Preparations Advance for Next Round of Tungsten ExperimentsAugust 16, 2016
For the past month, the research team has been preparing for a second and final series of experiments with the two tungsten electrodes, before moving on to the planned combination of tungsten cathode and beryllium anode later in the fall (see next story). In the course of these preparations, we’ve come across a possible route to removing the tungsten oxides layers that have increased impurities in the plasma. Work by Alenka Vesel and colleagues at the Jožef Stefan Institute, Ljubljana, Slovenia showed that simple microwaves can remove the oxides. The microwaves heat a hydrogen fill gas to 900 C allowing the hot hydrogen atoms to react chemically with the oxygen in the tungsten oxide, forming water vapor. As the reaction occurs, over the course of 10 seconds, the water vapor can be pumped out, preventing the oxides from re-forming when the gas cools.
LPP Fusion Chief Scientist Eric Lerner is not certain this method will work as the microwaves will not be evenly distributed in FF-1 vacuum chamber, so uneven heating may lead to uneven oxide removal. “In your microwave oven, which is heated by the same device, a magnetron, that we’ll be using, heating is evened out by a rotating turntable. But we don’t have that in FF-1’s vacuum chamber,” he explained. But the new method is certainly worth a try and will be tested in September.
In the meantime, the team has been upgrading the instrumentation used to record and understand FF-1’s operation. The fast ICCD camera has been out of action during the recent experiments. This turns out to be due to excessive radio-frequency noise that has affected the triggering of the camera and the transmission of images over a USB cable. To address this issue, Research Physicist Syed Hassan has installed a new wave generator to trigger the camera, and installed isolating transformers which remove noise from the power supplies to both the wave generator and the ICCD itself. In addition, the team has replaced the standard USB cable from the ICCD with a fiber-optic link and upgraded the shielding on the USB cables connecting the fiber-optic converter to the computer that records the images, which is located outside the experimental room. Finally, new optics are being installed to allow the ICCD camera to image the region where the discharge starts, on the anode near the insulator. This will allow the team to determine how well pre-ionization has reduced the vaporization of the anode material, another source of impurity.
A second upgrade involves a thorough calibration of the sensitivity and wavelength accuracy of the optical and UV spectrometer. This calibration will allow the research team, with the help of outside experts, to determine quantitatively how much tungsten and oxygen is in the plasma.
Finally, Electrical Engineer Fred van Roessel and System Administrator Jose Varela, working with Chief Information Officer Ivy Karamitsos are improving the data processing that analyzes the data from the photomultiplier tubes (PMTs) that measure x-rays from the experiment.