Measurements of Fusion-Protons Anisotropy around the Pinch Axis within High-Current PF-1000 Experiments 1-2

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M. J. Sadowski , A. Malinowska , K. Malinowski , K. Czaus , R. Kwiatkowski , E. Skladnik-Sadowska , J. Zebrowski1, L. Karpinski2, M. Paduch2, M. Scholz2 and W. Stepniewski2 1

The Andrzej Soltan Institute for Nuclear Studies (IPJ), 05-400 Otwock-Swierk, Warsaw, 2 Institute of Plasma Physics and Laser Microfusion (IPPLM), 01-497 Warsaw, Poland

Abstract. The paper describes measurements of fast protons produced by D-D fusion reactions during high-current discharges within the PF-1000 facility operated with the deuterium filling at 27 kV, 480 kJ. The measurements were performed by means of a set of pinhole-cameras equipped with PM-355 track detectors shielded by 80-µm-thick Al-filters, which eliminated fast primary deuterons and protons of lower energy (< 3 MeV). Those cameras were placed at different angles around the pinch axis. The obtained proton images showed a distinct angular anisotropy, which was explained by an influence of local magnetic fields connected with a filamentary structure of the plasma column during the fast proton (and neutron) emission.

Experimental setup The PF-1000 machine [6] was equipped with coaxial electrodes of 460 mm in length, 230 mm and 400 mm in diameter, respectively. The inner electrode was a copper cylinder, and the outer electrode consisted of 12 SS-tubes of 80 mm in diameter each. The discharges were triggered at the initial pressure of 2.1 hPa D2, and they were supplied from a 1.32-mF condenser bank, charged to 23-27 kV, 350-480 kJ. To measure the azimuthal distribution of the fast fusion-protons, small pinhole cameras equipped with PM-355 nuclear-track detectors were applied. Since during the previous studies within the PF-1000 facility the fusion-protons were measured at different angles to the axis in the horizontal plane [5], in the recent studies particular attention was paid to measurements around the z-axis. The proton measurements were carried out during two experimental campaigns: in autumn 2008 and in autumn 2009. During the first campaign the use was made of 8 identical pinhole cameras, which were oriented towards the center of the pinch column and fixed upon a circular support at different angles around the z-axis. During the second experimental campaign, in order to increase accuracy and amount of the collected data, the use was made of 20 identical pinhole cameras, which were placed at different angles upon the circular support shown in Fig.1.

Fig.1. View of the PF-1000 electrodes and supports of the pinhole cameras, which were used for measurements of fusion-produced protons at different angles to the z-axis and around that axis.

Experimental results

A quantitative analysis of those images, as performed by means of an automatic optical scanning microscope, has also confirmed the appearance of the strong azimuthal anisotropy. Results of the analysis of the fusion-proton images shown in Fig. 3, are presented in Fig. 4.

Fig. 4. Histograms of fusion-protons tracks, which were obtained from the analysis of detectors shown in Fig. 3 after their 6-hours etching.

Although the applied detectors were exposed to different PF-1000 discharges the observed azimuthal anisotropy of the fusion-protons has shown a quasi-periodical character. That effect has been explained as an effect of current filaments observed in PF-type discharges [2-3]. It was proved experimentally that filamentary structures of a current sheath and pinch column are quite reproducible, e.g., in the PF-360 experiment after a series of shots there were observed distinct spots on an edge of the inner electrode, which could be caused by reproducible current filaments only. A good macroscopic reproducibility was also observed in X-ray pinhole pictures of the pinch column in a high-current POSEIDON experiment [2]. In many experiment the filamentary images were very similar for discharges performed under identical experimental conditions. Therefore, in our research particular attention was paid to a role of local magnetic fields generated by different filaments [8]. We considered different configurations, i.e. a homogenous pinch column, the configuration of 6 or 12 linear filaments, and a funnel-like configuration. It was shown that for the pinch configuration, which contains linear filaments, fast fusion-protons can rotate close to the pinch column and be emitted in the upstream- or downstream-direction. Some fusion-protons can even rotate several times before leaving the pinch surrounding [9]. Detailed computations of fusion-proton trajectories showed that the azimuthal distribution of the fusion protons can have a number of distinct peaks corresponding to the number of the current filaments, as shown in Fig. 5.

