Energy and Power En gi neering, 2011, 3, 444-449
doi:10.4236/epe.2011.34055 Published Online September 2011 (
Copyright © 2011 SciRes. EPE
Study of Luminous Emission from a Coaxial Plasma
Discharge Device in the Presence of External Transverse
Magnetic Field
Tarek M. Allam, Hanaa A. El-Sayed, Hanaa M. Soliman
Plasma Physics and Nuclear Fusion Department, Nuclear Research Center, AEA, Cairo, Egypt
Received January 8, 2011; revised February 8, 2011; accepted March 10, 2011
The experimental investigations in this paper are focused on the study of luminous radiation emission from
coaxial plasma discharge device and the effect of applied transverse magnetic field Btr on it. The experiment
was done in (1.5 KJ - 10 KV) coaxial plasma discharge device. The discharge is operated in Nitrogen gas at
pressures from 1 to 2.2 torr. Helmholtz magnetic coils are placed outside the coaxial electrodes with its axis
at a distance = 3 cm from the coaxial electrodes muzzle, then Btr with a maximum induction 0.85 T is ap-
plied perpendicularly to the expanded plasma from the coaxial electrodes muzzle. The diagnostics used in
the measurements include a Rogowsky coil and a photomultiplier tube equipped with light collimator. The
experimental results showed that the maximum intensity of luminous radiation is detected at axial distance
(side view) z = 8 cm and gas pressure, P = 2.2 torr. It also showed that the maximum value of axial luminous
plasma zone velocity = 2.383 × 106 cm/s at z = 11 cm and P = 1.4 torr. In mode of presence of external Btr,
the investigations have shown that, at P = 1.4 torr the maximum intensity of luminous radiation (detected at
end-view position) is reduced by 17%, the full width at half maximum, FWHM of luminous radiation signal
is increased by 40 times, while the luminous radiation signal is delayed by ta = 438 µs. In two modes of op-
eration ta and FWHM have approximately a minimum values at P = 1.4 torr.
Keywords: Coaxial Discharge, Luminous Radiation, Transverse Magnetic Field, Energy Losses
1. Introduction
Several studies were made in the past to detect the influ-
ence of applied external magnetic fields on the plasma
behavior in coaxial plasma devices [1-5]. In coaxial
electrodes discharge of 0.5 kJ [1] the angular velocity of
plasma current sheath PCS in the interelectrode dis-
charge region was increased by addition of an externally
excited axial magnetic field of 500 G field along the co-
axial electrodes. In coaxial plasma discharge of 4.5 kJ [2].
It has been observed that, with the application of external
axial magnetic field along the coaxial electrodes of 3 kG,
the PCS was reduced from a complex multilayers struc-
ture to a single layer. If an externally excited axial mag-
netic field of (102 - 103 G) was introduced at the end of
the central electrode of 6 kJ coaxial discharge [3], the
decay rate of PCS was showed down. Study the effect of
applied magnetic nozzle fields on coaxial plasma dis-
charge showed that, the nozzle field will push the plasma
through the field so that the plasma leaves the coaxial
discharge without losing significant axial momentum [4].
Coaxial plasma discharge devices operating in a quasi-
stationary mode, turbulences in plasma flow occur causing
deformation of central electrodes. By using a ferromag-
netic insert an additional magnetic field was generated in
the coaxial electrodes. As a result, plasma turbulence and
consequent wear of the anodes were strongly reduced [5].
A transverse magnetic field which was trapped ahead
of the current sheath will reduce the ejection rate of
plasma which occurred during the collapse stage of
plasma focus discharge. This reduction should lead to a
more uniform plasma of large dimensions [6]. The PCS
at the coaxial electrodes muzzle had a conical shape with
a thin luminous column on the axis. If transverse mag-
netic field of 0.1 w/m2 was applied between the coaxial
electrodes at muzzle of the gun, the conical shape be-
comes more blunt and the central pinch becomes thicker
due to the magnetic flux trapped in it [7]. A transverse
magnetic field of 1 kG applied at the coaxial muzzle im-
pedes the motion of the PCS, and caused an increase in
the soft X-ray emission intensity and the PCS become
more stabilized [2,8]. Expanded PCS from the coaxial
electrodes muzzle restricted by a transverse magnetic
field of 280 G applied directly after a coaxial muzzle,
and the maximum velocity of expanded plasma was de-
creased by 33%. Also the expanded plasma was con-
tained by applied transverse magnetic field [9].
