Telltale spectra and more in September reportSeptember 22, 2013
This update is part of the September 23rd, 2013 Focus Fusion progress report. Download the full report in PDF format here, for the following additional updates:
- Open letter for fusion funding gains support
- LPP and FFS begin planning for crowdfunding effort
- Design of monolithic tungsten cathode finalized—to cure impurities
- LPP participates in Plasma 2013 conference, releases database
Join the discussion of this report on the Focus Fusion Society forums.
Spectra confirm metal impurities in FF-1 plasma
Our research team believes that the main problem impeding higher density and yields in LPP’s FF-1 device is metal impurities in the plasma. We have estimated the level of these impurities in various ways, but have not until now actually measured them. Now, thanks to a new optical-UV spectrometer from Ocean Optics, we have measured the impurities and confirmed what they are and their amounts. This important step forward was taken with the able help of summer Research Associate Kyle Lindheimer, a student at Penn State University, under the direction of LPP Lab Director Derek Shannon.
With an optical spectrum, elements can be identified by the characteristic wavelength of light they emit in concentrated “lines” that show up as peaks in the spectrum.
Figure 1. In this spectrum, recorded on Sept. 11, 2013, the X-axis is the wavelength of light in nanometers while the Y-axis shows relative intensity. The tallest peaks are identified as those of deuterium, the main fill gas. The second largest peaks are silver, and the smallest ones are copper. While we need to and will obtain additional spectra, current calculations from the relative strength of these lines (area under the peaks) indicates that about 2% of the ions in our plasma are silver and perhaps 0.5% are copper. Since silver ions are 54 times as massive as deuterium ions, as much as half the total mass of the plasma may be silver. This is consistent with earlier simulations that showed the plasma sheath moving much slower than anticipated. This is also more than enough silver to disrupt the plasma filaments and prevent higher density plasmoids and more fusion yield. Our solution is to eliminate the silver and copper in the electrodes and replace them with tungsten.
Figure 2. Doing the twist: A recent photo of the pinch in our FF-1 device gives some of the clearest evidence yet for the kinking that forms the plasmoid (bright central spot). See the spiral-staircase (helical) patterns above and below the plasmoid, where the current filaments kink up.