A pulsed high-current plasma beam under external and self-induced magnetic confinement in a linear device

Abstract

The previously developed governing equations for Magnetic Inertial Confinement Fusion, which combines the advantages of both magnetic and inertial confinement approaches, are improved to analyse a plasma beam in a linear device assisted by an external magnetic field. The equations are applied to simulate a steady state plasma beam sustained by a DC power supply as well as a transient beam generated by a separate pulsed discharge superimposed on the steady-state plasma. The calculated increase of plasma density during the pulse from the steady-state condition is compared with measurements using a laser interferometer at a relatively low voltage supply of 150 V for the pulses. The numerical and test results are found to agree within 20%. When the voltage rises, plasma instability is observed. This issue is inherent due to the use of a solid positive target electrode that blocks the plasma flow in the axial direction. As a remedy, additional tests were carried out using a hollow target electrode in a two-circuit design (to permit free gas flow in the axial direction) by replacing the DC power with transient, pulsed, highvoltage sources for plasma initiation and beam formation. These enhancements were successful in suppressing the instabilities. The peak plasma density was calculated at ∼10 22 m −3 for confinement times of the order of 1 ms. These results lie between the extremes for the current leading approaches yet are achieved for a more compact and inexpensive linear device.

Enhanced density in a stabilized high-current plasma beam

“…Figure 2 shows a typical image. This type of stable image was not observed in the previous experiments described in [23]. This indicates that the additional self-generated magnetic field in the small chamber was successful in stabilizing the plasma beam through magnetic confinement.…”

“…In previous experiments, (e.g. figure 14 of [23]), the plasma beam generated by the second circuit was observed to pass through the hollow target that led to the design of the third circuit. However, the current needed to achieve magnetic compression (or pinching) was still not reached due to instabilities at higher voltages.…”

“…Previous work [20,21] has demonstrated the efficacy of the application of a prior axial B field, in combination with laser heating in a high power Z-pinch device, to produce higher D-D fusion neutron yields. Other recent studies [22,23] were similar in their use of an external magnetic field to stabilize a high-density plasma beam during high-current pulses in a linear device to achieve Z-pinch conditions. However, instead of external laser-heating, high voltage was used to form the plasma.…”

“…where α is a constant to be determined, W m and n m are known thickness and plasma density at a measurement location. On the other hand, plasma density at an arbitrary location may be calculated using the relationship given in equation ( 22) of reference [23], i.e.…”

“…However, instead of external laser-heating, high voltage was used to form the plasma. In the work described in [23], the magnitude of the axial B field (0.3 Tesla) was insufficient to suppress plasma instabilities under high-current conditions. In the present work, this issue is addressed by means of a novel magnetic confinement approach, stabilized by an external magnetic field, described below.…”