The purpose of the present paper is to examine the
behavior of luminous radiation emission from a PCS in
axial phase (side view) and the effect of applied trans-
verse magnetic field upon it (end view).
2. Experimental Arrangement
The coaxial plasma discharge device used in this paper
consists of coaxial discharge chamber, energy storage
system, the electrical power supply, the vacuum system
and the gas flow inlet system [10].
A schematic diagram of the coaxial plasma discharge
device is shown in, Figure 1. The device consists of two
stainless steel electrodes, inner and outer electrodes, with
diameters and lengths of 5 cm, 8.9 cm and 13 cm, 60 cm
respectively. The inner and outer electrodes are insulated
from each other by a tubular Perspex insulator of 1.5 cm
length. A rectangular glass window of length 40 cm and
width 0.4 cm parallel to the cylinder’s axis is used for
optical observation.
A capacitor bank of 30 µf, 10 kV is used to deliver a
maximum discharge current of 68 kA at charging
voltage of 10 kV from the power supply. The device is
connected to the condenser bank via a spark gap switch
and 12 coaxial cables. A high voltage pulse generator is
used to trigger the spark gap switch, which in turn dis-
charges the condenser bank. The inner electrode is nega-
tively polarized with respect to the outer electrode. Fig-
ure 2 shows the electrical circuit of the coaxial plasma
discharge device. The annular space between the two
coaxial electrodes is admitted with nitrogen gas with
pressure ranging from 1 to 2.2 torr after evacuation of
discharge chamber to a suitable air pressure 10–3 torr.
A pair of Helmholtz coils each of 21 turns with outer
diameter 24 cm and distance between centers is 12 cm is
placed on each side of the outer electrode of coaxial sys-
tem and a current of 2.413 KA passes through the two
coils to set a transverse magnetic field of 0.85 T.
A photomultiplier tube with wavelength range (400
nm - 700 nm) is used in this work to give information
about the luminous emission intensity from the coaxial pla-
sma discharge. A slit of dark glass pipe (light collimator)
is used to select a part of the illumination of the plasma
sheath luminous zone and a cable of optical fiber is used
to transmit the emitted light from the pipe to the pho-
tomultiplier slit. Figures 3 and 4 show the experimental
arrangement of the array of photomultiplier and the dis-
charge chamber in cases of side and end view respectively.
The data of experimental works were taken from an
average of approximately from 5 to 7 shots for each gas
pressures and axial distances under consideration.
3. Experimental Results and Discussion
The first part of this work deals with the behavior of lu-
minous radiation emission from PCS in axial direction
along the coaxial electrodes (side view).
In this study the nitrogen gas pressure is the dominant
parameter which affects the luminous radiation emission
from plasma sheath and the other parameters of device
under consideration are remain constant.
The maximum amplitude of the luminous radiation
intensity, Irad as a function of axial distance, Z and at
different nitrogen gas pressure is shown in Figure 5. It
can be seen from this figure that, the enhancement of Irad
is clear at P = 2.2 torr, z = 8 cm from the coaxial elec-
trodes breech, but Irad has a minimum value at the coaxial
Figure 1. Coaxial plasma discharge device.
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Figure 2. Electrical circuit of the coaxial plasma discharge
Figure 3. Experimental arrangement (side-view) of the ar-
ray of photo-multiplier to the discharge chamber.
Figure 4. Experimental arrangement (end-view) of the ar-
ray of photo-multiplier to the discharge chamber.
electrode muzzle, z = 12 cm and P = 2.2 torr. Intensity of
luminous radiation, Irad for all values of gas pressure and
at Z = 12 cm is almost the same and it has approximately
a minimum value, this may attributed to particle diffu-
sion in freely ionized gas and three body recombination
processes, also an increase of plasma temperature and
velocity at this position. In general increased or de-
creased of Irad is related to different parameters such as
plasma temperature and density as well as plasma veloc-
ity and energy losses from plasma.
