Using the plasma noise spectrum to measure the parallel temperature in a nonneutral plasma

A simple, non-destructive diagnostic for the temperature of a non-neutral plasma is desirable as plasma lifetimes increase and the confined species become more exotic. If the confinement system includes an isolated wall sector, the motion of the charges beneath that sector will result in a noise signal that can be directly related to the velocity distribution of the confined particles. Care must be taken to differentiate the resulting plasma signal from the instrumental noise spectrum and the strong signals from plasma oscillation modes. The theoretical basis of the relationship between the noise spectrum and the temperature as well as experimental results are presented. INTRODUCTION A simple, non-destructive parallel temperature diagnostic is desirable as plasma lifetimes increases and the confined species become more exotic. Thus, we have develop a way to use the plasma noise spectrum from a isolated wall sector to measure the parallel temperature of the plasma. FIGURE 1. Schematic of our Malmberg-Penning Trap with ring numbers To measure the plasma noise spectrum of a plasma, we use an isolated wall sector commonly find in a Malmberg-Penning Trap. Fig. 1 illustrates how our MalmbergPenning Trap is configured. In this trap, Rings 1 and 9 are used to confine our electron plasma. This trap operates in the normal fill-prepare-experiment-dump cycle. Typical parameters for this trap are the electron density n ~ 10cw~, confining potential of (])c ~ —150 V, an axial magnetic field of Bz ~ 700 G, an electron thermal energy of kT ~ 1 eV, and an average axial transit time of about ~ 1.5ys. [I] Our noise spectrum diagnostic basically consists of a wall sector in this trap, a charge sensitive pre-amp, and a spectrum analyzer. CP606, Non-Neutral Plasma Physics IV, edited by F. Anderegg et al. © 2002 American Institute of Physics 0-7354-0050-4/027$ 19.00 271 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 128.187.97.22 On: Tue, 01 Apr 2014 20:14:32 NOISE SPECTRUM THEORY The basis of this diagnostic is that the motion of the charges beneath the wall sector induces a noise signal that is directly related to the velocity distribution of the confined particles. To better understand this signal, we developed a simulation to study the plasma noise spectrum. This simulation calculates the induced charge on a wall sector by the particles using the solution for a cylindrical green function. We also assumed in our simulation that these particles were not interacting with each other and that we had hard walls as the confining potentials. To better understand this signal, we began by looking at what one electron does under a wall sector. As an electron passes underneath the wall sector, it induces a charge in that sector, as a function of time. This signal is measured as a pulse in our detector. The amplitude of the pulse is related to the charge that is induced on the wall sector by the electron. This amplitude does not change with velocity. In contrast, the width of this pulse is a function of velocity of the electron under the wall sector. As the velocity of the electron under the sector increases, the width of the pulse decreases. By Fourier transforming this signal, we find that the frequency spectrum is also a function of velocity of the electron. As the velocity of the electron increases, it lowers the transit time of the electron under the wall sector, which translates to higher frequencies in the corresponding spectrum. For a collection of electrons in our plasma, the revelant velocity is the thermal velocity of the electrons, which is related to the temperature of the plasma in the direction parallel to the confining magnetic field.