Correlation of Radiation and Electron and Neutron

Correlation of Radiation and Electron and Neutron Signals at PF- 1000 P. Kubes1, J. Kravarik1, P. Barvir1, D. Klir1, M. Scholz2, M. Paduch2, K. Tomaszewski2, I. Ivanova-Stanik2, B. Bienkowska2, L. Karpinski2 3, L. Ryc2, L. Juha3, J. Krasa3, M. J. Sadowski2,4, E. Skladnik-Sadowska4, L. Jakubowski4, A. Szydlowski4, A. Malinowska4, K. Malinowski4, H. Schmidt5 1) CTU Prague, Technicka 2, 166 27 Prague, Czech Republic, [email protected] 2) Institute of Plasma Physics and Laser Microfusion, 23 Hery, 00-908 Warsaw, Poland 3) Institute of Physics AS Czech Republic, Na Slovance 2, 182 21 Prague 8, Czech Republic 4) The Andrzej Soltan Institute forf Nuclear Studies, 05-400 Otwock-Swierk n. Warsaw, Poland 5) ICDMP, 23 Hery, 00-908 Warsaw, Poland Abstract. At the signals of x-rays usually 2 peaks were observed. The first peak corresponded to the time of the minimum diameter of the imploding plasma sheath (pinch phase) recorded by the visible frames. The second peak occurred 150-200 ns later at the time of the development of instabilities. High-energy electrons registered in the upstream and downstream directions differed in the intensity (ratio 3:1) and in the time of production. Their peaks correlated with x-rays. The energy of neutrons and time of their generation were determined by time-of-flight method from the pulses of seven scintillation detectors positioned in the axial direction. At the rise-time, each neutron pulse has registered downstream energies in range of 2.7–3.2 MeV. The final part of neutron pulse has isotropic energy distribution with energies up to 2.6-2.7 MeV. The evolution of the neutron pulses correlates with the visible frames. The first pulse correlates with the fast downstream zipper-effect of the dense plasma in the pinch and with the forming of the radiating ball-shaped structure at the bottom of the dilating plasma sheath. The second neutron pulse correlates with the second pinching and exploding of the plasma of lower density and with existence of the structure of the dense plasma positioned at the bottom of the dilating current sheath, similarly to the first pulse. The neutrons have a non-thermal beam-target origin. A possible influence of the zipper-effect on the acceleration of deuterons and on the plasma heating is discussed.

Keywords: plasma focus, fast energy particles generation, neutron production. PACS: 52.58.Lq

EXPERIMENTAL SET UP AND DIAGNOSTICS The reason why the plasma focus discharges are studied is a high efficiency of the x-ray, high-energy electron and ion and (if one uses deuterium as a filling gas) neutron production. A detail research of fusion processes at the plasma focus device had been performed on the Poseidon in Stuttgart [1-3]. The results presented in this paper were performed within the PF-1000 facility at the maximum collector current of 1.5-1.8 MA. The radiation from the visible to hard x-ray range was measured with temporal-, spatial- and energy-resolution. We employed one soft xray microchannel-plate (MCP) detector (4 quadrants and gating time of 2 ns), which was shielded with a 5.2-µm polyester (C8H8) to transmise the radiation in a window of 200-300 eV and above 600 eV. A PIN-silicon diode, covered with 3-µm-thick aluminized mylar-foil, detected x-rays in the same spectral window as MCP detector. Four optical frame cameras with a gating time of 1 ns and the inter-frame separation of 10 - 20 ns imaged the emitting plasma in the 10 nm visible spectral window around 589 nm. The Ne102a scintillator covered with 10-µm-thick Alfoil detected x-rays in the range above 4 keV. Fast electrons (energies above 50 keV and below 300 keV) were registered with three Cerenkov detectors (made of diamond or rutil crystals) located downstream (along the current sheath movement) and upstream (behind the hole of 5-cm-diameter in the top of the anode, in opposite direction to the current sheath movement) and side-on. Seven scintillation probes Ne102 with photomultipliers were used to

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perform the detailed time-resolved measurements of the hard x-ray and neutron emission. They were situated downstream (at distances of 7.0 m and 16.3 m) and upstream (at distances of 7.0 m, 16.3 m, 30.3 m, 44.2 m and 58.3 m). For neutron yield measurement indium and silver-activation counters were applied.

