G. Bekefi

Rippled-Field Magnetron

The rippled‐field magnetron is a novel crossed‐field millimeter wave source in which electrons move in nearly circular paths under the combined action of a radial electric field, an axial magnetic field, and an azimuthally periodic wiggler magnetic field, Bw cos (Nϑ) oriented transversely to the flow. Estimates are given of the frequency and growth rate of the free‐electron laser (FEL) type of instability excited in this smooth‐bore magnetron configuration.

Millimeter wave emission from a rotating electron ring in a rippled magnetic field

We report measurements of millimeter wave emission from a rotating relativistic electron ring (2 MV, 1 kA) in which electrons move in quasi‐circular orbits under the combined action of a uniform axial magnetic field and an azimuthally periodic wiggler magnetic field. We observe radiation at frequencies above 91 GHz, at power levels in excess of 200 kW.

Coherent radiation from a relativistic electron beam in a longitudinal, periodic magnetic field

The longitudinal wiggler, beam interaction is a novel source of submillimeter wave radiation. It comprises a relativistic electron beam gyrating in a longitudinally rippled, periodic magnetic field of the form B = ẑ[B0+B1 sin(k0 z)], where k0 = 2π/l and l is the period. Axial and transverse electron density bunching gives rise to a convectively unstable wave that propagates along the guiding magnetic field ẑB0. Its growth rate has been computed for the case of a cold beam.

EFFECTS OF CONTACTS ON THE EMISSION FROM INDIUM ANTIMONIDE

The low‐field microwave emission from InSb subjected to parallel electric and magnetic fields is strongly dependent on the type of electrical contact attached to the sample. It is suggested that the radiation is a localized phenomenon due to high‐field regions or due to a Schottky barrier.

CYCLOTRON RADIATION OF AN Hg PLASMA GENERATED BY AN ELECTRON BEAM

The microwave radiation from a plasma generated by an electron beam fired into an unionized low-pressure mercury vapor in a uniform magnetic field was measured. The emission is shown to depend on collective motions of the particles. A microwave radiometric method was used. (R.E.U.)

A FREE ELECTRON LASER PUMP PRODUCED BY MAGNETIC DIFFUSION

Magnetic diffusion through a spatially periodic assembly of metal conductors yields a large‐amplitude pump for use in free‐electron lasers. Experiments and computations which test the performance of the pump are presented.

ANOMALOUS DIFFUSION AND RESISTIVITY OF A TURBULENT WEAKLY IONIZED PLASMA.

The plasma loss rate and certain transport coefficients of a dc discharge in the presence of turbulent argon‐gas flow are found to vary strongly with Reynolds number (Re). The loss rate increases by a factor of 15 as Re is increased from zero to 6300.

Permanent magnet helical wiggler for free‐electron laser and cyclotron maser applications

A permanent magnet, ‘‘bifilar’’ helical wiggler for use in free‐electron lasers and cyclotron masers has been designed, assembled, and measured. It is composed of a cylindrical array of staggered samarium‐cobalt bar magnets, transversely magnetized and held in place in an axially grooved hollow metal cylinder. High‐quality helically polarized fields of several kilogauss can be readily achieved.

Energy distribution in relativistic electron beams from field‐emission diodes

Foil and foil‐less field emission diodes (current ∼10 kA, voltage ∼1.5 MV) used in free‐electron Raman lasers are studied experimentally with the view of reducing the perpendicular energy of the emitted beam. Microwave emission, attributed to a cyclotron instability in the beam, is used as the diagnostic. Removal of the outer portions of the beam leads to a significant reduction in the radiated intensity, indicating an overall improvement in beam quality.