The contribution of tungsten oxides to the plasma was clearly evidenced in the optical-NIR spectra obtained from slits pointed at the anode above the region of the pinch. These showed a clear peak at 777 nm, typical of oxygen.

This is for a typical shot in the tungsten series. It is more difficult to estimate the range of impurity. The near-UV spectra showed tungsten lines that were quite consistent from shot to shot as measured as a fraction of the continuum intensity. The minimum-intensity lines were only about 10% less than the mean. But the variability of run-down time, discussed below, indicated a wider range of impurity, with the minimum calculated impurity values roughly 60% below the mean. The Lee simulation fits indicate an intermediate minimum value of about 35% below the mean.

These results are consistent with a reduction in impurity mass by about a factor of three and thus a factor of 10 reduction in impurity by ion number, given the mass ratio of tungsten to copper. Due to tungsten's greater, the reduction of thefactor is then from about 17 to 9, thus moving only from the high to medium impurity level.

We also measured a reduction in the effect of impurities on the speed of the run-down phase. Using S. Lee's simulation model,shots from both the copper-electrode shots and the tungsten anode shots were fitted, allowing the external inductance to be varied in the simulations. We had previously measured the external inductance of the circuit using high-pressure N shots to be 26 nH. However, to fit the copper electrode results, the simulation inductance had to be increased to 40 nH, while the tungsten result could be fit with an external inductance of 30–32 nH.

This was confirmed following the firing run by direct examination of the anode. During a 125-shot run with silver-coated copper anode, an easily measurable reduction of over 200 μ m was observed in the anode diameter in the 0.8 mm wide region near the insulator. No such reduction was measurable with a 150-shot run with the tungsten anode, putting an upper limit on the erosion of about 25 μ m. In addition, we used a Fourier analysis of audio produced by rubbing the two anode surfaces near the insulator. This showed a roughly 3-fold reduction in roughness at long (100- μ m) wavelengths for tungsten relative to copper, again confirming a substantial reduction in erosion.

We found evidence that the new conditions did indeed reduce the erosion from the anode near the insulator. First, the dip in current that occurred during the first 100 ns of the current rise was substantially reduced in the new shots relative to those with copper electrodes and no pre-ionization (Fig.). On average, the total energy lost in these dips declined by 50% with the new conditions as compared to the old, which clearly reflected a reduction in energy absorbed by vaporization and ionization of material from the anode surface.

B. Energy transfer and fusion yield

2 21, 122706 (2014). 2. P. Kubes, M. Paduch, J. Cikhardt, J. Kortanek, B. Cikhardtova, K. Rezac, D. Klir, J. Kravarik, and E. Zielinska, Phys. Plasmas, 122706 (2014). https://doi.org/10.1063/1.4903906 The reduction in impurity was accompanied with an improvement in conditions at the pinch. The “early beam,” an increase in current immediately before the strong decrease in current at the time of the pinch, had been observed consistently with the copper electrodes.However, this phenomenon was essentially eliminated with the tungsten electrodes.

11 neutrons. We also increased mean yield for a sequence of 10 shots by 66% to 1.5 × 1011 neutrons (see Table et al. for comparison values from other researchers. 28 29(8), 1255 (1989). 28. H. Herold, A. Jerzykiewicz, M. Sadowski, and H. Schmidt, Nucl. Fusion(8), 1255 (1989). https://doi.org/10.1088/0029-5515/29/8/002 We used both pure deuterium and deuterium-nitrogen fill gases. With the first shots with pure D and arcing pre-ionization, we achieved a 56% increase in best-shot fusion yield compared with the previous copper electrodes, to 2.5 × 10neutrons. We also increased mean yield for a sequence of 10 shots by 66% to 1.5 × 10neutrons (see Table I ). For comparison, the peak yield was 50% higher than the yield of any other plasma focus device at 1.2 MA and double the yield of any other PF for a bank energy of 60 kJ. See Heroldfor comparison values from other researchers.

