A Technical Introduction to Plasma Physics

Plasma Physics Overview

Generally in our research we have a spacecraft immersed in a magnetized, cold plasma with electrons and various species of ions; in this case a cold fluid theory can be used to describe many aspects of the behavior of the plasma. In such a plasma, the electrons spiral about the magnetic field lines, and move along field lines relatively unconstrained. The much more massive and less mobile ions may be viewed as a stationary background of positive charge or may engage in limited motion depending on the time scale of the process under consideration. There are cases in which we also must account for the detailed electron and ion velocity distributions, in where we must employ a kinetic description which acts as a correction to the cold fluid approach and can account for plasma wave-particle interactions and particle acceleration. Such processes occur in a vast variety of space plasmas such as in the high energy, accelerated electron plasma above the aurora, the solar wind plasma streaming past the earth, and various plasma populations found in within the earth's magnetosphere and tail region. In these regions a multitude of plasma waves are found, including Langmuir waves, lower hybrid emissions, ion and electron cyclotron waves, and Alfvin waves. The Berkeley physics group sends instruments out to many of these regions to make plasma and wave measurements (see Figure).

Instrumentation

The Berkeley space physics group employs two types of detectors which make complementary measurements of space plasma processes. Various particle detectors measure the energy spectra of ions and electrons. Of fundamental importance for a meaningful analysis of the data are the velocity distributions of particles relative to the external magnetic field, defining a perpendicular and a parallel velocity. Thus some of our electron and ion energy spectrometers also resolve the angular distribution of the particles with respect to the magnetic field. To measure waves in space plasmas, we construct long stacer booms with spherical probes near the ends; the potential difference between pairs of probes then gives the electric field in a given direction. For both plasma and field measurements we must consider carefully the effects of a non-zero spacecraft potential relative to the plasma, plasma-probe coupling issues, and contamination by sunlight and particles from photo-emission off of spacecraft surfaces to name just a few.

Electrostatic Analyzers

Electrostatic analyzers (ESA's) resolve particle energy spectra and angular distribution simultaneously. A sweeping potential between the two concentric hemispheres allows particles within a specific energy range to enter the detector undeflected. Micro-channel plates (MCP's) are then used to amplify each particle impact into a shower of secondary electrons which are detected by position sensitive anodes, thus resolving the polar angle from which the particle arrived. Typical energy ranges are ~1 eV to 60 keV for electrons and ~1 eV to 20 keV for ions, at ~64-256 ms time resolution (voltage sweep time).

Fast Electron Spectrograph

The FES is a magnetic sector energy spectrometer used for high time resolution measurements of the distribution of magnetic field aligned electrons. The instrument magnetic field curves the trajectories of incoming electrons onto the MCP imaging plane as shown. The integration time for an energy channel can be as low as ~1 ms, making this detector useful for measuring rapid fluctuations and modulations in the precipitating electron fluxes such as those found in the Earth's northern aurora. The energy range is typically 100 eV - 48 keV.

Electric Field Probes

Electric fields in space plasmas are measured using spherical probes mounted on the ends of long "stacer" booms. By measuring the potentials on a pair of spheres separated by the booms yields the electric field along that boom direction. Small pre-amplifiers inside the probes minimize plasma sheath and probe-plasma coupling effects. The electric field booms are deployed as shown in one of our typical auroral rocket vehicles. The potential difference between combinations of spheres 1 through 6 yields the electric field along that axis where E12 is the electric field between probes 1 and 2. Other combinations of sphere potentials yield electric fields both parallel and perpendicular to the auroral magnetic field. These measurements are further processed with an array of onboard signal processing electronics, including wave spectrum analyzers, digital signal processors, a burst memory, and filtering electronics to isolate specific frequency bands between 0 - 5 MHz.