Absorber for wakefield interference management at the entrance of the wiggler of a free electron laser
A method for managing the broad band microwave and TeraHertz (THz) radiation in a free electron laser (FEL) having a wiggler producing power in the electromagnetic spectrum. The method includes placement of broadband microwave and TeraHertz (THz) radiation absorbers on the upstream end of the wiggler. The absorbers dampen the bounced back, broad band microwave and THz radiation returning from the surfaces outside the nose of the cookie-cutter and thus preventing broadening of the electron beam pulse's narrow longitudinal energy distribution. Broadening diminishes the ultimate laser power from the wiggler. The broadband microwave and THz radiation absorbers are placed on either side of the slot in the cookie-cutter that shapes the wake field wave of the electron pulse to the slot shape of the wiggler chamber aperture. The broad band microwave and THz radiation absorber is preferably a non-porous pyrolytic grade of graphite with small grain size.
1. A method for managing reflected wake field energy in the form of broad band microwave and TeraHertz (THz) radiation in a free electron laser (FEL) having a wiggler producing laser light and containing a cookie-cutter for shaping the wake field wave to the wiggler chamber aperture, comprising:
a nose portion on the upstream end of the cookie-cutter, the nose portion including two flat surfaces, a planar face on the cookie-cutter perpendicular to each of the flat surfaces, and a slot in the nose portion in axial alignment with the wiggler chamber aperture; and
covering the planar faces of the cookie-cutter with a broadband microwave and TeraHertz (THz) radiation absorber wherein the absorber is a non-porous pyrolytic grade of graphite with small grain size.
2. The method for managing broadband microwave and TeraHertz (THz) radiation of claim 1 wherein the graphite includes a porosity of between 10 and 20%.
3. The method for managing broadband microwave and TeraHertz (THz) radiation of claim 1 wherein the graphite includes an average grain size of between 5 and 20 μm.
4. The method for managing broad band microwave and TeraHertz (THz) radiation of claim 1 wherein the planar faces of the cookie-cutter are of semi-circular shape and the broad band microwave and TeraHertz (THz) radiation absorbers are in the shape of semi-circular disks.
5. The method for managing the broad band microwave and TeraHertz (THz) radiation of claim 4 wherein the semi-circular disks further comprise:
a flat side; and
an arcuate side.
6. The method for managing the broad band microwave and TeraHertz (THz) radiation of claim 5 wherein the semi-circular disks further comprise:
a planar upstream surface;
a planar downstream surface; and
a plurality of bolt holes therein.
7. The method for managing the broad band microwave and TeraHertz (THz) radiation of claim 4 further comprising:
providing a vacuum chamber boundary and a two-sided bimetallic CF flange having a copper central portion; and
conveying absorbed heat beyond the vacuum chamber boundary by attaching the copper central portion of the two-sided, bimetallic CF flange to the cookie-cutter.
8. The method for managing broad band microwave and TeraHertz (THz) radiation of claim 1 wherein the cookie-cutter is machined from a single piece of oxygen-free high conductivity (OFHC) copper.
9. The method for managing broadband microwave and TeraHertz (THz) radiation of claim 1 further comprising a means of shaping the wake field wave of the electron pulse through the wiggler.
10. The method for managing broad band microwave and TeraHertz (THz) radiation of claim 6 wherein the means of shaping the wake field wave of the electron pulse through the wiggler includes providing the slot in the nose portion with a width that is at least 3 times the height of the slot.
11. The method for managing the broad band microwave and TeraHertz (THz) radiation of claim 1 further comprising:
a planar downstream surface on the THz absorber; and
a sheet of indium foil sandwiched between the planar downstream surface of the THz absorber and the corresponding planar face of the cookie-cutter.