EXPERIMENTAL RESULTS AND DISCUSSION The soft x-ray peak at a PIN detector signal corresponds to the instant of a pinch phase with the minimum diameter recorded in visible-radiation frames taken with the optical frame camera. The peak of the PIN detector signal is assigned as time zero for each discharge. The time of this peak also correlates to the minimum of a current derivative. Signals from the PIN detector have usually 2 peaks. The first pulse has a rise-time of 20-30 ns. The second pulse (of the considerably lower intensity) has the maximum ca. 120-170 ns later. At the time of both pulses we usually observe the intense emission of hard x-rays above 50 keV. The hard x-rays have usually the maximum at the first soft x-ray peak. The hard x-ray emission lasts during 300 - 700 ns. High-energy electrons registered upstream and downstream differ in the intensity (with the ratio 3:1) and in the time of generation. The upstream pulse is detected 0-30 ns later than the downstream one and its maximum correlates with the first peak of soft x-rays. The peak of the second electron pulse is usually small one. A signal of electrons detected in the radial direction (perpendicularly to the axis of the PF configuration) is very week in comparison with both axial directions. An example of pulses from the shot no. 5055 is shown in Fig. 1. The production of fusion neutrons correlates with the x-ray emission, and in particular with hard x-rays of energy above 50 keV. The first neutron pulse (with the initial mean energy downstream equal to 2.7 – 3.0 MeV) starts together with the x-rays, i.e. 20 – 30 ns before the xray peak. At the same time the energies downstream are about 2.0 – 2.2 MeV. The maximum of neutron emission is observed ~ 30 ns after the x-ray peak. During the decrease of the first neutron pulse, the velocities of neutrons observed upstream increase. The final portion of the neutrons observed upstream has energies up to 2.6-2.7 MeV. The FWHM of the first neutron pulse is 50-70 ns. The second neutron pulse contains the number of neutrons 3-10 times higher than that in the first one. The energies of the neutrons measured downstream reach 3.0-3.2 MeV. The later temporal evolution of the second pulse is similar to that of the first pulse, hence velocities in upstream direction gradually increase with time. The final portion of neutrons has component upstream with energies up to 2.6-2.7 MeV. The FWHM of the second pulse is evaluated as 70-100 ns. The start of the emission of x-rays, electrons and neutrons correlates with the formation of an intensively radiating region of the pinch positioned about 2 cm in front of the anode top (2a). The intensity of this radiation imaged with filtered visible frames is a scale of plasma density. At this time –20 ns one was able to observe the start of the right part of the downstream zipper-effect in the radiating (dense) plasma region up to the distance of 6 cm from the anode front with the velocity of ∼ 3 x106 ms-1. This phase is finished at t = 0, i.e. in the time when the peak of soft x-rays appears and the narrow radiating pinch column of the minimum diameter equal to ∼ 5 mm and 5 cm in length is observed. At t = 0 we observe a continuation of the zipper-effect, i.e. motion of the left boundary of the dense pinch downstream (velocity ∼ 1 x106 ms-1) while the right boundary does not shown practically any motion (2b-2c). This phase is ended by the formation of a dense spherical structure of ∼ 1 cm in diameter situated 6-8 cm from the anode end, at the top of the focus region and at the bottom of the dilated current sheath (2c-d). Its lifetime was about 30-50 ns. The plasma density in the observed dense localities is evaluated from the laser interferograms presented in [1]. The parameters of the cited experiment and the obtained results were similar to ours. The plasma density of the most intensively radiating axial plasma-region, as seen in the recorded frames, reached 1019 cm-3. The MCP frames (3) recorded the XUV radiation above 600 eV within a window of 200-300 eV (the same as PIN detector. The images of the first pinch (3a – c) are 4-8 cm in length and 1-2 cm in diameter. This diameter is relative stable and considerably higher than that recorded in the visible frames. Hence, one can deduce that the XUV radiation was generated in the hot layers with a lower density, surrounding the dense pinch recorded in the visual frames. It should be noted that in the MCP frames the zipper-effect was not observed. Within the period t=50-100 ns (3d) the intensity of radiation decreased and we recorded only a contour of cooling pinch showing m=0 instabilities (2d). The final phase of the development of instabilities of the pinch column started at ~ 100 ns. In Figs. 2e-2h and Figs. 3e-3h we recorded the second pinch, i.e. the implosion of the week radiating surface that surrounded dark regions (of low temperature and density) of the first pinch. During this phase, the pinch diameter decreases and intensity of XUV with the plasma density increases. At t = 120 -130 ns the hot and dense second pinch was formed. This final phase of the pinch development correlated with the second maximum of x-rays. The explosion of the

