W.J. Brown

Improved photonic bandgap cavity and metal rod lattices for microwave and millimeter wave applications

We report an experimental and theoretical study of a photonic bandgap (PBG) cavity with improved coupling of the TE/sub 10/ rectangular waveguide mode into the cavity. The 17 GHz PBG cavity is built with a triangular array of metal rods with a defect (missing rod) in the center. The TM/sub 010/-like defect mode is the operating mode for this cavity. In the experiment, critical coupling was achieved by removal or by partial withdrawal of some rods, a result that was verified by simulations. We also report simulation results of PBG structures in metal rod lattices useful for vacuum microwave electron devices. The bandgaps for the fundamental and higher-frequency oscillations in the lattices are determined. These results show that PBG cavities are very promising for applications in active and passive devices at microwave and millimeter wave frequencies.

High power operation of a 17 GHz photocathode RF gun

We report the first operation of a 17 GHz RF photocathode electron gun. This is the first photocathode electron gun to operate at a frequency above 2.856 GHz. Such electron guns have the potential for achieving record high values of electron beam quality. The 112 cell, π-mode, copper cavity was tested with 5–10 MW, 100 ns, 17.145 GHz pulses from a 24 MW Haimson Research Corp. klystron amplifier. Klystron power is stable to within ±5% up to 15 MW. Conditioning resulted in a maximum surface field of 250 MV/m, corresponding to an on-axis gradient of 150 MV/m. Dark current of 0.5 mA was observed at 175 MV/m, consistent with Fowler-Nordheim field emission theory if a field enhancement factor of about 100 is assumed. Electron bunches were generated by a regenerative laser amplifier that produces 2 ps, 1.9 mJ pulses at 800 nm with ±10% energy stability. These pulses were frequency tripled to 46 μJ of UV, and then focused on the wall of the cavity. Preliminary beam measurements indicate 0.12 nC bunches were produ...

Photonic bandgap structure based accelerating cell

We present detailed calculations of photonic bandgap (PBG) accelerating cavities including estimation of the important effects of an input coupler. The PBG structures consist of a triangular lattice of metal rods containing a single defect on axis. The operating frequency is selected to be 17.1 GHz. The accelerating mode is a quasi-TM/sub 010/ mode localized near the defect. We analyzed the excitation of the cell using a rectangular waveguide with aperture coupling. Both the design of the PBG cell and the design of the input coupling were varied in order to optimize the design. Results are compared with a conventional TM/sub 010/ cavity design. Ohmic loss is found to be comparable for the different cavity designs. The results indicate that the PBG cavity can have the following advantages relative to a conventional cavity: rarified spectrum of modes; oversized dimensions; and simplified input coupling with no frequency shift.

Experimental results of the MIT 17 GHz RF gun

We report on experimental results of a 17 GHz RF photocathode electron gun. This is the first photocathode electron gun to operate at a frequency above 2.856 GHz. The 1.5 cell, π-mode, copper cavity was tested with 50 ns pulses from a 17.150 GHz klystron amplifier built by Haimson Research Corp. A Bragg filter was used at the RF gun to reduce the reflection of parasitic modes back into the klystron. Coupling hole theory in conjunction with cold test measurements was used to determine the field profile in the RF gun. The particle in cell code MAGIC was used to simulate the beam dynamics in the RF gun. With power levels of 4 MW, the on axis electric field at the cathode exceeds 300 MV/m, corresponding to an average accelerating gradient of 200 MV/m over the first half cell of the gun. Breakdown was observed at power levels above 5 MW. Electron bunches were produced by 20 μJ, 1 ps UV laser pulses impinging on the RF gun copper photocathode and were measured with a Faraday cup to have up to 0.1 nC of charge. ...

Design, analysis and cold test of a 17 GHz RF Gun

We analyzed and cold tested a 17 GHz 1-1/2-cell RF gun cavity excited through two coupling holes in the broad wall of a rectangular waveguide. An equivalent circuit theory and an advanced field theory were developed to describe the excitation of an 1-1/2-cell RF gun cavity. SUPERFISH was used to calculate the majority of the equivalent circuit elements as well as the field theory parameters. The matching values of magnetic polarizabilities of the coupling holes were determined by comparing the theory with the measurements. The field theory results were used to model the electric field distribution and accelerating gradient in the RF gun cavity. From this analysis we concluded that the RF gun cavity support gradients as high as 300 MV/m at the cathode without breakdown.

Production of high brightness electron beams with a 17 GHz RF gun

We report on beam measurement results from the MIT 17 GHz RF gun experiment. Tests of a 1.5 cell RF gun have been completed, with bunch charges up to 0.1 nC and beam energies up to 1 MeV produced. The normalized rms emittance of the beam after 35 cm of transport from the gun has been measured to be about 3 /spl pi/mm mrad for a 50 pC bunch. This agrees with PARMELA simulations at these beam energies, and can be shown to correspond to a beam brightness of about 80 A/(/spl pi/mm mrad)/sup 2/ at the gun exit. Results of a 2.4 cell 17 GHz RF gun are also reported. This gun has been built and cold tested and high power RF conditioning has begun. The gun is designed to produce a 2 MeV beam with peak accelerating gradients of 200 MV/m. After emittance compensation, simulations predict a 0.1 nC bunch with a normalized emittance of 0.5 /spl pi/mm mrad can be produced, corresponding to a beam brightness of 800 A/(/spl pi/mm mrad)/sup 2/.