Investigation of the Microstructure of Ion Beams Emitted from PF-1000 at Different Angles to the Z-Axis 1

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E. Skladnik-Sadowska , M. J. Sadowski , K. Czaus , K. Malinowski , R. Kwiatkowski , J. Zebrowski1, M. Paduch2, M. Scholz2, P. Kubes3, I. E. Garkusha4 and A. Talebitaher5 1

The Andrzej Soltan Institute for Nuclear Studies (IPJ), 05-400 Otwock-Swierk, Warsaw, Poland 2 Institute of Plasma Physics and Laser Microfusion (IPPLM), 01-497 Warsaw, Poland 3 Czech Technical University (CVUT), 166-27 Prague, Czech Republic 4 Institute of Plasma Physics, NSC KIPT, 61-108 Kharkov, Ukraine 5 Plasma Radiation Sources Laboratory, NIE NTU, 637616 Singapore

Abstract. The paper describes diagnostics of fast ion beams emitted from the PF-1000 facility operated at 21-27 kV, 290-480 kJ. The use was made of pinhole cameras equipped with 0

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PM-355 track detectors and placed at different angles to the discharge axis. Measurements performed at 0 , as well as those at 60 showed a complex spatial structure of the fast ion beams. 0 The ion measurements, which were for the first time performed in the upstream direction (at 180 ), have proved that some fast deuteron beams are emitted also in the upstream direction.

Experimental setup During the experimental campaign in 2009 the PF-1000 machine was equipped with coaxial Mather-type electrodes, as described earlier [1]. Plasma discharges were triggered at the initial pressure po = 0.75-2.1 hPa D2, and they were supplied from a 1.32-mF condenser bank charged to 21-27 kV, 290-480 kJ. The maximum discharge current amounted to about 1.5-1.8 MA. Measurements of the fast ions were carried out by means of ion pinhole cameras (IPC) with PM355 track detectors and different absorption filters. The cameras were placed at different angles: 0 0 at 0 (along the z-axis) - at a distance of 162 cm from the electrode outlets, at 60 at a distance of 74 0 cm from the center of the pinch column (observed at z = 4 cm), and at 180 (in the upstream direction) - at a distance of 74 cm from the electrode outlets. The cameras were introduced into the PF-1000 chamber through a T-pipe and a valve, which made possible to exchange detectors without disturbing the vacuum.

From the presented macro-photographs one can easily see that the tubular bunch of highestenergy deuterons and protons was surrounded by numerous high-energy micro-beams. 0 It should be noted that the ion pinhole (IPC7-II) measurements at 60 were carried out simultaneously with a mass- and energy-analysis of ions emitted along the z-axis, which were performed with a Thomson mass-spectrometer [4]. Some results are shown in Fig. 4.

Experimental results The pinhole cameras were equipped with internal supports which made possible to place the PM-355 track detector at different distances behind the input diaphragm. In some cases there were used two detectors: the first one with a small hole in the center, and the second for the recording of the central ion beams, as shown in Fig. 1. IPC

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4 cm

4 cm

Fd= 0,5 mm

Fig. 1. Picture of the PF-1000 electrode outlets, a scheme of the ion pinhole camera and images st nd of ion beams, which were recorded along the z-axis upon the 1 and 2 detectors irradiated 10 during two 290-kJ discharges with the total neutron yield Yn = 6 x 10 .

Fig. 4. Map and histograms of high-energy protons and deuterons, which were recorded with a o pinhole camera placed at 60 and equipped with PM-355 detector and a 10-µm Al.-filter.

The presented histogram shows that at 600 the most deuterons behind a 10-µm Al-filter had energies of the order of 1000 keV, while the most protons of the order of 850 keV. Those values were in agreement with the energy spectra measured along the z-axis [4]. Since in many PF experiments there was observed strong anisotropy in the fusion-neutron emission, some researchers suspected that a portion of the fast deuteron population might be accelerated and emitted also in the upstream direction. That suspicion was confirmed by measurements of backward-oriented high-energy deuteron beams in a small-scale NX2 experiment [5]. Therefore, in the recent PF-1000 experiments particular attention has been paid 0 to the ion pinhole measurements at 180 , i.e. behind the collector plate in the upstream direction. Some results are presented in Fig. 5. IPC18 0um Al

IPC20 3um Al

IPC21 6um Al

The PM-355 detectors, after their irradiation and etching, have demonstrated that the emitted ion beams have a complex spatial structure and they consists of many micro-beams. A divergence st of the central micro-beams, as estimated from a comparison of the dimensions of a hole in the 1 nd -3 detector and the image on the 2 detector, was about 4 x 10 sr. A comparison of the electrode projections with the ions tracks suggest that the recorded ion beams originated mainly from the pinch column and regions between the outer electrode tubes. To study the spatial structure of the high-energy deuteron and proton beams, the pinhole cameras were equipped with PM-355 track detectors shielded with absorption filters made of Al-foils of different thickness. Some examples of such measurements are shown in Fig. 2.

