• N. Sereno, M. Borland, K.C. Harkay, Y.L. Li, R.R. Lindberg, S.J. Pasky
  ANL, Argonne

One proposed injector for the X-ray Free Electron Laser Oscillator* uses a 100 MHz thermionic rf gun to deliver very small emittances at a 1 MHz rate**. Since the required beam rate is only 1 MHz, 99\% of the beam must be dumped. In addition, back-bombardment of the cathode is a significant concern. To address these issues, we propose*** using a laser to quickly heat the surface of a cathode in order to achieve gated thermionic emission in an rf gun. We have investigated this concept experimentally using an existing S-band rf gun with a thermionic cathode. Our experiments confirm that thermal gating is possible and that it shows some agreement with predictions. Operational issues and possible cathode damage are discussed.

*K. J. Kim et al., Phys. Rev. Lett. 100, 244802 (2008)
**P. N. Ostroumov et al., Proc. Linac08, 676-678.
***M. Borland et al., these proceedings.

• M. Borland, B. Brajuskovic, R.R. Lindberg, N. Sereno
  ANL, Argonne

The proposed injector design for the X-ray Free Electron Laser Oscillator* uses a 100 MHz thermionic rf gun in order to obtain beams with very small emittances at high repetition rates**. The required beam rate is only 1 to 10 MHz, so 90 to 99\% of the beam must be dumped. In addition, back-bombardment of the cathode is a significant concern. To address these issues, we propose using a laser to quickly heat the surface of a cathode in order to achieve gated thermionic emission in an rf gun. This may be preferrable to a photocathode in some cases owing to the robustness of thermionic cathodes and the ability to use a relatively simple laser system. We present calculations of this process using analysis and simulation. We also discuss potential pitfalls such as cathode damage.

*K. J. Kim et al., Phys. Rev. Lett. 100, 244802 (2008).
**P. N. Ostroumov et al., Proc. Linac08, 676-678.

• X.W. Dong, M. Borland, G. Decker, K.-J. Kim, J.G. Power, N. Sereno
  ANL, Argonne

The proposed X-ray Free Electron Laser Oscillator* requires an ultra-low emittance gun that generates continuous electron bunches at 1 to 10 MHz. Recently, T. Shintake raised the possibility of using a pulsed triode gun with a thermionic cathode. In this paper, we investigate the feasibility for such a gun as part of an injector producing normalized emittances in the 0.1 μm range with 2 ps rms duration for 50 pC/bunch. We also explore some implementation concepts.

*K. J. Kim et al., Phys. Rev. Lett. 100, 244802 (2008)