Tungsten passes erosion, first thermal shock tests
This story is part of LPP’s November 4th, 2013 newsletter, available in PDF format here. Join the discussion at the Focus Fusion Society here.
New laboratory tests on the existing tungsten plate in FF-1’s cathode (outer electrode) have increased LPP researchers’ confidence that the planned all-tungsten electrodes will both cure the impurity problem and survive many shots. In the first test, the LPP team measured how much erosion had occurred on the tips of the tungsten teeth where the current starts to flow. These tips suffer from the highest erosion of any parts of the cathode, because of the high concentration of current to which they are exposed. The team compared enlargements of photographs of the tip of the teeth taken when the plate was first installed in November, 2012, with photographs taken this month, after some 340 shots had been fired.
As seen in Figure 1, the tip started out with a 400-micron radius curve. After the 340 shots, the curve remained the same, within the estimated 15-micron accuracy of our measurements. Since any erosion would have flattened the curve, the same way wear blunts a knife-edge, the erosion is estimated at 15 microns or less. Photos of several different teeth failed to show any measurable erosion. If erosion from the anode is approximately as great, an upper limit on total impurity from the tungsten amounts to only about one tungsten ion for every 8,000 deuterium ions, well within the limits that LPP has calculated will be acceptable and have no significant effect on the plasma’s properties. By contrast, the present silver-plated copper electrodes erode so rapidly that there is about one impurity ion for every 50-70 deuterium ions. So the tungsten electrode is expected to lead to a 100-fold drop in impurity levels.
Figure 1: At left, a tungsten tooth in November, 2012; at right, a tooth in October 2013
A second test was to see if the tungsten plate suffered small-scale microcracks due to the thermal shock of being heated suddenly by the plasma. To increase the total energy that the tungsten was exposed to, the LPP team attached four more capacitors to the FF-1 circuit, bringing it back up to the maximum of 12 capacitors. We fired four shots at 35 kV and then disassembled the electrodes. We observed no microcracking. Others’ research with tungsten has shown that microcracking either starts in the first few shots, or is postponed for several hundred shots. So this test is also encouraging. However, we will soon do another round of tests at 40 kV, close to the 45 kV maximum power of FF-1.
We have now received a bid on the manufacture of our tungsten electrodes, so we expect to have them in hand by February, 2014, assuming adequate finances.
