Reliable firing of bank with 10 capacitors
(2) CommentsFrom LPP’s December 30, 2010 report. On the last day of firing this year, we finally achieved reliable firing with 10 capacitors—all fired together six times in a row with no pre-firing.
Remaining issues
We now are confident that we understand the remaining two problems of the switches: their pre-firing and the breakage of the spark plug insulators. Unfortunately it has taken us a total of three months to get this understanding, from the time that we solved the first problem of achieving simultaneous firing in late September.
We simply have not had enough reliable shots to optimize the pressure and axial field to obtain the higher yields we are seeking. While we are able to continue firing the bank in its present configuration, to go to all 12 capacitors firing at 45 kV will require some additional work on the switches. But we know what we must do.
We ran into a number of problems that had to be resolved in turn.
More capacitors, more pre-firing
Perhaps most importantly, we learned from our experiments that the pre-firing inevitably gets worse with more capacitors. This is because the pre-firing is caused by a slow breakdown of the gas related to a phenomenon called “corona discharge”. With more capacitors attached, the power supply’s fixed output charges the bank more slowly, so the switches stay longer at high voltage. Because of this problem, the present configuration of the switches will not work with all 12 capacitors attached. Ten capacitors is our current maximum.
However we know the cure for this. We’ll move the electrodes in the switches further apart (see the switch diagram post for further explanation). We will be doing this in our general redesign over the next few months.
Pre-firing from worn tungsten trigger rods concentrating field
In addition, if the tungsten trigger rods get worn down so they are too thin or too rough, the field gets too concentrated, and this also leads to pre-firing. Temporarily, we have replaced the thinnest rods and are sanding the others carefully on a regular basis. In our re-design, we will use much thicker rods—probably one-quarter-inch diameter instead of one-eighth inch.
Ruggedization to end mechanical and electrical breakdown of insulators
We also had trouble eliminating breakage. While our large stabilizer block prevented any cracking of the insulator above the plate, the rapid movement of the tungsten rods was still breaking the Lexan insulators near the tip. After several tries, we have used a solid cylinder insulator to provide maximum strength. So far, they have lasted 40 shots with only two cracking, so this is adequate for now, although too soon to tell their real lifetime. Again, we know the solution to the mechanical breakage: making the rods thicker so they bend less, and making the insulators thicker so they are stronger. These changes require replacing the top plates of all the switches and making the spark plug holes larger. This hole size has limited our past efforts with the spark plugs.
Finally, to prevent the electrical break-down of the insulators, we also have to make them thicker. All of this can be calculated, based on the experiments we have done, and we hope to complete design work very soon, probably early in January. However, there are considerable ordering delays on some items, such as the tungsten rods, and some additional testing will be needed, so realistically we will not complete the new switches until March. Until then, we will be running with the existing 10–capacitor bank. Derek Shannon is ably assisting while Dr. Subramanian is on vacation.
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Comments
For a more in depth discussion, start a thread in the forums.How many spare insulators are there on average?
Some thoughts from my high voltage engineering education:
The so called “corona discharge” refers to a weak glowing discharge occuring on sharp edges because of locally very high field strengths. The discharge normally stays small and doesn’t lead to a breakdown even when the voltage becomes bigger, because it is only a very local effect and when the voltage doesn’t change very fast a space charge is generated in front of the edge, which homogenizes the field.
This phenomenon (corona discharge -> space charges -> field homogenization) usually helps to achieve higher breakdown voltages at frequencies 50 Hz or below. So this cannot be the reason for the pre-firing.
In your case it’s probably just the statistical nature of the breakdown process. An electric breakdown in air occurs when the voltage or the electric field strength, respectively, is high enough AND when a free charge carrier which initiates the breakdown is available, e.g. throuh ionization by natural radiation. The latter is the statistical process. It means that it becomes more likely that a breakdown occurs the longer the voltage is applied. So it’s normal that you have pre-firing when you charge longer. What are the actual charge rates?
Anyway, the solution to this problem is simple: make the air gap larger. But then you get problems with the triggering. Unfortunately I don’t know much about trigatrons to help you here. There are a lot of papers about that:
http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=556570
They use a mid-plane triggered trigatron. Could this be of any help in your case?
For simultaneous triggering of several switches alternative triggering methods with electron beams or lasers would be better. But this will be most likely above your budget. Perhaps you ask the guys at Sandia’s if they can give you some spare parts from their system
-> http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=941917
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