Experimental Study of the Variation of Neutron Emission Anisotropy in a Filippov-type Plasma Focus Facility

S. Goudarzi1, R. Amrollahi2, M.V. Roshan1
1. Nuclear Fusion Research Center, Atomic Energy Organization of Iran,
North Kargar Ave, PO. Box 14155-1339, Tehran, Iran
2. Amir Kabir University of Technology, Dept. of Physics, Hafez Ave, Tehran, Iran
This paper presents the results of experimental studies of the variation of spatial anisotropy in neutron emission with pressure and
discharge energy in a 90 kJ Filippov-type Plasma Focus device. The working gases are D2 and D2+1%Kr. The results of our
experiments have shown that the anisotropy factor decreases with increasing the pressure and/or energy. Furthermore, it has been
observed that by using D2+1%Kr as working gas, the variation in anisotropy factor with pressure and/or energy is relatively high, but
by using D2 it changes slowly. The highest neutron yield has been achieved by using D2+1%Kr, and we have studied the correlation
between neutron yield and anisotropy factor for this case at 16 kV discharge voltage and different pressures, at pressures around
optimum, the anisotropy factor increases with neutron yield, but at low and high pressures the anisotropy factor doesn’t change
significantly with yield.
Keywords: Plasma Focus, neutron yield, anisotropy factor
PACS: 52.59.Hq
Introduction
One of the most important characteristics of the neutron emission in Plasma Focus devices is a marked spatial anisotropy
in neutron emission that is very important in determination of the neutron production mechanism in these devices and is
inconsistent with thermonuclear mechanism for neutron production [1-5]. In this paper, the results of the study of
variation of spatial anisotropy in neutron emission in “Dena” Filippov-type Plasma Focus (90 kJ, 25 kV, 288 μF) with
working conditions are presented. On the basis of these results, some discussions have been made about the neutron
production mechanism in this device.
Experimental set-up
Our experiments were performed with “Dena” Filippov-type Plasma Focus device (90 kJ, 25 kV) shown in Fig.1. The
description of the device and its diagnostic systems has been reported elsewhere [6, 7]. Three Geiger-Muller (G-M)
counters were used for time-integrated neutron flux measurements. One G-M tube which is surrounded with an indium
foil (0.23 mm thickness) and placed in a polyethylene moderator and located at an angle of about 45° with respect to the
anode axis at a distance of about 30 cm from the center of anode is used for measuring the total neutron yield. The two
other counters are surrounded with silver foils (0.3 mm thickness) and placed in polyethylene moderators. One of them is
located at end-on position and the other at side-on position with equal distances from the pinch (about 90 cm). All of
counters have been calibrated respect to each other before the experiments.
Results and discussion
A series of discharges were carried out under different working conditions to study the variation of neutron emission
spatial anisotropy. For each working condition, 8-10 shots have been recorded and their average values are used for our
analysis. In these experiments the most neutron yield has been achieved with using D2+1%Kr as working gas and a conic
2
insert anode. Therefore, the major part of our experiments have been done with D2+1%Kr, however, some experiments
were done with pure D2.
In Figs. 2, 3 one can observe the variations of neutron emission anisotropy factor A= n(0o ) ϕ / n(90o ) ϕ with pressure at
16 kV discharge voltage with using D2 and D2+1%Kr as working gases, respectively. It is seen that in both cases the
anisotropy factor decreases with increasing the pressure, but the range of its variation in using D2+1%Kr (from 1.72 at
low pressures to 1.3 at high pressures) is more than the case of D2 (1.59 to 1.46). These facts show that the role of
beam-target interaction mechanism in neutron production decreases with increasing the pressure and there are some
differences between neutron production mechanisms for using D2 and D2+1%Kr.
In Figs. 4, 5 the variations of anisotropy at angle of 45° ( n(45o ) ϕ / n(90o ) ϕ ) with pressure have been shown for using D2
and D2+1%Kr as working gases, respectively. In both cases this parameter varies in a narrow range around 1 (for using
D2 from 0.9 to 1.05 and in the case of D2+1%Kr from 0.9 to 1.1), that is an evidence for weak role of beam-target
interaction mechanism in neutron generation in regions far from anode axis.
In Figs. 6, 7 the variations of anisotropy factor and n(45o ) ϕ / n(90o ) ϕ with pressure at 14 kV discharge voltage and using
D2+1%Kr have been plotted, respectively. It is seen that both average and maximum values of them are increased in
comparison with 16 kV discharge voltage, this is an evidence in favor of increasing the role of beam-target interaction
mechanism in neutron production.
For study the correlation between the total neutron yield and anisotropy factor in fixed working conditions, the variations
of anisotropy factor versus neutron yield from shot to shot using D2+1%Kr at 16 kV discharge voltage and initial
pressures of 0.68, 0.84 and 1.18 torr have been shown in Figs. 8-10, respectively. The results indicate that there is no
clear correlation between total neutron yield and anisotropy factor, however, it is seen that at pressures around optimum
(0.84torr), when the neutron yield increases the general behavior of anisotropy factor is increasing which shows that the
contribution of beam-target interaction mechanism in neutron production increases with the neutron yield, at lower and
higher pressures with changing the neutron yield the anisotropy factor does not change noticeably, it can be concluded
that the neutron production mechanism is relatively constant in these pressures.
In Figs. 11, 12 one can observe the variations of anisotropy factor and n(45o ) ϕ / n(90o ) ϕ , respectively, with discharge
energy at a constant pressure (0.84 torr) and D2+1%Kr as working gas. It is seen again that the values of n(45o ) ϕ / n(90o ) ϕ
are near to 1, in Fig. 11 it is seen that at low discharge energies (less than 20 kJ), the anisotropy factor has relatively high
values and decreases sharply with increasing the discharge energy, and at higher energies (20-37 kJ) it changes slowly. It
seems that at low energies the beam-target interaction mechanism is dominant and with increasing the discharge energy,
the contribution of this mechanism in the neutron production would reduce.
Conclusions
The observed values of anisotropy factor in our experiments were not high (in comparison with reported values up to 4
especially in Mather-type devices [5]), they are more than the expected values for moving boiler model and less than
predictions of beam-target interaction mechanism [8]. From these results, it can be deduced that both thermonuclear and
non-thermonuclear mechanisms are always present in neutron production, but the contribution of each mechanism
strongly depends on working conditions such as initial pressure, discharge voltage and gas composition. The measured
values of n(45o ) ϕ / n(90o ) ϕ show that the role of beam-target interaction mechanism in the regions far from the anode axis
is not considerable.
References
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