Electrode Design

High magnetic field strength in the plasma focus is essential to get efficient fusion power generation. In the focus, the smaller the electrodes the higher the magnetic field. Also, a small anode will increase the speed of the current sheet rundown, which seems to enhance the efficiency of energy transfer into the plasmoid.

However, if very small electrodes are to be used, there is the possibility that the heat generated by the electric current could melt part of the electrode, or that stress created by the magnetic fields or by thermal expansion could over-stress the electrode and cause it to mechanically fail. To study these possibilities and to help determine how small the electrode can be, Focus Fusion Society member Doug Olsen has volunteered to carry out a series of computer analysis, which have already begun. Olsen is a senior engineer with Northrop Grumman Corporation.

The heating problem is made more significant because the electric current carried by the electrode is concentrated in a 30-micron thick layer at its surface. This “skin depth” is characteristic of any rapidly changing current, and in the planned focus experiments, the current will vary at about 250 kHz. So, even though copper and other metals have high conductivity, the thin surface area has sufficient resistance to generate considerable heat. In turn this causes a sudden expansion of the outer layers, which can squeeze the rest of the electrode like a tube of toothpaste. In the meantime, the pinch forces, caused by the interaction of the current in the magnetic fields, is also pushing the outer layers inward.

Olsen’s preliminary studies have indicated that the first electrode design, with an anode only 2.5 mm in radius, is almost certainly too small. The studies should soon determine the minimum safe size that can be used in the next set of experiments.