LPP and PPRC collaborate on pre-ionization experimentsApril 3, 2014
The FF-1 anode, which is plated with 0.001 inches of silver, shows a ring of erosion near the end of the insulator (which has been removed along with the cathode). On the right side, where deposits have been cleaned away, the copper color shows clearly where a ring of silver has been vaporized and measurements show about 0.12 grams eroded in 125 shots. On the left side, not cleaned, the copper is deposited lower on the anode, covering up silver below.
LPP’s research team, having identified impurities vaporized from the electrodes as the main obstacle to higher yields, has been attempting to account theoretically for all sources of vaporization, so as to eliminate them. In January, the team looked more closely at the material eroded from around the anode near the insulator (see photo). Two things seemed surprising: the amount of material—about 1 mg per shot, or half of all the impurities in the plasma; and the fact that the vaporization occurred right at the start of the pulse, when the current flow is the weakest. No possible mechanism seemed to account for so much erosion so fast.
However, a literature search turned up the answer: runaway electrons. Runaway electrons occur when very strong electric fields, such as in lightning bolts, accelerate electrons moving through a mainly neutral gas. If the field is strong enough the electrons gain more energy between each collision with an atom than they lose in the collision, thus speeding up to high energy. In FF-1, electrons gains as much as 3 keV of energy, slamming into the anode and depositing enough heat energy to vaporize the silver plating and some of the copper underneath. Once the plasma is fully ionized and its resistance drops, the high accelerating fields no longer exist, so the runaway electrons stop—But by then, the plasma has already been contaminated.
There are two solutions to this problem. One is simply increasing the initial pressure of the gas, so more collisions occur. This will happen with FF-1 as it approaches peak current. But for now, an additional solution is needed: pre-ionization. In this technique a small current breaks down the plasma resistance before the main pulse passes through—smoothing the way, as it were. The small pulse has too little energy to cause runaway electrons, and by the time the main pulse comes through, the resistance that can sustain the large electric field is gone. Experiments by plasma focus groups in Pakistan and elsewhere had good results with pre-ionization.
To directly test if pre-ionization can eliminate the “ring around the anode” erosion, LPP’s collaborators at the Plasma Physics Research Center (PPRC) in Tehran, Iran are conducting experiments using this technique on their 2-kJ DPF device. At the same time, LPP is doing preliminary tests of pre-ionization techniques on FF-1. Together, the experiments should be able to show how to eliminate this source of erosion prior to FF-1’s next round of experiments with tungsten electrodes.