Pinch punctuality sparks international projectNovember 5, 2013
Pinch timing measures impurities and sparks international project
Recent research at LPP has shown the great importance of eliminating impurities in the plasma in order to obtain higher densities and fusion yields. These impurities, created by vaporization of the electrode materials, add enough mass to the sheath of plasma that carries the current that it slows its travel down the electrodes and delays the time of the pinch. This pinch is where the current merges together to form the plasmoid. Recent calculations by LPP Chief Scientist Eric Lerner showed that this delay in pinch time, which is very easy to measure, could provide a simple estimate of the amount of impurities. Simply put, the more impurities present, the longer the time to the pinch for a given current and fill-gas pressure.
A computer simulation of the run-down time using the model of Dr. Sing Lee, one of the pioneers of plasma focus research, is a good way to predict what the time to the pinch will be for a given mass of plasma. By adjusting the “fill pressure” in the model so that the predicted run-down time matches the observed time, we can determine the total mass in the plasma. If we then subtract the actual fill pressure of deuterium from the pressure in the model, we get a measure of the mass of impurities.
For FF-1, this exercise indicates impurity mass that is 50-70% of the mass of the deuterium, comparable with estimates made from spectroscopy and other methods. Significantly, the difference between predicted and observed time-to-pinch started to occur back in March 2010, when the first evidence of the “early beam” phenomenon showed up. This early beam was later identified as the signal that part of the current was not merging into the plasmoid, greatly reducing density and fusion yield. In fact the “short pulses”, which we now understand were those without large impurities, had ten times the fusion yields of the “long pulses” which did have impurities.
What caused the impurities to suddenly turn up? At that time, improvements in switch functioning were increasing FF-1’s current. When current density (current per unit area) passed a critical threshold, around 2 MA/cm2, electrode evaporation started to occur, releasing the impurities.
These measurements are so easy to do and to analyze that they can be done on any plasma focus device. Last month, Lerner circulated the proposal that other groups duplicate these measurements to the members of the International Center for Dense Magnetized Plasmas, an international group of plasma focus researchers. Initial reaction was very favorable, and new results should be emerging in the coming months. It will be very interesting to see if there is the same critical current density separating the machines with impurities from those without and if there is the same large difference in fusion yield.