Field Asymmetry and Thrust Control in the GDM Fusion Propulsion System
[Abstract] One of the notable attributes of the Gasdynamic Mirror (GDM) fusion propulsion system is the fact that it lends itself readily to thrust control due to asymmetry in the magnetic field configuration. GDM is a magnetic plasma confinement device that has been proposed as a potential propulsion system. It differs from the standard “collisionless” mirror in that it operates at a much higher plasma density and is generally designed with a large aspect ratio to provide plasma stability. When a plasma is injected into such a device, the electrons tend to escape through the mirrors more readily due to their small mass, leaving behind an excess of ions and correspondingly a positive electrostatic potential. Such a potential accelerates the ions while slowing down the electrons until both species drift outward at equal rate. Of special importance to its value as a propulsion device is the effect of magnetic field asymmetry on such an electrostatic potential. Numerical simulations have been carried out to study this phenomenon and to quantify the role of asymmetry on the overall performance of the system. Experiments will be conducted in order to validate the theoretical and simulation models and to provide a basis for further assessment of GDM as a propulsion device with variable thrust capability.
Propulsive Capability of an Asymmetric GDM Propulsion System
*† The role of the ambipolar potential in the propulsive capability of the Gasdynamic Mirror (GDM) has been studied previously. The electrostatic potential arises as a result of the initial rapid escape of the electrons due to their small mass, leaving behind an excess of ions and creating a positive electric potential that slows down the electron escape while speeding up the ions until their respective axial diffusion are equalized. As a result, the ion confinement time is reduced while the ion escape energy is increased by an amount equal to the potential, translating into increased thrust and specific impulse. Previous studies, however, have only considered a GDM configuration with equal mirror ratios at both ends such that on average half of the ions escape through either end of the GDM. For a thruster it is desirable to bias the direction of ion escape to the thrusting end of the GDM. This can be achieved by properly controlling the mirror ratio at both ends such that on average a specified fraction of ions escape through the thrusting end. Due to this asymmetry, the magnitude of the ambipolar potential is changed, and as a result, the ion confinement time and escape energy are affected resulting in different propulsive capabilities compared to the case of a symmetric system.
Measurement and Demonstration of Thermal Expansion Coefficient
There are many classroom demonstrations, such as the ball-and-ring experiment, for showing the existence of thermal expansion of solids. The coefficient of linear expansion α (= increase in length divided by the original length times the change in temperature) may be measured in high school laboratories using a number of methods.1–4 In this paper we describe an especially simple experiment for determining α for a metal rod. It is based on the well-known optical lever method and is suitable for either a laboratory experiment or a classroom exercise.