It is shown that a secondary electron emission-capacitive probe can determine the plasma potential when Te ≥50 eV. The probe is wideband (1 Hz to greater than 20 MHz) and relatively simple to operate. The Phaedrus-B tandem mirror plasma where Te ∼40–60 eV and n≂5×1012 cm−3 is used to verify this technique.
Electrostatic end plugging is observed in a completely axisymmetric, three cell tandem mirror under conditions where the central-cell plasma density is always larger than the end-cell density. A factor of 4 increase in the central-cell density, to a maximum of 1.2×1013 cm−3 with simultaneous plasma beta of 13%, occurs upon application of the end plugging potential. Ion confining potentials of 25 V and 80 V at the two ends of the device, respectively, result in a factor of 2.5 increase in the axial confinement time for Tic =40 eV in agreement with the collisional flow model for ion confinement. The non-Boltzmann ion confining potential is caused by electron heating in the end cells by rf near the ion-cyclotron frequency. The initial central-cell density rise is caused by an increase in the ionization rate that occurs because of an increase in the electron temperature. The density remains high throughout the end-cell heating pulse as a result of increased particle confinement time. There is no nonambipolar radial ion loss in the core plasma (r≤16 cm) but inward radial transport of ions is observed at a rate consistent with ion–neutral collisions and a radial electric field in the negative radial direction. Steady-state thermal-barrier-like potential dips that are in agreement with the Boltzmann model for potentials are observed in the transitions between the central cell and the end cells.
A new method of plasma potential measurement which makes use of secondary electron emission is presented. A differential secondary emissive probe is employed consisting of two similar single probes, each with surfaces that have different secondary emission coefficients. The differential emission current drives a feedback control circuit (previously used with heated probes) to serve as a sensor of plasma potential. When the secondary emission coefficient of a single probe is greater than unity, the floating potential of that probe is also shown to be a good indication of plasma potential.
High bandwidth capacitive probes are useful tools for potentials measurements in the tokamak scrape off layer. An improved capacitive divider probe design with very high frequency bandwidth is shown. The gain of this system is between 0 and -8 dB for frequencies from 0.1 I-12: to more than 100 MHz, with the -3 dB point at approximately 40 MHz. The probe structure, circuits, performance, and experimental results from the Phaedrus-T tokamak ar,e given.
In this paper we present investigations of two kinds of aperture effects on charged particles in the presence of a magnetic field. The first is an experimental study of the plasma potential profiles in front of an aperture. Plasma potential is measured with emissive probes. The interaction between the plasma and the solid wall of the aperture results in a nonuniform potential profile. Our experiments show that suitable aperture biasing can smooth the electric field in front of the aperture. The second is the transmission of electrons through a finite thickness aperture. The experimental results of the latter are compared to results of a single particle orbit model. It is shown that transmission measurements can be used to estimate the electron perpendicular energy in the magnetic field..
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