From the arrival time of maximum amplitude of (Irad)
data, the average luminous plasma zone velocity at the
point of observation can be estimated during the axial
phase. The variation of average luminous zone velocity
VL as a function of axial distance z at a different gas
pressures is illustrated in Figure 6. It indicates that VL
gradually increases with increasing of axial distance, z to
reach a maximum value at the coaxial muzzle for all
values of gas pressures, except at P = 1 torr and at axial
distance from 5 cm to 9 cm, this situation probably due
Figure 5. The maximum amplitude of the luminous radia-
tion intensity as a function of axial distance at different gas
Figure 6. Axial distribution of plasma shea th luminous zone
to a some nonlinear phenomenon break the original be-
havior shown at P = 1.4, 1.8, 2.2 torr, which need a fur-
ther investigations. In general VL(max) is detected at P =
1.4 torr during the axial rundown phase, rather than P = 1
torr, due to inefficient of Snowplough behavior [3] i.e.
mass and current shedding effect fm, fc respectively at P =
1 torr play an important rule for PCS motion.
Variation of VL and Irad are plotted against the gas
pressures and different z as shown in Figure 7, this re-
sults verified that, at a distance approaches to muzzle end
the behavior of plasma sheath luminous zone velocity
with a gas pressure has an opposite version with respect
to the behavior of luminous zone intensity for most val-
ues of gas pressures. This behavior may due to, at axial
distances closes to coaxial electrodes the average plasma
sheath luminous zone velocity has a peak value for most
values of gas pressures under consideration, this behav-
ior may be due to less losses of plasma energy and in-
creasing of plasma temperature, then a reduction of lu-
minous radiation is occurred.
The second part of this study is concerned on the ef-
fect of applied external transverse magnetic field of
maximum induction Btr 0.85 T upon the luminous ra-
diation emission from PCS (end-view).
Figure 7. Variation of axial velocity and intensity of plasma
luminous zone versus gas pressure.
A Helmholtz magnetic coils have been placed outside
the outer electrode. The axis of them is located at Z = 3
cm from coaxial electrodes muzzle to produce Btr across
coaxial plasma discharge device and to detect the effect
of Btr on plasma flow from coaxial electrodes muzzle.
In this experiment, the slit of the collimator is oriented
at the end view of outer electrode axis at Z = 69 cm from
the breech to view optically through a glass flansh the
common end view of luminous radiation emission inten-
sity from PS, this system is shown in Figure 4.
The effect of applied Btr on the maximum value of lu-
minous radiation intensity as a function of gas pressure is
illustrated in Figures 8 and 9, these figures demonstrated
that in case of Btr = 0, Irad is increased with gas pressure
to reach a maximum value at P = 1.4 torr as Irad α P0.79
and then it decreases as Irad α P–0.34 but in case of applied
Btr, Irad α P1.62 in the range from P = 1 to 1.4 torr and Irad
α P–1.5 in the range from 1.4 to 2.2 torr.
Figure 8. (a, b) Variation of the maximum value of plasma
luminous intensity with gas pressure in case of (normal
mode operation).
Figure 9. (a, b) Variation of the maximum value of plasma
luminous intensity with gas pressure in case of applied ex-
ternal Btr.
The above relations illustrated that Irad is increased and
decayed with gas pressure with fast and slow rate respec-
tively in case of applied Btr than that in normal case of
Figure 10 shows the percentage ratio of the plasma
luminous radiation intensity in the two cases of operation
with respect to gas pressure. This result demonstrated
that Btr causes a reduction of Irad and the maximum per-
centage ratio of Irad (with Btr) and (without Btr) 17% is
detected at P = 1.4 torr and the minimum value of it
9.5% is obtained at P = 2.2 torr. Variation of arrival time,
ta of common plasma luminous radiation signal with gas
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Figure 10. Percentage ratio of the maximum value of pla-
sma luminous radiation intensity in cases of applied Btr and
normal mode operation versus gas pressure.
pressure in two modes of operation is shown in Figure
11. These variations have approximately the same be-
haviors for all values of gas pressure except in the range
from 1.2 to 1.4 torr the behavior has an opposite version.
