Resistance measurements show way to higher currentsJuly 24, 2012
Resistance measurements show way to higher peak current in FF-1
Peak current is one of the key parameters in getting to higher yield, as for every doubling of current we expect roughly a 30-fold increase in fusion energy. Studies by LPP electrical engineer Fred Van Roessel have indicated that in the recent shots in May, where we experienced arcing problems, the resistance of the device was much higher than usual. If we confirm that this resistance is in fact due to arcing, then our steps to eliminate arcing can boost total current delivered over 2 MA—close to our design goal– even with the switches we are now using.
The resistance—a measure of how much of the current is dissipated as heat—reduces the current delivered to the plasma. It can be measured by looking at the wave of current that occurs when there is no pinch. (The pinch and subsequent formation of the plasmoid greatly complicate matters by consuming a lot of energy.) Van Roessel fitted theoretical curves (see figure 1) to the measured ones. The faster the current wave decreases, the more the resistance. He found that in the May shots, resistance was as high as 17-20 mOhms, while in earlier shots, with less arcing, resistance was only 5-6 mOhms. In theory, the metal parts of FF-1 that carry the current, the switches and capacitors should have no more than 2 mOhms resistance.
While these resistances may seem small, at FF-1’s very high current, they are very significant. At 1 MA, a resistance of 20 mOhms creates a voltage drop of 20 kV, half the entire charging voltage of the capacitors.
If, as seems likely, the change in resistance is due to arcing, we believe that the total elimination of arcing, which we are working on now, may drop the resistance to as low as 2 mOhms. In that case, simulation programs developed by Dr. Sing Lee predict that with full power, shorter electrodes, and our existing switches, FF-1 will produce over 2.3 MA, over twice our current output. New, faster switches now under design for LPP by Raytheon will get us the rest of the way to our goal of 2.8 MA.
Figure 1: FF-1’s resistance calculated by matching calculated and observed curves.