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second pinch is probably very fast and we had no evidences of it upon the frame images. We recorded only a dense structure positioned ~ 8 cm in front of the anode, at the bottom of the dilated current sheath, similar to the structure after the first pinch. The neutrons produced in the second pulse t = 130 – 200 ns correlated with the second pinch, with its explosion. At the second pulse we have no evidence about zipper-effect probably due to small intensity of the radiation and small plasma density. 10 cm

anode

dI/dt

PIN

a) -30 ns

b) 0 ns

soft-X

e-down c) 30 ns

d) 50 ns

e-up

hard-x

e) 130 ns

f) 140 ns

n

-100

0

100

200

300 g) 160 ns o)

[ns]

FIGURE 1. Oscilloscope signals

h) 170 ns

FIGURE 2. Visible frames

anode

10 cm

a) -20 ns

e) 100 ns

c) 30 ns

b) -10 ns

g) 140 ns

f) 130 ns

d) 80 ns

h) 180 ns

FIGURE 3. MCP frames

The possible influence of the zipper-effect on the neutron production in the first pulse has 2 independent supports: (i) The initial velocity of zipper-effect ~ 3 x106 ms-1 corresponds to the kinetic energy of deuterons of ~


100 keV. According to [7], we need even the deuterons with kinetic energy of 100 keV in order to generate 3-MeV neutrons by the beam-target mechanism. (ii) The number of particles in zipper-effect volume ~ 5x1018 enables existence of the number of fast deuterons ~ 5x1017 with energies ~ 100 keV. This number agrees with estimations presented in [2]. If the number of particles in the target will be 1018, then for the cross-section of 3.7x10-26 m-2 of D – D reaction (for 50 keV deuterons) we obtain the total neutron yield 2x1010 in agreement with observations. Then the zipper-effect can play an important role in the acceleration of deuterons for beam-target mechanism of neutron production. The most of the neutrons has anisotropy distribution of energies, while the part of neutrons with isotropic distribution of energies (in both pulses) constitutes 20 – 30%.

ACKNOWLEDGMENTS This research has been supported by the research programs No. 6840770016, No. 1P04LA235, No. 1P05ME761 and No. LC528 of the Ministry of Education of the Czech Republic and the GACR grant No. 202-03-H162.

REFERENCES 1. 2. 3. 4.

A.V.Batyumin et al, Soviet Journal of Plasma Physics 16, 597-605 (1990). V. S. Imshennik, Soviet Journal of Plasma Physics 18, 349-71 (1992). D. McDaniel (private communication). H. Schmidt, Plasma Focus and Z-Pinch, Proc. II Latin American Workshop on Plasma Physics and Controlled Thermonuclear Fusion, edited by R Krikorian 1, (1987), pp. 1-30. 5. U. Jäger, H. Herold Nuclear fusion 27, 407-423 (1987). 6. R. Schmidt, H. Herold Plasma Physics and Controlled Fusion, 29, 523-534 (1987). 7. O. A. Anderson et al, Physical Review 110, 1375-1387 (1958).
