This thruster development has been a perpetual ass kicking, but I'm apparently a glutton for punishment. Going for the 6th ignition attempt of the AHT1-PQ Pico Hall Thruster. Everything is pumping down, let's see if this new mod does anything. Test updates in the thread below!
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For this test, I've increased the channel width by drilling out the pole cap piece, which in turn also gives me a deeper channel. I've also increased spacing a bit between the anode and inner cathode, and slightly increased magnetic field strength to compensate.

2:01 AM ยท Oct 2, 2021

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I actually have no idea how much fuel is remaining I'm the thruster. At this point that's not a big deal. I think from now on I'm going to try to attempt ignition with the residual background gases in the mTorr range first and operate this as a standard anode layer ion source.
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If I can't get this ignited as a base level anode layer ion source at ignition pressures, then fuel won't help much here. If I can get a stable plasma, then I'll start trying to introduce fuel flow at full test levels.
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This will be an interesting balance, turning on the diffusion pump and trying to keep chamber pressures around a few mTorr, either by choking the foreline pump, or just unplugging the diffusion pump and letting pressure rise, then turn it back on to pump down again.
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I should however have at least enough fuel to run this test. It does seem however that I have to change the foreline trap zeolite fill about every 2 years since it rapidly saturates with Adamantane, making pumpdown increasingly difficult.
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On the plus side, the weather is really starting to cool down, so the basement is cooler now, meaning my cooling system will run better and keep the diffusion pump and baffle at a better cooling temp.
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Ok, with a fresh zeolite fill the chamber has roughed down very rapidly, and I can begin warming up the diffusion pump. Since I will be starting testing at higher pressure this time, I won't have to wait for full pumpdown, so testing should begin fairly soon.
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Ok pumpdown is still terrible. The absorbed gas load in the diffusion pump oil is so bad pressure has already risen from 50mTorr to over 150mTorr during warmup. It's starting to come down now, but not really a great start.
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So the plan is when I hit around 10mTorr during pumpdown I'm going to apply power to the anode, see if I get any plasma. If I do, great, I'll see at what pressure it extinguishes at as pressure drops. If not, then I'll have to play it by ear.
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Well, I'm at 10mTorr and have the anode power at full tilt, but no plasma. I guess that's a good sign in that fuel flow is needed, though maybe spacing is too much. I've turned on sublimation heater power to max, see if I can get some fuel going to the head.
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Ok it's already shot down to the mid 10^-5 Torr range. So much for my original plan. Guess I'll just be going the standard route of flowing fuel until the pressure rises enough for ignition.
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Chamber pressure is not rising as typical with sublimation heater power at full tilt. I might be out of fuel, which means a complete teardown to refuel. I'll turn off the diffusion pump and see if I can get to some pressure where I can ignite something.
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Been attempting to rapidly cycle the pressure up briefly. With the diffusion pump unplugged, pumping still happens for quite a while since it is still hot. I briefly shut the foreline valve, watch pressure rise, check for signs of ionization, then open the valve. So far nothing.
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System is approaching 10^-3 Torr. No plasma, but the anode supply has sagged from 700V unloaded to 630V now. There is some current flow, but no plasma happening.
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Ok now approaching 30mTorr. I think it's pretty clear at this point nothing is going to happen. Most likely anode spacing is now too far, even for this high pressure.
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Playing some dangerous games artificially pushing it to 100mTorr by choking the foreline, yet nothing. No plasma, not even a board flashover, which always happens at this pressure. The residual gas is not enough - I need an easily ionizable gas, like the fuel I'm using.
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I have to shut it down. Absolutely nothing happened except that if anything I have at least verified that chamber pressure effects alone do not contribute to the plasma. I need enough pressure with the Adamantane gas to achieve plasma.
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