TABLE I. Results are compared between copper electrodes, tungsten with pure D, and tungsten with D-N mix with and without corona discharge pre-ionization. Mean values are shown for the best ten-shot sequence. Statistically significant (at 1% level) differences from the copper results are shown in bold and results obtained outside the ten-shot sequences are shown in parentheses.

Electrodes material (condition) Gas Total pressure (Torr) Number of shots Neutron yield (single-shot) × 1011 Neutron yield (mean 10 shots) ×1011 Neutron Yield s.d. (%) Mean ion energy (single-shot) keV Mean ion energy (mean 10 shots) keV Copper D 2 24 10 1.6 0.9 55 135 (170) 77 Tungsten D 2 18-20 8 2.5 1.5 38 200 71 Tungsten (No Pre-ionization) D 2 + N 2 18 7 1.9 1 41 124 78 Tungsten (Pre-ionization) D 2 + N 2 18 10 1.7 1.4 14 225 (240) 124

Using D-N mix and corona-discharge pre-ionization, we almost matched the increase in mean yield for pure D, increasing this by 56%, although we did not match the increase in single-shot yield.

23 48, 745 (2006). 23. S. Ahmad, S. S. Hussain, M. Sadiq, M. Shafiq, A. Waheed, and M. Zakaullah, Plasma Phys. Controlled Fusion, 745 (2006). https://doi.org/10.1088/0741-3335/48/6/003 In addition, very significantly, with corona-discharge pre-ionization (CDP) we observed a four-fold decrease in shot-to-shot variability over a ten-shot run as compared with copper electrodes and nearly 3-fold reduction as compared with the pure D results with arcing pre-ionization. These results are quite similar to those obtained using pre-ionization with much smaller-current PF devices. The large reduction in variability strongly indicates that much of the well-known shot-to-shot variability in PF devices is due to unsymmetrical breakdown, which pre-ionization greatly reduces.

The D-N, CDP shots also produced a 40% increase in mean ion energy for a single shot (June 7, 2016, shot 6), to a record of 240 ± 20 keV for any PF device. This is a record for confined ion energy in any fusion-fuel plasma using any device. A second shot (June 2, 2016 shot 8) was measured with E i of 225 keV. We observed as well a 61% increase over the best copper results in E i averaged over 10 consecutive shots with the same conditions.

29 19, 032704 (2012). 29. E. J. Lerner, S. K. Murali, D. Shannon, A. M. Blake, and F. Van Roessel, Phys. Plasmas, 032704 (2012). https://doi.org/10.1063/1.3694746 et al. in 2015. 30 22, 062705 (2015). 30. P. Kubes, M. Paduch, J. Cikhardt, B. Cikhardtova, K. Rezac, D. Klir, J. Kravarik, J. Kortanek, and E. Zielinska, Phys. Plasmas, 062705 (2015). https://doi.org/10.1063/1.4922483 These results exceeded a previously reported record in 2012 of 170 keV for confined mean ion energy in the same PF device with copper electrodes.It also exceeded the 100–200 keV gyrating ions reported by Kubesin 2015.

i by time-of flight measurements of neutrons at 11 m and 17 m from the device axis. The TOF PMTs were located at the height of the end of the anode, perpendicular to the machine axis. They had a collimated field of view of only 2 cm vertically from the tip of the anode. As described previously, 7 7. H. Schmidt, in Proceedings of 3rd International Workshop on Plasma Focus (1983), pp. 63– 66 . 4 11. We measured mean ion energy Eby time-of flight measurements of neutrons at 11 m and 17 m from the device axis. The TOF PMTs were located at the height of the end of the anode, perpendicular to the machine axis. They had a collimated field of view of only 2 cm vertically from the tip of the anode. As described previously,the spread in neutron velocities in the perpendicular direction must be due to high-energy ions which are confined within the central plasmoid produced at the tip of the anode (Fig.). The duration of the neutron production, measured both by projecting back the TOF signals and by a third PMT located at 1.34 m, is 40 ns, and the neutron yield was 1.6 × 10

30 22, 062705 (2015). 30. P. Kubes, M. Paduch, J. Cikhardt, B. Cikhardtova, K. Rezac, D. Klir, J. Kravarik, J. Kortanek, and E. Zielinska, Phys. Plasmas, 062705 (2015). https://doi.org/10.1063/1.4922483 In these shots, as in previous workwith FF-1, the neutrons observed cannot be mainly produced by an axial beam passing through either a dense plasmoid or through the background gas. Such a beam would produce a strong asymmetry in neutron distribution, with far more neutrons in the axial, down-beam direction than horizontally. We observed no such large asymmetry.