Fig. 2. Images of ion beams emitted from similar 290-kJ PF-1000 shots, 0 as recorded at 0 , at a distance of 162 0 cm (upper row), and at 60 , at a distance of 74 cm (lower row). Beam images were obtained behind different filters and showed deuterons of energy: (left) ED > 220 keV, (middle) ED > 380 keV, (right) ED > 700 keV. The contribution of protons, as determined from measurements with a Thomson spectrometer [4], was negligible (about two orders of magnitude lower).

From an analysis of the recorded tracks it was determined that for a single PF-1000 shot at the 9 pinhole input placed along the z-axis (at a distance of 162 cm) there were recorded about 2.4 x 10 2 9 2 0 deuterons/cm of energy > 380 keV, and 1.0 x 10 deuterons/cm of energy > 700 keV. At 60 the 9 deuteron fluxes at the pinhole input (placed at a distance of 74 cm) amounted to about 9 x 10 2 8 2 deuterons/cm of energy > 380 keV and 1.2 x 10 deuterons/cm of energy > 700 keV. An analysis of macro-photographs of the obtained tracks delivered information about the spatial microstructure of the investigated deuteron beams, as shown in Fig. 3.

Fig. 3. Macro-photographs of the highestenergy deuterons (> 700 keV) and protons (> 525 keV) beams recorded at 0 0 , as taken from different parts of the image IPC 9 (shown in Fig. 2) at different magnifications.

Fig. 5. Ion pinhole images from a camera placed at z = -74 cm (behind an axial channel in the inner electrode), as obtained from a single shot #8408 with Yn = 1.5 x 1011 (left), 3 shots #8409-8411 with 11 11 Yn = 1.8 x 10 ( middle) and another single shot #8412 with Yn = 1.1 x 10 (right). Parts of the image were analyzed at larger magnifications (lower row).

The presented image constitutes the first experimental evidence that high-current PF-1000 discharges can emit some population of high-energy deuterons in the upstream direction. On the basis of the recorded tracks it was estimated that at the pinhole input (placed at z = -74 cm) 8 the fluxes of deuterons of energy > 30 keV, > 380 keV and > 700 keV amounted to about 1.4 x 10 -2 7 -2 6 -2 cm , 9.2 x 10 cm and 2.2 x 10 cm , respectively, i.e. they were about two orders of magnitude lower than those emitted along the z-axis.

Summary and conclusions TThe most important results of this study can be summarized as follows: 1. The ion measurements with pinhole cameras within the PF-1000 facility confirmed a complex spatial structure of the emitted high-energy deuteron and proton beams; 2. The analysis of the recorded tracks made possible to estimate numbers of the accelerated and emitted primary deuterons of energy > 380 keV and > 700 keV; 3. The emission of such high-energy deuterons constitutes an experimental evidence that non-linear phenomena occurring in a PF pinch column induce strong electrical fields within the dense plasma volume; 4. It has for the first time in the PF-1000 experiment been demonstrated that some portion (at least about 1%) of the primary deuterons can be accelerated and emitted in the upstream direction. One can conclude that the ion studies, and particularly time-resolved measurements should be continued in order to collect more information about ion acceleration processes.

References [1] M. Scholz, B. Bienkowska, M. Borowiecki, et al., Nukleonika 51 (2006) 79-84. [2] K. Malinowski, E. Składnik-Sadowska, M.J. Sadowski, et al., AIP CP812 (2006) 256-259. [3] M.J. Sadowski, V.A. Gribkov, P. Kubes, et al., Physica Scipta Vol. T123 (2006) 66-78. [4] K. Czaus, E. Skladnik-Sadowska, K. Malinowski, et al., Proc. PLASMA-2010 – this issue. [5] M.V. Roshan, P. Lee, S. Lee, A. Talebitaher, et al., Phys. Plasmas 16 (2009) 074506.