Higher pressure DPF – would it work?
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Aeronaut.
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August 25, 2010 at 2:32 pm #895
FerretParticipantI’m interested in simplifying the whole DPF installation. The first thing I would throw out is the vacuum pump and additional tubing. So I’m curious whether DPF can be achieved at normal atmospheric pressure. Is there a maximum pressure for it to work?
What problems may appear at higher pressures? One I can think of is higher voltage required for the capacitors in order to discharge through the electrodes gap. Some other possible ones would be instabilities in the plasma flow and plasma focussing – what do you think about them? Are they going to appear?
Next, there is the influence higher plasma densities will have on the ion-electron beams coming out of the plasmon. The beam energies will be partially lost to the surrounding plasma. I have no idea how much “partially”.
So, what do you think about this topic?
August 25, 2010 at 3:01 pm #7879HenningParticipantYou’ll need to pump out the atmospheric pressure anyway to replace it with fuel. Nitrogen and oxygen aren’t great fusion candidates.
Then the pinch wouldn’t occur with higher pressure. For D-D normally 7 torr (9 mbar) is used. FF reached 40 torr (53 mbar) accidentially. See https://focusfusion.org/index.php/site/article/simultaneous_firing_at_last_first_pinch_above_1_mega-amp_record-high_p/
The pressure of the fill gas is quite a parameter to tweak (one of the many variables), and there is no sound theory how to calculate it. An empirical simulation is available though, the so called Lee Model: http://www.plasmafocus.net/
August 25, 2010 at 10:25 pm #7890TulseParticipantWhy doesn’t the pinch occur at higher pressure?
August 26, 2010 at 12:03 am #7894AeronautParticipantTulse wrote: Why doesn’t the pinch occur at higher pressure?
It can, but has to be offset by higher capacitor bank voltage and the resulting increased current and magnetic fields crushing the plasmoid. At this point LPP’s nailing down the parameters which are relatively easy to repeat from shot to shot. Later on, these settings can speed up scientific confirmation in other labs.
Achieving reliable 3MA currents may be a short to mid-term set of engineering challenges.
August 26, 2010 at 3:21 am #7901TulseParticipantI guess my question was more basic — what is it about higher pressures that makes a pinch harder to achieve?
And is it a big issue to run vacuum equipment? I would think that kind of tech is very well developed and close to off-the-shelf — am I incorrect? Does it add a lot of complexity to the gear?
August 26, 2010 at 4:56 am #7902AeronautParticipantTulse wrote: I guess my question was more basic — what is it about higher pressures that makes a pinch harder to achieve?
And is it a big issue to run vacuum equipment? I would think that kind of tech is very well developed and close to off-the-shelf — am I incorrect? Does it add a lot of complexity to the gear?
More fuel gas (pressure) requires more air and spark, nearest I can translate it to our level. I used to work in a place with lots of vacuum pumps which evacuated test chambers that were roughly 1 cubic meter, maybe a little smaller, in a few seconds. These monsters are not small, light, cheap, or cool-running. While they are required, I question the need for them on every shot if the machine were running around 1 khz, preventing the gas from condensing.
August 26, 2010 at 8:34 pm #7909FerretParticipantThe functioning of a DPF device is controlled by the so-called “drive factor”: D = I / ( a sqrt(p) ), where I is the peak current, a the anode radius and p is the pressure inside the device. For optimum neutron yield in deuterium, D = 78.46 kA / ( cm sqrt(mbar) ). In order to use the normal atmospheric pressure p = 760 Torr instead of 7 Torr, p has to increase 100 times. Thus, either I increases 10 times too or a decreases 10 times. I would go for a decrease of a. Say a = 0.2 cm (anode diameter of 0.4 cm), a catode 0.5 cm in radius (1 cm diameter) and p = 1000 mbar, it follows I = 0.5 MA. Up to now it should work with deuterium fuel at normal atmospheric pressure. For decaborane, maybe a higher I is necessary, say 1-2 MA.
Of course, there are problems with such a device. For one thing, the anode has to be a solid bar, i.e. no cooling hole in the center. That way it could take some of the mechanical stress from the plasmoid. Next, it may not last long. As Mr. Olsen found out, the current passing through it and the X-rays will vaporize a layer at its surface each time the device is fired. But it should take a few pinches.
One advantage of having a smaller anode is that the device needs far less energy. If I’m correct, the energy scales as a^3, thus a 10 times decrease of a means a 1000 times decrease for the energy. Instead of, say, 5-10 kJ used in other devices, only 5-10 J would be required. Thus, the capacitor bank may be reduced to just one capacitor.
That is, if the pinch does occur. Why would it not occur? This is a real question, not rethorical.
There is one possible reason: the flow of plasma along the anode may be disrupted by the higher pressure, somehow. Is it the case? Or maybe other reason?
PS: The capacitor fires at about 10 kV in air, quite a high voltage. Its capacity: about 100 nF or 0.1 uF.
August 27, 2010 at 12:36 am #7910AeronautParticipantI see you’ve done your homework, Ferret. I can tell you (or you can read my posts) to see that Eric’s got it figured just about to a T. Have you run the numbers of the FF rig? The anode diameter is about the minimum it can be cooled yet not distort under magnetic fields.
August 28, 2010 at 8:47 pm #7969FerretParticipantI played a bit with the Lee Model simulation, just to get used to it. It complained about pressures higher than 20 Torr, so I erased the complaint from the program. Next it complained about the run time being too long compared to the discharge time. I played a bit with the parameters until the axial run time was OK. This is what I got: cathode radius 5 mm, anode radius 2 mm, anode length 2.1 mm so that current I is at its maximum at the end of the axial phase, charging potential 120 kV (not 10 kV as I wanted it), pressure 760 Torr (I squeezed it out of the anode length 🙂 ), maximum current 0.5 MA. I used L = 5 nH, C = 0.1 uF and r0 = 1/4 sqrt(L/C) = 1.77 mOhm. The fuel was Deuterium. The only products I see are Joule heat and Bremsstrahlung radiation, the last being 0. The plasma went to a temperature of 4.5E6 (Kelvins I guess).
This is just the first shot. I’ll have to play with it a bit more. At least the simulation took those parameter values (except for the pressure, I forced it into it). The voltage is far too high for what I aimed.
Next week I’ll try a few more guesses and see what I’ll get.
August 29, 2010 at 2:38 am #7973AeronautParticipantBeen a while since I went through the entire Lee package, but it’s not intended for aneutronic fusion per se. What did you think about the formulae? Seems all of them are fractions/ratios. Given a critical axial speed, have you come up with a step by step method which systematically computes the ideal dimensions for each of the parts? I’m no mathemagician, but I do remember that every dimension and parameter is inter-related. I also remember that the Lee simulation and Lerner’s theory (see Tech Papers under the Learning Center link on the homepage’s nav bar) are not always in agreement.
These SWAGs might put your simulator in FoFu1’s ballpark: C~30uF; Vmax~100kV (with dramatically shortened cap design life as voltage increases) L~27nH. You might be able to guestimate the electrode and vacuum chamber specs from the picture galleries. These are copper, designed for operation in deuterium and some mixes of heavier gasses.
Happy hunting! :coolsmile:
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