November 17, 2014
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Physics of Plasmas, the leading journal in the field of plasma physics, has published LPPFusion’s new paper on Runaway electrons as a source of impurity and reduced fusion yield in the dense plasma focus. The paper, by Chief Scientist Eric J. Lerner and Chief Research Officer Hamid R. Yousefi, was published online October 22, 2014 less than a month after it was submitted for peer-review. Physics of Plasmas had published a previous LPPFusion paper on record-breaking ion energies in 2012.
The new paper describes the evidence that runaway electrons are a key cause of vaporization of electrodes in the dense plasma focus device, an idea first reported on LPPFusion’s website in April of this year. 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, when the current pulse is just starting and the gas in the device is mostly neutral, very large fields build up as the electrons try to push their way through the resisting gas. With very few electrons able to move, the ones that do have to travel fast to carry a given current. The fast-moving runaway electrons gain as much as 3 keV of energy, slamming into the anode and depositing enough heat energy to vaporize some of the metal. This vaporized metal becomes a major impurity in the plasma, disrupting the formation of plasma filaments and leading to lower density in the plasmoid that the current generates. Lower density in turn leads to much lower fusion yield.
This runaway mechanism is a second main source of impurities, the first being arcing between different pieces of the electrodes. While one-piece, monolithic electrodes will eliminate all arcing, more steps need to be taken to eliminate the runaway electrons. The most important is 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, there are lots of free electrons ready to move. With many electrons, the current can be carried with each electron moving slowly and thus having little energy. Thus runaway electrons don’t occur in the main pulse either. High pressure in the gas, which make collisions of electrons with atoms more common, can help to prevent runaways as well.
Pre-ionization is a bit like deliberately creating a traffic jam. Runaway electrons are like cars on a highway at mid-day. There are fewer cars passing a given point but at a higher speed. These faster- moving cars, like the runaway electrons, are carrying more energy. At rush hour, there are far more cars passing a given point per minute, but they all move at a slower speed. Pre-ionization, by creating lots of free electrons, an electron ”rush hour”, allows a higher current with slower moving electrons, eliminating the fast runaways.
The paper will be available for free download from Physics of Plasmas’ only until Nov.21, 2014.
See this full article here.
LPP and PPRC collaborate on pre-ionization experiments
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.
See this full article here.
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