Also, from this figure, it must be noted that, ta (with Btr)
is much greater than the corresponding values in case of
normal mode of operation, Btr = 0. Figure 12 shows the
relation between the full width at half maximum (FWHM)
of photomultiplier signal and the gas pressure in pres-
ence and absence of Btr. It can be seen from this relation
that, when Btr is applied (FWH M) has a greater value
than that in normal mode of operation. The minimum
values of ta and FWHM are detected approximately at P
= 1.4 torr for two cases of operation while a maximum
values of them are obtained at P = 1 torr (Btr = 0) and in
the range from 1.8 to 2.2 torr (Btr = 0.85 T) respectively.
In general Figures 10-12 illustrate that, the plasma flow
from the coaxial electrodes muzzle interacted with ap-
plied transverse magnetic field, then the plasma flow is
contained and restricted by this magnetic field, also its
motion is impeded [9] i.e. ta and FWHM in presence of
Btr are greater than in normal mode operation, Btr = 0.
Also a decrease in luminous radiation emission intensity
in presence of Btr may due to particle diffusion and three
body recombination processes, then presence of Btr
causes a decrease in energy losses from plasma and
plasma temperature raises may be expected.
4. Conclusions
The experimental work was done in a coaxial plasma
discharge device with a nitrogen gas at pressure varying
from 1 to 2.2 torr.
The study of the luminous radiation emerging from a
PCS during the axial phase is one of the most important
features because it is the most efficient channel of energy
losses from the plasma. In this study the luminous
plasma radiations emission are visualized on two direc-
tions (side on and end on) of coaxial plasma device by
using a photomultiplier with collimator system. The
Figure 11. Arrival time in case of normal mode operation
and case of applied external transverse magnetic field Btr
versus gas pressure.
Figure 12. Full width at half maximum in cases of normal
mode operation and applied external magnetic field versus
gas pressure.
measurements were carried out in absence and presence
of external applied transverse magnetic field of induction
0.85 tesla.
The axial distribution of Irad and VL were detected
along the coaxial electrodes and at a different values of
nitrogen gas pressures. The obtained experimental results
reveal that, Irad has a peak value at a distance closes to
coaxial electrodes muzzle, approximately at 8 cm and 10
cm for most values of gas pressures under consideration
afterwards it decreases with increasing of axial distance
to reach a minimum value at the muzzle, specially at P =
2.2 torr. From the above results, one can concluded that,
luminous radiation emission, Irad is due to excitation col-
lision processes. Increasing of the luminous radiation
intensity with axial distance can be contributed to the
increase of plasma density when the PS scrapes the rest
gas on its way along the coaxial electrodes while Irad
decay at the muzzle is almost attributed to particle diffu-
sion in fully ionized gases and three body recombination
processes. In general an increase or decrease of Irad is
related to different physical processes such as plasma
temperature and density, plasma velocity and energy
losses from plasma. Also experimental results illustrate
that VL is increased gradually with increasing of axial
distance for all values of gas pressure under considera-
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tion except at P = 1 torr and at axial distance from 5 cm
to 9 cm, the behavior of VL versus Z may probably due to
some nonlinear phenomenon break the original behavior
shown at P = 1.4, 1.8 and 2.2 torr, in future a more in-
vestigations will be carried out to study this behavior.
Moreover the dependence of Irad and VL on gas pressures
and at different axial distance, Z demonstrated that, at a
distance near the end of a coaxial electrodes at (Z = 8 cm -
11 cm), Irad has a vise versa behavior with respect to VL,
this may due to an increase of axial PCS force at axial
distances mentioned previously, then an average lumi-
nous PCS front velocity is increased and consequently a
low intensity of Irad is occurred during these distances as
a result of less losses of plasma energy and increasing of
plasma temperature.
Experimental results of variation of ta and FWHM
with a nitrogen gas pressures, illustrated that ta and
FWHM (in presence of Btr) are greater than the corre-
sponding ones for (Btr = 0). Also, a decrease of Irad in
presence of Btr is detected.
The conclusion obtained from the above results are as
follows, applied Btr causes a reduction of luminous ra-
diation emission intensity, Irad, delayed the arrival time,
ta and increased of FWHM of PCS luminous zone signal
i.e. Btr applied caused an impedes of the motion of PCS
along the coaxial electrodes system for all values of ex-
perimental parameters under consideration. Then in gen-
eral the energy losses from the plasma in the form of
luminous radiation was decreased (with applied of Btr)
and an increase in plasma temperature may be expected.
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