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Study of the formative phase of a low pressure, high voltage Z-pinch

University of British Columbia

The formative phase of a 40 kV Z-pinch has been investigated in the pressure range from 10 mtorr to 80 mtorr in hydrogen. The energy spectrum of the electrons on the axis of the vessel, the spatial distribution of the current at the face of the anode, the total discharge current and the voltage across the discharge have all been monitored. For the pressures examined, the formative phase lasts a few hundred nanoseconds. At the initiation of the discharge, a current of electrons with energies in excess of 20 keV is observed on the axis. As time progresses, the average energy of the electrons decreases to a few keV and the current density increases to about 300 amps/cm². At pressures of 30 and 50 mtorr-H(2), the electron velocity distribution appears to relax to a Maxwellian [formula omitted]. This result is in agreement with the theoretical description of a weakly ionized gas in a strong electric field. However, it should be noted that the influence of plasma-turbulence has been neglected. The current to the central region of the electrode initially carries more than 50% of the total current through the discharge. This observation indicates that ionization initially occurs throughout the entire discharge vessel. However, shortly after the current of energetic electrons reaches its maximum value, the current to the central region of the electrode disappears. Simultaneously, a transient voltage of up to 20 kV appears across the discharge and the rate of increase of the current exhibits a sharp drop in value. The combination of these phenomena has been interpreted as the formation of the boundary layer. The boundary layer or current sheet is not completely formed until the end of the formative phase. Measurements of the pinch time support this conclusion. At low pressures, the time of formation of the boundary layer is considerably shortened by the enhancement of ionization at the wall. This is caused by the presence of the magnetic field of the current flowing in the central region of the vessel. The experiment has led to better comprehension of the mechanism of boundary layer formation in low pressure Z-pinches. The overall understanding of ionization in Z-pinch discharges has been improved and can be extrapolated to higher pressures. In addition, the observations indicate that the detailed nature of the current sheet collapse can be strongly affected by the formative phase.

Text

2011-05-04

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http://hdl.handle.net/2429/34263

Physics

University of British Columbia

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