POLARIZATION CONTROL OF MICROWAVE EMISSION FROM HIGH-POWER RECTANGULAR CROSS-SECTION GYROTRON DEVICES

Results are summarized of experiments on a gyrotron utilizing a rectan gular-cross-section (RCS) cavity region. The major issue under investi gation is polarization control of microwave emission as a function of magnetic field. The electron beam driver is the Michigan Electron Long Beam Accelerator (MELBA) at parameters: V = 0.8 MV, I-diode = 1-10 kA , I-tube = 0.1-0.5 kA, and t(e)-beam = 0.4-1.0 mu s. The annular e-bea m is spun up into an axis-encircling beam by passing it through a magn etic cusp prior to entering the RCS interaction cavity, Experimental r esults show a high degree of polarization in either of two orthogonal modes as a function of cavity fields. The RCS gyrotron produced peak p owers of 14 MW in one polarization (TE10) and 6 MW in the cross-polari zed mode (TE01). Electronic efficiencies for this device reached as hi gh as 8% with transverse efficiency of 16%. Experimental results on th e beam alpha (alpha = V-perpendicular to/V-//) diagnostics, where alph a is the ratio of the e-beam's transverse velocity to its parallel vel ocity, agree well with the single electron trajectory code. MAGIC code results are in qualitative agreement with microwave measurements. Mic rowave emission shifts from the dominant fundamental mode polarization (TE(10)square), to the next higher order mode polarization (TE(01)squ are) as the solenoid magnetic field is raised from 1.4-1.9 kGauss. Fre quency measurements using heterodyne mixers support mode identificatio n as well as MAGIC code simulations.