Comparing neutron measurements by cross-calibrated bubble detectors at 4° and 90° from the axis, we found an asymmetry for 10 shots of only 1.3 ± 0.1 towards the axial direction. By contrast, if the neutrons were produced by a beam interacting with the 1.6 m-long column of deuterium in the vacuum chamber and drift tube, we would expect more than 100 times as many neutrons at the 4° than at the 90° detector. Thus, only confined ions that are gyrating within the plasmoid, not moving in an axial beam, can produce the neutrons observed.

29 19, 032704 (2012). 29. E. J. Lerner, S. K. Murali, D. Shannon, A. M. Blake, and F. Van Roessel, Phys. Plasmas, 032704 (2012). https://doi.org/10.1063/1.3694746 19 cm−3 and for that density, 250 keV ions will take a few microseconds to thermalize, far longer than the 40 ns lifetime of the plasmoids. Of course, there is no reason to believe that these ions are Maxwellian in velocity distribution, and therefore we do not refer to the ion mean energy as a temperature. In a previous work,we have calculated the density of the plasmoids as about 3 × 10cmand for that density, 250 keV ions will take a few microseconds to thermalize, far longer than the 40 ns lifetime of the plasmoids.

On the other hand, there is also no evidence in our results for two populations of ions with widely different E i . The distribution of neutron arrival times is sufficiently close to the expected Gaussian curve that we can exclude, for a wide range of hypothesized parameters, a relatively cold, dense background plasma colliding with a 500 keV fast ion population to produce collisions with 250 keV average energy. The colder ions would produce a central, sharply peaked distribution of neutron arrival times while the hot ions colliding with each other would produce greater-than Gaussian wings to the arrival time distribution. We observed neither. However, a very-low-energy background plasma within the plasmoid with E i < 7 keV would produce too few neutrons to be observed, so we cannot exclude that possibility. Neither do we see any positive evidence for it.

For comparison, we performed 6 shots with no pre-ionization but with the same D-N mix. While the number of shots was limited, the mean fusion yield, yield variability, and E i were essentially identical with that for the copper electrodes. The difference between mean fusion yield of the 10-shot pre-ionization run and the 7-shot no- pre-ionization run was significant at the 2 sigma level. For E i , only the D-N CDP shots had significant increase over copper, with both the no-preionization D-N mix and arcing-preionization pure D having the same mean E i as copper.

et al., 31 44(6), 968 (2016). 31. H. Bruzzone, H. N. Acuña, M. O. Barbaglia, M. M. Milanese, R. Miklaszewski, M. Paduch, E. Zieliñska, and A. Clausse, IEEE Trans. Plasma Sci.(6), 968 (2016). https://doi.org/10.1109/TPS.2016.2562038 Energy transfer to the pinch also increased significantly from the copper to tungsten electrodes. The average voltage spike at the time of the pinch increased by 120% from 37 to 81 kV and the maximum spike for a single shot increased from 45% from 62 to 90 kV. Based on inductance calculations using the formulas of Bruzzonewe calculated that maximum energy transfer into the pinch increased to about 8 kJ. For energy transfer, there was no significant difference among the preionization and no-preionization conditions.

After the first 100 shots, which was after 50 shots with pinches, fusion yield began to deteriorate, reverting to about the same levels as copper electrodes by the time the series ended at 150 shots. The last shots showed a return as well of the early beam phenomenon and an increase in the early dips in current. While the reason for the decline is not determined, it seems likely that the increased roughening of the anode surface probably led to increasing asymmetry in breakdown and therefore in the current sheath and subsequent pinch.