Switch reliability allows higher fusion yield

With our new ability to reliably control the number of capacitors firing, and thus to control to within 4-5% the current produced by the bank, we can adjust the gas fill pressure in the chamber to optimize yield.  As a result, on September 30 and October 1, we were able to get two shots with fusion yields above 10¹¹ neutrons for the first time since late March.

DPF researchers have long known that it is important to match the time that the pinch occurs with the time that the current from the capacitors peaks.  Last month, we plotted the fusion yield for FF-1 against the time of the pinch (see figure 1).  We found that for the shots in March and early April, the yield was tightly correlated with pinch timing, and there was a sharp peak right around the time of greatest current, close to 1.8 microseconds.  All the high-yield shots had short pinch times, and none of the lower yield shots did.  The same pattern was followed at the higher pressures and currents that we used in September, but the whole curve was shifted upwards by about a factor of 5.  This shift implies a good scaling with current to the fifth power.

What was particularly significant was that only shots with the axial field had the short pinch times.  In shots without the coil turned on, or without the permanent magnetic field that in March was induced in the vacuum chamber, shots that should have had short pinch times (based on the amount of gas and the current) did not pinch at all.  In our most recent shots, which have been with no axial field, we observed the same phenomena, with pinches at a little more than 2.0 microseconds alternating with non-pinch shots.  As seen in Figure 1, the new shots follow the same trend as the earlier ones.
While we are not certain of the theoretical explanation yet, it appears that the correct axial field allows a pinch to occur at a time very close to the peak current, while this is not possible without the field.  In our experiments this month, we intend to vary the axial field together with the gas pressure, to achieve pinching at a time very close to that of peak current.  We expect in this case to avoid the “hiccupping” shock waves observed in the lower yielding shots (described in last month’s report) and move up to a yield approaching or exceeding 10¹² neutrons (over 1 joule of fusion energy).

Figure 1.  Neutron yield vs. the time from the start of the pulse to the pinch.  The neutron count is shown as the logarithm of the neutron count in thousands.  A level of 2 corresponds to 10¹¹ total neutrons.  Small blue dots are shots at 10 torr fill pressure in late March and early April, showing a close fit to the empirical scaling line.  The other symbols are from shots in August and September at 18-24 torr.  The large red squares are the most recent shots at the end of September.  The higher pressure shots follow the same trend line as the 10-torr shots, but with higher yield, implying a scaling of about I⁵.