AN EXPERIMENT TO MEASURE COLLISIONLESS RADIAL TRANSPORT OF ENERGETIC ELECTRONS CONFINED BY A DIPOLE MAGNETIC-FIELD

A new laboratory terrella has been constructed in order to study collisionless radial diffusion of particles trapped within a dipole magnetic field. Unlike previous laboratory terrella, Columbia's Collisionless Terrella Experiment (CTX) aims to reproduce the process of wave-induced radial transport and does not try to simulate magnetospheric structure. CTX achieves very low collisionality using a large vacuum tank with fast getter pumping and microwave heating of energetic trapped electrons. The first experiment planned for CTX is the direct measurement of stochastic radial diffusion induced from wave-particle drift resonances. A ring of energetic electrons (i.e., an ''artificial radiation belt'') is formed during electron cyclotron resonance heating after the neutral gas pressure has been increased. When the source of neutral gas is switched off, the density of cooler, background plasma decreases, the fractional density of energetic electrons increases, and a spectrum of hot electron interchange instabilities will be excited causing rapid radial transport. These low-frequency instabilities have frequency near the drift frequency of the hot electrons, omega approximately omega(dh), and induce radial motion while preserving the cylclotron and longitudinal adiabatic invariants, mu and J. Since omega(dh) decreases rapidly with equatorial radius, L-2, a single frequency creates a ''drift island'' but does not induce radial diffusion. Only as the intensity of several frequencies increase will the Chirikov condition for chaos be satisfied initiating collisionless radial transport. This article describes the motivation for the CTX experiment and illustrates the procedures to be used to measure the intensity and spectrum of fluctuations generating chaos, the rate of radial transport, and the evolution of the density and pressure profiles. Because of the success of similar experiments conducted earlier in a long-thin magnetic mirror, these dipole experiments can be performed with a high degree of confidence. An example from these earlier experiments is presented.