interaction compact x-ray source at the University of Hawaii at Manoa is a unique approach to an ICS which employs an FEL as the laser source. We have measured a total average flux of 3.0 × 10 5 photons=second with an average brilliance of 2.0 × 10 7 photons=s mm 2 mrad 2 0.1% of bandwidth (BW) with a peak energy of 10.9 keV from the source. While these results are modest in comparison to the standards set by other IC sources, upgrades to the system have the potential to increase the total average flux to 9.2 × 10 11 photons=second with an average brilliance of 1.9 × 10 12 photons=s mm 2 mrad 2 0.1% BW: comparing more favorably to other sources. We discuss the scientific program, the progress made in design and development, and the achievements of the source to date. We also outline future upgrades and integration needed to yield an enabling source for emerging high brilliance x-ray applications.

An amplitude and phase compensation system has been developed and tested at the University of Hawai'i for the optimization of the RF drive system to the Mark V Free-Electron Laser. Temporal uniformity of the RF drive is essential to the generation of an electron beam suitable for optimal free-electron laser performance and the operation of an inverse Compton scattering x-ray source. The design of the RF measurement and compensation system is described in detail and the results of RF phase compensation are presented. Performance of the free-electron laser was evaluated by comparing the measured effects of phase compensation with the results of a computer simulation. Finally, preliminary results are presented for the effects of amplitude compensation on the performance of the complete system.