Dense Plasma Focus - Operation

Operation

The charged bank of electrical capacitors (also called a Marx bank or Marx generator) is switched onto the anode. The gas breaks down. A rapidly rising electric current flows across the backwall electrical insulator, axisymmetrically, as depicted by the path (labeled 1) as shown in Fig. 1. The axisymmetric sheath of plasma current lifts off the insulator due to the interaction of the current with its own magnetic field (Lorentz force). The plasma sheath is accelerated axially, to position 2, and then to position 3, ending the axial phase of the device.

The whole process proceeds at many times the speed of sound in the ambient gas. As the current sheath continues to move axially, the portion in contact with the anode slides across the face of the anode, axisymmetrically. When the imploding front of the shock wave coalesces onto the axis, a reflected shock front emanates from the axis until it meets the driving current sheath which then forms the axisymmetric boundary of the pinched, or focused, hot plasma column.

The dense plasma column (akin to the Z-pinch) rapidly pinches and undergoes instabilities and breaks up. The intense electromagnetic and particle bursts, collectively referred to as multi-radiation occur during the dense plasma and breakup phases. These critical phases last typically tens of nanoseconds for a small (kJ, 100 kA) focus to around a microsecond for a large (MJ, several MA) focus.

The whole process, including axial and radial phases, may last, for the Mather DPF, a few microseconds (for a small focus) to 10 microseconds (for a large focus). A Filippov focus has a very short axial phase compared to a Mather focus.