Investigation of Strong Guiding Effects in Helicon Propagation

Abstract

Measurements were made of the phase, attenuation, and coupling constants of circularly polarized m=±1 traveling helicons in n-type InSb cylinders at 54.2 MHz. Values of kHa between 0.25 and 3.0 were achieved by varying the temperature from 81° to 373°K and the magnetic field from 0 to 11 kG. The quantity kH is the infinite-medium phase constant and a is the cylinder radius. Theoretical calculations were made extending earlier work to the values of kHa and ωcτ covered experimentally, where ωc is the cyclotron frequency and τ is the collision time. The results differ from the earlier work, and in particular, it is found that the m=×1 modes do not have the same polarization near the cylinder axis. Transverse magnetoresistance and Hall measurements were made to determine kHa and ωcτ. The agreement between experiment and guided theory was found to be relatively good. Both of the guided helicon modes excited had higher phase velocities and much larger attenuation constants than the values calculated from infinite-medium theory. These results indicate difficulty in achieving the conditions necessary for predicted two-stream helicon instabilities.