Capacitive Probe for Plasma Potential Measurements in a Vacuum diode and Polywell Device

Transcription

1 Capacitive Probe for Plasma Potential Measurements in a Vacuum diode and Polywell Device Scott Cornish, Matt Carr, David Gummersall, Joe Khachan Fusion Studies Group

2 Introduction This presentation will focus on: The development of a capacitive probe diagnostic to measure the plasma potential of a non-neutral, predominantly electron plasma with magnetic fields The application of this probe, to test its capabilities, in the well understood case of a planar vacuum diode. The measurement of the potential well formation in a Polywell device. 2

3 Capacitive Probe What makes a capacitive probe? There is no direct contact between the plasma and the probe electrode. The probe electrode capacitively couples to the plasma, which forms the secondary electrode. Capacitive probes can be large and give bulk properties of a plasma or be made small enough to give local information [1],[2]. 3

4 Capacitive Probe How does a capacitive probe work? The outside of the glass is bombarded with high energy electrons this causes secondary electron emission [3]. The glass charges up to the plasma potential. This voltage capacitively couples to the inner electrode. The electrons must have sufficient energy or the probe will not work. Figure(1) Secondary electron emission coefficient for Pyrex glass, for electron energy. [4] 4

5 Capacitive Probe Experimental setup: The probe is attached to a follower circuit which acts as a capacitive divider. Improves frequency response, reduces voltage by 5950 times. The amount the primary to secondary voltage is attenuated by is measured by wrapping the probe with foil [2] and applying a voltage. Figure(1) Shows the Voltage of the foil and the probe 5

6 Planar Diode Experimental setup 6

7 Planar diode Probe Response: Probe signal does not change above 5.36mA Diode current limited to a maximum10ma due to space charge. Similar behaviour observed for diode voltage of 4000V. Probe saturates at 20mA and space charge current is 40mA 7

8 Planar diode Experimental Results 1D Model [5] 8

9 Polywell Experimental setup: The Polywell [6],[7] is made of six aluminium spools that are joined at right angles with brackets. The coils have 25 turns and the spools have an outer diameter of 60mm Four 12V, 50W tungsten filaments are placed at the centre of four of the faces. 9

10 Polywell Capacitive probe measurement in the centre: The Potential Well is deeper for larger Polywell coil currents. The magnetic field gets absolutely deeper but relatively shallower as the electron energy is increased. The electron emission current was 150mA, 370mA, 680mA, and 1680mA 10

11 Polywell Potential Well depth for varying injection current: The potential well depth is strongly related to the injection current Further analysis to determine confinement time indirectly. 11

12 Polywell Radial Plasma Potential measurements The Magnetic field ranged from 0G in the centre to G at 10mm and 850G-3750G at 20mm. It is expected that the potential at 20mm is an underestimate of the true plasma potential. 12

13 Polywell Radial Plasma Potential Measurements The potential well depth is strongly related to the injection current Points at 20mm are expected to be underestimating the Plasma Potential due to a strong magnetic field. 13

14 Polywell Potential well behaviour The potential well depth is strongly related to the injection current Points at 20mm are expected to be underestimating the Plasma Potential due to a strong magnetic field. 14

15 Conclusions A capacitive probe is robust and will work in a non-neutral, non-maxwellian plasma with flows and can tolerate magnetic fields. The Capacitive probe is an ideal diagnostic for the Polywell. The depth of a potential well in a Polywell device is strongly related to the injection current. 15

16 References [1] Spatio-temporal measurements of Trichel corona discharge using capacitive probe diagnostic. Deepak K. Gupta, H. Ramachandran, and P. I. John. Rev. Sci. Instrum. 71, 406 (2000); doi: / [2] Secondary electron emission capacitive probes for plasma potential measurements in plasmas with hot electrons. En Yao Wang, N. Hershkowitz, D. Diebold, T. Intrator, R. Majeski et al. J. Appl. Phys. 61, 4786 (1987); doi: / [3] TOPICAL REVIEW Emissive probes. J P Sheehan and N Hershkowitz. Plasma Sources Sci. Technol. 20 (2011) (22pp) doi: / /20/6/ [4] The Secondary Electron Emission of Pyrex Glass. C. W. Mueller. J. Appl. Phys. 16, 453 (1945); doi: / [5] The Effect of Space Charge and Initial Velocities on the Potential Distribution and Thermionic Current between Parallel Plane Electrodes. Irving Langmuir. Phys. Rev. 21, (1923) [6] Low beta confinement in a Polywell modelled with conventional point cusp theories. Matthew Carr, David Gummersall, Scott Cornish, and Joe Khachan. Phys. Plasmas 18, (2011); doi: / [7] Forming and maintaining a potential well in a quasispherical magnetic trap Nicholas A. Krall, M. Coleman, K. Maffei, J. Lovberg, R. Jacobsen et al. Phys. Plasmas 2, 146 (1995); doi: /