Experimental Plasma Dynamics
Plasma thrusters create force by expelling a high-velocity stream of hot ionized gas called plasma. Much can be learned about the inner workings of the thruster by examining the dynamics of this plasma in and around the thruster. In the Isp Lab we specialize in measurement techniques designed to uncover the forces and energy flow within dynamic plasmas.
The measurement of plasma properties, such as electron temperature and density, can be obtained by inserting small electrically biased “probes” into the plasma region of interest. The voltage of the probe is varied while the current hitting the probe from the plasma is measured. Using these techniques we can ascertain how well our thrusters are utilizing electromagnetic energy and also how the exhausted plasma might interact with the spacecraft in orbit.
In some instances the physical presence of these probes disrupts the plasma in the area being measured, ultimately changing the properties that are being measured. Laser Thomson scattering (LTS) is a noninvasive measurement technique employed in the Isp Lab which can be used to determine the temperature and density of free electrons in a plasma using only light. We use high-power lasers to shoot a burst of photons of known frequency (color) into the plasma. These photons collide with free electrons in the plasma and are elastically scattered in all directions. We use a sensitive spectrograph to compare the color of the scattered photons to their original color coming out of the laser – small differences can tell us how fast the free electrons were moving and, from this, the plasma temperature.