Shot reproducibility of the self-magnetic-pinch diode at 4.5 MV

In experiments conducted at Sandia National Laboratories’ RITS-6 accelerator, the self-magnetic-pinch diode exhibits significant shot-to-shot variability. Specifically, for identical hardware operated at the same voltage, some shots exhibit a catastrophic drop in diode impedance. A study is underway to identify sources of shot-to-shot variations which correlate with diode impedance collapse. The scope of this report is limited to data collected at 4.5-MV peak voltage and sources of variability which occur away from the diode, such as sheath electron emission and trajectories, variations in pulsed power, load and transmission line alignment, and different field shapers. We find no changes in the transmission line hardware, alignment, or hardware preparation methods which correlate with impedance collapse. However, in classifying good versus poor shots, we find that there is not a continuous spectrum of diode impedance behavior but that the good and poor shots can be grouped into two distinct impedance profiles. In poor shots, the sheath current in the load region falls from 16%–30% of the total current to less than 10%. This result will form the basis of a follow-up study focusing on the variability resulting from diode physics.

Figure 1

Diagram of the downstream portion of the RITS-6 accelerator. The B-dot current monitors at positions F, IFEED, IBEAM, and DIODB are labeled.

Figure 2

Traces of the currents, voltage, diode impedance, and radiation pulse for two SMP diode shots with a 7-mm diameter cathode needle and a 7-mm AK gap. Shot 1441 (blue) demonstrates the desired diode behavior while Shot 1516 (red) has a premature radiation pulse termination due to a rapid drop in diode impedance. The IF (dashed) and OF (solid) currents are show in (a). The voltage at position F is shown in (b). The diode current is shown in (c). The diode impedance (solid) and radiation pulse (dashed) are shown in (d).

Figure 3

The IF and OF currents for SMP diode shots with a 7-mm diameter cathode needle and a 7-mm AK gap. The gray dotted lines are the average OF in the first 10 ns at peak power.

Figure 4

Simulation geometry of the downstream section of the RITS-6 accelerator, including the dustbin and diode. Contours of the sheath electron density at peak power are also shown on a logarithmic scale.

Figure 5

Measured voltages used to drive four simulations of the SMP diode.

Figure 6

Currents in the MITL, dustbin, and diode from a simulation of shot 1394 compared to data. The diode has a 8-mm diameter needle and a 8-mm AK gap. The diode impedance in the simulation is stable.

Figure 7

Currents in the MITL, dustbin, and diode from a simulation of shot 1441 compared to data. The diode has a 7-mm diameter needle and a 7-mm AK gap.

Figure 8

Currents in the MITL, dustbin, and diode from a simulation of shot 1509 compared to data. The diode has a 7-mm diameter needle and a 7-mm AK gap. The falling diode impedance is modeled using massive fluid electrons at the anode which drift away from the surface and effectively reduce the AK gap.

Figure 9

Currents in the MITL, dustbin, and diode from a simulation of shot 1519 compared to data. The diode has a 7-mm diameter needle and a 7-mm AK gap. The falling diode impedance is modeled using massive fluid electrons at the anode which drift away from the surface and effectively reduce the AK gap.

Figure 10

The magnitude of the OF current at peak power versus shot number. Good shots are shown in blue and poor shots in red. The values shown are averaged over the first 10 ns at peak power.

Figure 11

The pulse rise from OF versus shot number. Good shots are shown in blue and poor shots in red. The rise is calculated as the time to rise from 10% to 90% of the mean OF current.

Figure 12

The azimuthal asymmetry of the IFEED (top) and IF (bottom) currents versus shot number. The asymmetry is the difference between the highest probe value and the average, normalized by the average, $(I_{\max }-\overline{I})/\overline{I}$. Good shots are shown in blue and poor shots in red.

Figure 13

The IBEAM and DIODB currents from shots 1500 through 1519. All shots are 7-7.

Figure 14

The IBEAM and DIODB currents from shots 1520 through 1539. All shots are 7-7.

Figure 15

IBEAM versus the ratio DIODB/IBEAM from 53 7-7 shots. The peak currents are used.

Figure 16

The diode impedance from simulations of 8.5 and 12.5-mm diameter cathodes on a scaled time axis. (For the 12.5-mm cathode, $t\to 8.5/12.5t$.) The simulations include effects of expanding anode plasmas.

Figure 17

(a) The IBEAM diode currents from shots 1442, 1443, and 1460, which are 6-6, 7-7, and 8-8 configurations, respectively. (b) The same IBEAM currents with the time axes scaled by $t^{\prime }=t/g_0$.

Figure 18

Theoretical critical current and IBEAM for a time-dependent AK gap. Equation (1) using $v=1\mathrm{cm}/\mu \mathrm{s}$, $\alpha =1.1$, and $\gamma (t)=8+0.0625t$ where $t$ is in ns. The time axis has been shifted.