Continuous Wave Thermionic Copper RF Gun for Compact FELs

Compact free electron lasers (FELs) based on superconducting energy recovery linacs are poised to open a new market of tunable high power lasers. The primary impediment to realizing these FELs is an affordable electron source that operates continuous wave (CW) with moderate average current, moderate beam brightness and moderate peak current, and does not contaminate the superconducting linac. This SBIR will develop an electron source based on a thermionic cathode that is immersed in a copper radio frequency (RF) structure that accelerates and bunches the electron beam. The copper accelerating structure will operate in CW mode at 300-700 MHz and generate a 50-100 keV electron beam to match into the superconducting linac. Average beam currents of about 10 mA with peak current in the bunches at the undulator of about 5 A are required. The thermionic cathode will be a standard barium dispenser with average current densities up to 10 A/cm2, or a cerium/lanthanum hexaboride crystal for improved cleanliness, ultrahigh vacuum and higher current densities up to 100 A/cm2. The beam brightness from a thermionic copper RF gun would be adequate for terahertz and infrared FELs. High electric fields at the cathode with two or more harmonic frequencies will produce a low emittance beam in both the longitudinal and transverse direction with minimal electron back bombardment. A collimator and low energy buncher chicane will prepare the beam for injection into the superconducting energy recovery linac. The bunch length can be further compressed after the 5-10 MeV superconducting linac