Dense Plasma Focus Mini-F


This page was begun on 15 January, 2012 and will be updated as the project progresses. Please use the Forum to ask questions and to provide feedback.


Mini-F is small dense plasma focus device (3 kiloJoules) that is built around a 59 uF, 10 kV capacitor with a 35 nH internal inductance. The spark gap, chamber and electrode assembly are mounted directly on the capacitor to keep the overall inductance to a low level.

I began the project a bit over a decade ago and it progressed to the initial testing stage but with no adequate diagnostics. It was dismantled in preparation for our move to Maine and is just now being reassembled. The design is being enhanced to permit better diagnostic access.

An article on the Mini-F was published in Volume 7, Number 3/4 of the Bell Jar. That article is available here.

An essential resource to understanding plasma focus devices is the Institute for Plasma Focus Studies. This contains information on the world-wide plasma focus network, sponsored by the UN University - International Centre for Theoretical Physics (UN/ICTP). This page contains links to numerous papers as well as information on the numerical modeling tool developed by Prof. Sing Lee. This tool is in the form of an Excel spreadsheet with embedded macros.

Design Considerations

Mini-F has some simplifications as compared with some other 3 kJ class devices. Specifically, other devices use one or more capacitors that are cabled to the chamber with swinging cascade spark gaps. In my device, everything is mounted directly to the capacitor. Before progressing much further I requested a critique from Prof. Lee on my approach. He promptly responded with some suggestions along with a run of the model with my parameters. Input parameters and some outputs are listed below:

  • Capacitor: 59 uF, 10 kV, 35 nH internal inductance
  • R0: 4 mOhm (estimated)
  • L0: 90 nH (estimated)
  • Inner electrode radius: 1 cm
  • Outer electrode radius: 3 cm
  • Inner electrode length: 18 cm (in order to reach a peak speed of ~9cm/us at ~time of peak current which takes ~3.6us for the bank parameters)
  • Peak Current: 185 kA
  • Neutron output at 4 Torr deuterium: >1 x 107 neutrons/pulse

A copy of the preliminary Mini-F model is available here. Please note that it is a 5.5 MB download and it requries that you have Excel on your computer. When prompted, enable the macros in the spreadsheet. I have locked the cells on this copy. This run is based on deuterium as the working gas. If you want to tinker with parameters, download the Lee model from the Plasma Focus website. The model also contains parameter for several existing plasma focus devices.

General System Configuration

Vacuum System: 2-stage rotary vane pump, 20 mTorr base pressure

Gauging: 10 Torr (full scale) MKS type 722 capacitance diaphragm gauge

Pressure Control: Static with gas fed from cylinder via 2-stage regulator, orifice and valve.

Operating Pressure Range: 0.5 Torr to 5 Torr, depending upon gas.

Diagnostics Summary

I plan to follow the approached used by the UNU/ICTP-PF program. Details for most of the diagnostics are contained in the Laboratory Manual Experiments with the ICTP-UM 3.3 kJ Plasma Fusion Facility.

  • Current Measurement: Rogowski coil in the spark gap circuit.
  • Soft X-ray Output: Filtered PIN diode(s) using the BPX65 opto device.
  • Neutron Output: Indium foil activation with scintillator/PMT assembly.
  • Ion Beam: Thompson parabola spectrometer with CR39 track film.

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