During both experimental campaigns, to eliminate high-energy primary ions (mostly deuterons) and to record fusion-produced protons the use was made of PM-355 nuclear-track detectors shielded by absorption filters made of Al-foils of 80 µm in thickness. Those filters transmitted only fusion-protons of energy > 3 MeV and a small population of fast deuterons (of energy > 4 MeV). During the 2008-autumn campaign the shielded detectors were placed inside 8 pinhole cameras equipped with input diaphragms of 3 mm in diameter. Those detectors were exposed to 11 5 successive PF-1000 discharges with the total neutron yield of 1.7 x 10 neutrons. The etching of the irradiated detectors, which was performed under the standard conditions [7], delivered distinct images of fusion-proton sources, as shown in Fig. 2.

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Fig. 5. Examples of fusion protons trajectories computed for the configuration of 12 linear filaments and numerical modeling of the fusion-proton angular distribution around the z-axis.

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Fig. 2. Examples of color-enhanced images of the fusion-protons, which were recorded at different azimuthal angles during the PF-1000 experimental campaign in autumn 2008.

Fusion protons yield, a.u.

Taking into consideration the described influence of current filaments, the total yields of the fusion-protons recorded in the investigated PF-1000 experiments (see Fig. 4) were approximated by a regular sinusoidal curve presented in Fig. 6.

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A comparison of the obtained images showed a distinct azimuthal anisotropy, which seemed to have a periodical character. That conclusion was confirmed by a quantitative analysis of the proton images, which was performed by means of an optical microscope. In order to verify the findings about the azimuthal anisotropy of the proton emission, it was decided to perform a new series of measurements using 20 pinhole cameras, placed upon the cylindrical support described above. During the experimental campaign in autumn 2009 the PF-1000 machine was operated at 27 kV, 480 kJ. The shielded detectors, after their irradiation during 7 successive 11 discharges with the total neutron yield of 5.4 x 10 neutrons and subsequent etching, showed also distinct differences of images recorded at different angles, as shown in Fig. 3.

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Fig. 5. Examples of fusion protons trajectories computed for the configuration of 12 linear filaments and numerical modeling of the fusion-proton angular distribution around the z-axis.

Summary and conclusions The most important results of the described studies can be summarized as follows: 1. The fusion-proton images recorded in the PF-1000 facility showed a distinct azimuthal anisotropy; 2. The total numbers of the fusion-protons recorded by the pinhole cameras placed at different angles around the z-axis fit well to the 12-peak sinusoidal distribution; 3. This agreement suggests that in the investigated PF-1000 discharges there appeared a dozen or so of current filaments within the pinch column. One can conclude that the presented results are consistent with a model, which assumes a filamentary structure of the dense magnetized plasma column during the fusionproducts emission. Studies of this model should evidently be continued.

References

Fig. 3. Examples of color-enhanced images of the fusion-protons, which were recorded at different azimuthal angles during the PF-1000 experimental campaign in autumn 2009.

[1] W.H. Bostick, L. Grunberger, W. Prior, 3rd Europ. Conf. CF&PP, Utrecht 1969, I, p. 120. [2] M. Sadowski, H. Herold, H. Schmidt, M. Shakhatre, Phys. Letters 105A (1984) 117-123. [3] A. Bernard, H. Brudzone, P. Choi, H. Chuaqui, et al.., J. Moscow. Phys. Soc. 8 (1998) 93. [4] A. Malinowska, A. Szydlowski, J. Zebrowski, et al., AIP CP 812 (2006) 237-240. [5] A. Malinowska, K. Malinowski, M.J. Sadowski, et al., AIP CP 993 (2008) 353-356. [6] M. Scholz, B. Bienkowska, M. Borowiecki, et al., Nukleonika 51 (2006) 79. [7] A. Szydlowski, M. Sadowski, T. Czyzewski, et al., NIM B171 (2000) 379-386 [8] M.J. Sadowski, A. Malinowska, Czech. J. Phys. Vol. 56 (2006) B364-B370. [9] M.J. Sadowski, M. Scholz, W. Stepniewski, et al., Proc. 29th ICPIG, Cancun 2009, PA5-6.