Heating of the relativistic electron plasma in a levitated spherator

Abstract

Noncyclotron resonant microwave heating preferentially heats relativistic electrons in a plasma. In the FM‐1 device, a levitated spherator, plasma may be produced by cyclotron resonant microwave heating at a base pressure of 3 × 10−8 Torr in helium. Under these conditions, plasma lifetimes are of the order of 100 sec. Nonresonant microwave heating at power levels of up to 1.5 kW produced electron temperatures of up to 900 keV with 4 MeV electrons not at all uncommon, as measured by wall bremsstrahlung produced at the limiter. Anomlaous loss mechanisms drop the energy to about 140 keV within 200 msec after the turnoff of the nonresonant heating, and the temperature remains roughly this value for 100 sec. Nonresonant heating at the cyclotron harmonics is as efficient as heating below the fundamental, which is explained by observing that synchrotron emission may be used to compute the nonresonant absorption.