22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Plasma and neutral gas acceleration in a radial plasma source
G. Makrinich and A. Fruchtman
H.I.T. - Holon Institute of Technology, Israel
Abstract: In order to determine what the forces on the plasma in a Radial Plasma Source
(RPS) are, detailed measurements of profiles of the plasma variables along the acceleration
channel were performed for various gas flow rates. The measured plasma variables were
the plasma density, the electron temperature and the plasma potential. The profiles were
used for estimating the electric force on the ions and the plasma pressure. The estimated
electric force was found to be approximately equal to the measured force exerted by the
flow of the RPS on a balance force meter (BFM), as it should be, and to be considerably
larger than the force due to the plasma pressure.
Keywords: electric force, ion acceleration, plasma pressure
1. Introduction
Flowing plasma is generated in electric propulsion and
is often used for various industrial applications. In our
Radial Plasma Source (RPS) a mixed plasma and gas are
accelerated, plasma ions by electric force and the gas
particles by ion-neutral collisions. The plasma electrons
drift azimuthally across a magnetic field. We have shown
in recent years that the electric force on the flow is
increased by ion-neutral collisions [1-4]. The objective of
this work was to evaluate the electric force from the local
measured profiles of the plasma density, the electron
temperature and the plasma potential.
2. Method and Results
As already described in detail [1], the RPS, shown in
Fig. 1, consists of a ceramic insulator, a molybdenum
anode, a magnetic-field generating solenoid, an iron core,
a gas distributor and a cathode. The ceramic insulator is
composed of two annular disks and an axial segment
glued together. The outer diameter of each of the annular
disks is 77 mm, the inner diameter 30 mm, and the axial
distance between the two disks 5 mm. The molybdenum
cylindrical anode is of 48mm diameter, a 4.5 mm height
and 0.25 mm thickness. Together with the iron core, the
solenoid generates an axial magnetic field that is
concentrated at the outer edge of the iron core. For
neutralizing the ion-flow current, a cathode-neutralizer is
used, located 80 mm from the axis of symmetry of the
source. It emits electrons by heating a five-turn loop
filament of 10mm diameter and 15mm height, by a DC
current of 19 A. The loop is positioned inside a
molybdenum cylinder of 25 mm diameter, 45 mm height
and 0.25 mm thickness, to reduce heating other parts of
the source. Argon gas is injected through the gas
distributor. A voltage applied between the anode and the
cathode ignites a discharge and accelerates the plasma
ions radially - outward across the axial magnetic field.
In order to find out what the forces on the plasma are,
detailed measurements of radial profiles of the RPS
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Fig. 1. The Radial Plasma Source (RPS) schematically
(left) and during force measurement (right).
variables were performed. With the use of Langmuir and
emissive probes, radial profiles of the plasma density,
electron temperature and plasma potential were measured,
for various gas flow rates. The radial profiles were
measured for various gas flow rates through the anode
distributor with a magnetic field intensity of 136 G and a
discharge current of 1.9 A. The gas flow rate was varied
from 13 to 100 SCCM and, respectively, the pressure in
the vacuum chamber varied from 2.5 to 11.5 mTorr. The
applied voltage was decreased with the gas flow rate from
106 to 63 V. For each gas flow rate the force exerted by
the mixed ion-neutral flow exiting RPS on the balance
force meter (BFM) was measured.
The measurements were taken at five different radial
distances from the RPS axis of symmetry (28, 32, 38, 46,
and 70 mm). The probe was moved by a one-dimensional
positioning system. These measurements were used for
estimating the plasma pressure and the electric force on
the ions.
Fig. 2 shows three forces as a function of the gas flow
rate. The first force, denoted as 𝐹𝑒𝑒 , is the force by the
total mixed ion-neutral flow exiting RPS-1, derived from
measurements by the BFM outside the RPS, as described
above. This force is calculated as
𝐹𝑒𝑒 = (𝐹1 − 𝐹2 )2𝜋𝑟𝑏 /𝑐.
(1)
1
This evaluation is an overestimate of the force due to the
plasma pressure force. Even this overestimated force due
to the plasma pressure, 𝐹𝑝𝑝 , is smaller than 𝐹𝑒𝑒 , so that
plasma pressure cannot be the main source of 𝐹𝑒𝑒 . The
third force, denoted 𝐹𝐸 , is the electric force that was
estimated from
𝑟
𝐹𝐸 = 2𝜋𝜋 ∫𝑟 𝑐 𝑛𝑖 𝑒𝑒𝑒𝑒𝑒 ,
𝑎
Fig. 1. The force by total mixed ion-neutral flow (𝑭𝒆𝒆 ),
the force due to the plasma pressure ( 𝑭𝒑𝒑 ), and the
electric force (𝑭𝑬 ), all versus the gas flow rate. The
magnetic field is 136 G and the discharge current 1.9 A.
Here, 𝐹1 is the total force measured by our BFM, 𝐹2 is
the force exerted by the gas flow immediately after the
RPS discharge is turned off, 𝑟𝑏 is the distance between
the BFM and the axis, and 𝑐 is the width of the BFM
sensing plate (in the azimuthal direction). In our
experiment 𝑟𝑏 = 70 mm and 𝑐 = 20 mm. The height of the
sensing plate is 20 mm and is much larger than the
distance between the disks 𝑏 (which is 𝑏 = 5 mm). It is
assumed therefore that all the flow in the azimuthal angle
of the BFM impinges on the sensing plate and no ions or
neutral flow above or below it in the axial direction.
The second force denoted as 𝐹𝑝𝑝 is the force due to the
plasma pressure. This force is estimated as
𝐹𝑝𝑝 = 𝑘𝐵 𝑛0 𝑇𝑒0 2𝜋𝑅𝑠 𝑏,
(2)
where 𝑛0 and 𝑇𝑒0 are the plasma density and electron
temperature at the location of maximal plasma
pressure, 𝑅𝑠 is the RPS outward radius (𝑅𝑠 = 40 mm).
2
(3)
in which the plasma density and the radial electric field
measured locally (not shown here) were used. From
Fig. 2 we see that the electric force 𝐹𝐸 estimated from Eq.
(3) is in good agreement with the force 𝐹𝑒𝑒 calculated
from measurements by the BFM. Therefore, the electric
force is the significant force in imparting momentum to
the ion-neutral flow.
3. Conclusions
The electric force was evaluated from the local
measured profiles of the plasma density, the electron
temperature and the plasma potential. The evaluated
electric force was found to be similar to the measured
force and both considerably larger than the force due to
the plasma pressure.
4. Acknowledgments
This research has been supported by Israel Science
Foundation (Grant No 765/11).
5. References
[1] G. Makrinich and A. Fruchtman. Phys. Plasmas,
16, 043507 (2009)
[2] G. Makrinich and A. Fruchtman. Appl. Phys. Lett.,
95, 181504 (2009)
[3] G. Makrinich and A. Fruchtman. Phys. Plasmas,
20, 043509 (2013)
[4] G. Makrinich and A. Fruchtman. Phys. Plasmas,
21,
023505
(2014)
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