Axial Field Coil, New Switches AssembledDecember 24, 2018
LPPFusion’s research team, working steadily to prepare for the new beryllium electrode experiments, has accomplished two big tasks. First, Research Scientist Dr. Syed Hassan designed and installed the new Axial Field Coil (AFC) in the vacuum chamber. This copper wire coil controls the spin on the plasmoid in our Focus Fusion device. It carries a small DC current, which produces a magnetic field directed along the axis (thus the name). When electric current in the plasma inside the chamber interacts with that field, the resulting forces produce a spin in the plasma. During the shot, the currents in the plasma will induce high-frequency currents in the AFC, so it will also act as a sensor to detect how much spin is induced. Optimizing the spin, we expect, will optimize the density of the plasmoid and thus fusion yield.
The AFC is now protected by high-temperature materials (Figure 1), quartz glass coil, aluminum oxide ceramic connections (white) and titanium-nitride coated wire supports. Previously, the AFC was contained in a simple copper coil. Making the connections with the ceramic turned out to take a great deal longer than we had thought, but the improvements are worth the delay.
At the same time, Dr. Hassan installed upgraded versions of two other coil instruments—the Upper and Lower Rogowski coils. These coils measure the current in the ion beam that the plasmoid emits. The new coils will be paired with Langmuir probes—basically simple wires—that will also detect the ion beams. Since the two probes will react differently to the radio-frequency noise the device produces, they will together make it much easier to separate the signal from the noise and make more accurate measurements of the beam energy and current.
A second big task was the successful installation of two ceramic-protected switches (Figures 2 and 3). The ceramic disks protect the Mylar plastic underneath. The Mylar is needed to prevent a spark from running along the sides of the big plastic insulator, shorting out the switch. But in the past the Mylar degraded rapidly and failed after a hundred shots or so. The ceramic will protect it. In turn the ceramic is held in place by Lexan tabs. Two new switches have been put in place for testing. If they pass after 10 or so shots, we already have the parts to put the other six in place.
The team is now putting together the parts for the new vacuum system, which is the last step before final assembly of the electrodes onto the machine. The vacuum system includes new filters to prevent beryllium dust from escaping into the environment. It also includes a small “dump chamber” where exhausted gas from several shots can be kept. This will be important for the hydrogen-boron shots later in 2019. While the main fusion reaction produces harmless helium, a side reaction produces carbon-11, a radioactive isotope. This material decays very rapidly with a half-life of 20 minutes. So after 8 hours, no radioactivity will be left and the exhaust can be safely pumped out of the dump chamber.
Figure 3. Left: Bottom of switch without the Lexan insulator. Right: The new spark plug (which goes into the top of the switch) with reinforced Lexan insulator.
In addition to the above-described:
- Upgraded Axial Field Coil (AFC) and ion –beam Rogowoski coils (URC, LRC)
- New ceramic switches
- New vacuum system with filters and dump chamber
Other upgrades to our experimental equipment (some of which have been reported on previously) include:
- Redesigned vacuum chamber with enlarged viewing windows
- Quartz protective guards on two vacuum chamber windows to shield against deposition
- Hollow anode and new upper electron spectroscope and Rogowski coil to measure electron beams
- Remote control over all x-room switches and valves
- Improved electromagnetic shielding of our instruments and oscilloscopes