Prior NSF support was used to design and construct a new high performance toroidal trap for non-neutral plasmas (i.e. plasmas with a single sign of charged particle). In the past year, construction of the experiment was completed and early experiments conducted in a portion of the torus produced nearly steady state plasma conditions (several second confinement times). The present award permits the P.I. and his undergraduate students to explore the physics of long-lived toroidal non-neutral plasmas in much greater detail. The non-uniform character of the toroidal magnetic field in this experiment is predicted to ultimately limit the confinement to times on the order of ten seconds. Experiments will be conducted to identify the loss mechanism predicted by this as yet untested theory. Additional experiments will be conducted to identify the effect of the curved and non-uniform toroidal magnetic field on the natural modes of oscillation (i.e. waves) that propagate in non-neutral plasmas. In addition, the present award will fund development of instruments and techniques necessary to transition from (and diagnose) experiments in a portion of the torus, to confinement in the full torus.
Experiments in which non-neutral plasmas are isolated for long times so that they can be observed and manipulated provide excellent "wind tunnels" for close testing of plasma and neutral fluid theories; helping refine our fundamental understanding of naturally occurring fluid systems and plasmas and of laboratory plasmas that find application in society. Non-neutral plasmas can be generated reproducibly and confined for very long times in traps with a simple geometry (i.e. a cylinder). However, until construction of the present device was completed, attempts to trap non-neutral plasmas in more complicated magnetic field arrangements (such as a torus) have resulted in very short trapping times (of order 100 microseconds). No other existing experiment is poised answer fundamental questions about the criteria for stable equilibrium states, the toroidal effects on plasma dynamics, and the limitations on confinement for such systems. The activity funded under this award will provide research experiences (as well as scientific authorship and presentation opportunities) for undergraduate students, inspiring some of them to pursue graduate work in physics. The apparatus will also be used to recruit promising high school students to the Lawrence University physics program and expose the campus and local communities to the subject of plasma physics.
A promising route to fusion power for electricity production is the tokamak — a toroidal (or bagel-shaped) magnetic confinement device that confines hot plasma composed of positively charged ions and negatively charged electrons. Non-neutral plasma, on the other hand, which is composed of relatively cold particles all of one type (either positively or negatively charged), is typically trapped in a straight device that uses a combination of electric and magnetic fields. Such non-neutral systems are used to store antimatter and also serve as candidates for quantum computing elements and next generation precision time standards (clocks), among other uses. The project at Lawrence University, a four-year liberal arts college, might be characterized as lying "between a clock and a hot place," as it uses a toroidal magnetic field (like the tokamak) to confine and study pure electron (i.e., non-neutral) plasma. The apparatus, which was designed and constructed with prior NSF/DOE support, is uniquely positioned to explore untested fundamental aspects of the physics of charged particle collections in a curved and non-uniform magnetic field that closes on itself. The scientific goals of the project were: 1) to characterize (and compare to theoretical predictions) the nature of the steady state for long-lived electron plasma in a toroidal magnetic field, including detailed investigation of the various waves or oscillations that can propagate in the plasma and a study of the rate at which charge escapes from the trap, and 2) to develop the technical ability to load charge into a portion of the toroidal trap (i.e. a C-shaped arc) and then convert the trap to a full torus. Substantial progress was made on both of these fronts. There are two main categories of waves that propagate in non-neutral plasma. The lower frequency waves can be visualized as deformations of the plasma surface so that it looks like the flutes on a column with the flutes lined up with the direction of the magnetic field. The number of flutes can be any integer. Measurement of the frequency of these waves can be used to determine basic properties of the plasma such as the total charge and average density. By monitoring these parameters over time, experiments yield information on how rapidly the plasma expands and is lost and how that rate depends on the magnetic field strength, and the plasma density. The second type of wave is similar to the sound waves in an organ pipe. The plasma particles oscillate back and forth along the direction of the magnetic field (i.e. they are "longitudinal" waves). These higher frequency waves had not been observed in toroidal non-neutral plasma prior to the experiments associated with this project. Further experiments in which these longitudinal waves are observed may yield methods to measure the plasma temperature as well as information about fundamental interactions between the plasma particles and waves. Previous experiments in the device at Lawrence University were conducted in a "partial" or C-shaped trap that used electric potential gates at the ends of the ‘C’. Recently, two techniques were developed to produce fully toroidal plasma in which particles circulate around the entire 360° of the torus. The first technique employs a pneumatic piston that is pressurized using a fast solenoid-activated pneumatic switch to retract the tungsten filament (electron source) after charge has been loaded into the partial torus. After retraction (in 100 ms) the potential gates that confine plasma to a partial torus are relaxed allowing plasma to fill in the excluded region of the torus. This technique was used to measure the rate at which charge is lost in a full torus compared with the partial torus experiments. Charge escapes somewhat faster in the full torus compared to the partial torus. This is thought to result in part from the longer plasma length, but may also be enhanced by the accumulation of positive ions in the trap. The second method producing fully toroidal plasma is to place the filament near the edge of the trap and apply timed negative bias voltage to inject charge and then a positive voltage to suppress injection. This edge injection technique produces plasma that has an initially hollow charge distribution. The project provided research experiences (as well as scientific authorship and presentation opportunities) for undergraduate students. Undergraduate research experiences in this small college environment tend to provide more comprehensive research skills training than comparable experiences at larger institutions, thereby contributing in significant ways to developing the future workforce in technical fields. The apparatus is also used to recruit promising high school students to the Lawrence University physics program and expose the campus and local communities to the subject of plasma physics.
