LPP Fusion continues cleaning impurities

In May, the LPP Fusion research team’s ongoing effort to clean off the oxides from the electrodes of our FF-1 experimental fusion device has made progress, but is not completed yet. they have succeeded in doubling the temperature of the microwave-heated plasma that does the cleaning. They also have convincing evidence that the oxides are still the source of the great majority of the impurities in the plasma. But to remove all the oxides, we need to increase our microwave power to get still higher plasma temperature, which is what we are doing now.

In March, we initially upgraded the waveguide that connects the magnetron, which produces the microwaves, to the window of our vacuum chamber. When they connected the magnetron to the large quartz window, we were able to get a bright glowing plasma by heating deuterium gas at low pressure, between 0.1 and 0.5 torr (atmospheric pressure is 700 torr). They could use the optical spectrum obtained from that glow to measure the temperature. Hydrogen produces a series of well-known lines in its spectrum and by measuring the ratio of the intensity of these lines, we could measure the temperature of the electrons in the plasma. It turned out to be around 5,000 K. While that sounds hot for an ordinary microwave-oven magnetron, it was not hot enough. We knew from published papers that the best results for oxide cleaning require about 20,000 K. At that temperature, the electrons have enough energy to allow the hydrogen atoms to strip the oxygen away from the tungsten.

Getting hotter—The new microwave test with two magnetrons (right) shows a much hotter, brighter plasma than the earlier tests with one magnetron (left). But the heating is still unevenly concentrated towards the large quartz window (direction of brightest region on right) and more power is needed to spread the hottest regions to all parts of the electrodes.

In April, to add more power, they attached a second magnetron to the small window that is on the opposite side of the vacuum chamber from the quartz window. With both magnetrons operating, we got a much brighter glow and the spectrum indicated a doubling of temperature to around 10,000 K. In addition, once we started firing, they found that the yellow oxide color that had earlier appeared on the outer surface of a few vanes had disappeared.

They believe that oxides on the part of the electrodes farthest from their quartz window are not in contact with sufficiently hot microwave plasma to be eliminated. The problem is that the diameter of the small window on the other side is less than half the 12-cm wavelength of the microwave radiation we are using. That is too small to transmit the radiation well, so most of the radiation does not get through. To get the full 2 kW we need, we must
put all of the microwaves through the large quartz window. They are now working to do that. They will put two magnetrons on a Y-shaped connector to funnel their power together. They hope then to have the 20,000 K we need to eliminate the oxides everywhere.

Beryllium cathode needed to have nicks repaired

The beryllium cathode will be crucial to the next set of experiments, so getting it right is worth the time. The nicks would have created sharp surfaces that could concentrate the electric current and possibly disrupt the filaments that are needed for high performance.

Fund a shot

LPP Fusion is still raising money with their fund a shot program. For $75 you can fund the charging of one of our 12 capacitors for one shot, for $150, two capacitors and so on up to $900 for a full shot. Everyone who funds a given shot will be recognized in a list kept permanently on the website.