Our high field Penning-Malmberg trap, shown above, is capable of trapping large numbers of singly charged particles in a UHV environment. It was constructed to store, cool, and manipulate positron plasmas. However, our research with test electron plasmas has flourished and has made important contributions to nonneutral plasma science. The device is a Penning-Malmberg trap, just like the buffer gas trap; it confines particles along the magnetic field direction with static electrical potentials, and radially with a magnetic field. It can be shown that infinite confinement times are possible assuming perfect cylindrical symmetry and high-vacuum conditions. The trap uses a superconducting magnet to produce a uniform 5 T field. A closed-cycle refrigerator is used to cool the electrode structure to temperatures as low as 10 K (although this lower limit has not yet been reached). This will eventually allow us to create cold (positron) plasmas with energy spreads ~1 meV for high resolution beam experiments. Another feature of our trap is an arrangement of two sectored (i.e., 4-sector and 8-sector) cylindrical ring electrodes that allow us to apply an electric field to the trapped plasmas that rotates in the plane perpendicular to the magnetic field.

THE EXPERIMENT

fill techniques Currently we are working with test electron plasmas injected with a standard electron gun. We have developed various techniques to optimally trap electrons by manipulating electrode voltages. We plan to soon attach our buffer-gas positron accumulator to the high-field trap and use similar techniques to trap positrons. beam production By manipulating the electrode potentials, we have the ability to produce mono-energetic beams with a wide range of energies. These beams have an energy spread of the order of the temperature of the plasma. The charged particles cyclotron radiate and come into thermal equilibrium with the surrounding electrodes, thereby cooling plasmas to the electrode temperature which will be arranged to be as low as 10 K. This technique can produce cold, energy resolved beams with energy spreads as small as 1 meV. These beams will be attractive for many applications.

limits on plasma density & total particle number
The total number of particles that can be confined is limited by plasma space charge. Ultimately, there is a density limit of particles in a Penning-Malmberg trap, the so-called Brillouin limit, but well below this density. However, plasmas create high enough space charge potential to "squirt out" of the confinement potential. By increasing the confinement voltages we can increase both the plasma density, and total number of particles in the trap. We have recently constructed amplifiers that can reach 1 kV, and have operated at comparable values of plasma space charge. We plan to push this space charge limit to investigate the trap storage properties in this regime of plasma densities and total particle number. A future multi-cell trap, described below, could improve dramatically the limits on total particle number.

rotating wall compression
Using the sectored rings, we apply a rotating quadrupole potential (i.e., one that is stationary in a frame rotating about the magnetic axis). We are able to work in a regime in which plasmas will come to thermal equilibrium in this frame, such that the plasma rotation frequency is very close to that of the quadrupole potential. Since the plasma rotation frequency is directly proportional to the density of the plasma, by applying rotating quadrupole potentials at different frequencies, we can set adjust densities of our plasmas. This rotating potential [a so-called "rotating wall" (RW)] applies a torque to the plasma to counteract the drag torques due to trap asymmetries that cause unwanted, outward plasma transport. This RW technique allows us to conveniently increase and set plasma density, and it also significantly increases plasma confinement times.

off axis manipulation of plasmas
Recently we have developed techniques to excite off-magnetic-axis orbits of our plasmas, by exciting so-called diocotron modes. These modes involve the plasma center rotating in a circular orbit around the axis of the electrode structure. These modes can be excited and damped by the application of a sinusoidal electrical signal to one sector of a RW electrode. The phenomenon of "autoresonance" is exploited to lock the diocotron frequency to that of the applied signal. Due to the nonlinear nature of the diocotron mode, the off-axis distance of the plasma is determined by the frequency. Thus, by setting the frequency, the radial position of the plasma can be controlled. Similarly, the phase of plasma rotation locks to the applied signal, so that the plasma position in the plane perpendicular to the magnetic field can be precisely controlled. The adjacent figure shows plasmas excited to the same radial displacement, D, then dumped at specific phases of rotation 90⁰ apart.

multicell trap for "massive positron storage" Design is currently underway for a new electrode structure that consists of multiple Penning-Malmberg cells. These multiple cells are arranged both in the magnetic field direction and perpendicular to it. This will greatly increase the number of particles that can be trapped for a given confinement voltage. The current design goal is a 95 cell trap (5 cells in the B-field direction and 19 hexagonally close-packed cells in the transverse direction) capable of storing 1012 positrons for weeks. Shown in the figure is a test electrode structurre designed to test features such as off axis confinement, that are critical to the development of a multicell device.

ion mass spectroscopy
There are other possible experiments that could be performed using this trap. The first is investigating the use of positrons to perform ion mass spectrometry. Potentially positrons are able to ionise large molecules without producing small fragments that are common using electron impact techniques. This could provide a valuable tool in mass spectrometry applications, allowing better identification of large molecules. In the high magnetic field environment, heavy ions and ion fragments can be investigated, leading to a better understanding of the potential of this type of instrument.