Download Effects of magnetic field gradient on cylindrical hall ion source

4
Effects of Magnetic Field Gradient on Ion Beam Current
in Cylindrical Hall Ion Source①
TANG Deli，ZHAO Jie，WANG Lisheng，PU Shihao，CHENG Changming，CHU Paul K.
A Hall ion source is a crossed electric and magnetic fields
The variable magnetic gradient is produced in the axial
device that emits ions that are accelerated in a quasi-neutral
discharge channel as shown in Fig.2. The positive magnetic
plasma. Typically, a strong electric field is established in the
gradient results in an upstream discharge channel and a
discharge region with a strong magnetic field. The magnetic
negative magnetic gradient forms the downstream one. The
field is large enough to magnetize electrons but not ions. The
maximum axial magnetic field appears below the annular
electrons experience an E×B flow in the azimuthal direction
anode, which is different from that in a conventional end Hall
when electrons move across the magnetic field. Therefore a
ion source.
Hall current is formed due to closed drifting of the electrons in
the E × B fields. Due to recent advances in magnetohydrodynamic methods for plasma acceleration, accelerators
employing closed drifting electrons in a plasma discharge in
crossed fields are practically promising. A Hall ion source with
a conical hollow anode and a circular open end was introduced
by Kaufman in 1987. In a conventional end Hall ion source, the
Fig.1. Schematic diagram of the cylindrical Hall ion source with a
magnetic field in the circular discharge region has mainly an
axial profile and decreases significantly from the main area of
magnetic field profile.
1——Discharge channel, 2——Cathode - outer magnetic pole, 3——
about 1 kG to the discharge exit with only about several tens of
Gauss. Therefore, the end Hall ion source has a negative
Annular anode, 4——Permanent magnet, 5——Cathode - inner magnetic
pole, 6——Inner shield, 7——Outer shield, 8——Back shunt.
magnetic gradient in the discharge channel, thereby making a
large number of axial electrons flowing upstream to the gas
distributor but resulting in low ionization efficiency of atomic
particles in the discharge region.
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Experimental setup
A Hall ion source with magnetron hollow cathode
discharge has been investigated in our laboratory. The ion
source which is schematically depicted in Fig. 1 consists of an
annular anode and a cylindrical hollow cathode enclosed by
magnetic poles and inner shield. The magnetic field is
Fig. 2. Axial magnetic field strength distribution.
produced by permanent, back shunt, inner and outer magnetic
poles. A cylindrical magnetic ring is shortened and centrally
inserted as an inner magnetic pole. The cylindrical，high
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Results and discussion
magnetic permeability tube in lieu of a rod magnet found in a
For a variable magnetic field gradient in the discharge
conventional end Hall ion source enhances the magnetic field
channel, the time-average force of non-uniform magnetic field
close to the annular anode in the discharge channel. A mirror
on electrons moving in a circular orbit can be calculated from
magnetic field profile in the discharge channel is formed and
the electron momentum equation:
1
0＝en∇ϕ－nkTe ∇B－∇ (nkTe )
B
the radial component at the open end exit is enhanced.
① Supported by the National Natural Science Foundation of China（10675040）
This paper has been published in Journal of Applied Physics, 2007, 102（12）
：123305
148
（1）
produces an electron trap in the axial direction. The axial
electrons are reflected between the inner and outer pole
cathodes along the axial magnetic field lines similar to a cold
cathode Penning discharge. Therefore, the axial electron loss is
reduced and more efficient ionization can be established by
multiple collisions among the oscillating electrons.
In our experiments, the effects of the magnetic field
profile on the performance of the cylindrical Hall ion source
are investigated. The ion beam current can be measured
downstream from the open exit by an electrostatic planar probe.
Fig.3. Saturation ion beam current versus discharge current.
The saturation ion beam current is measured by a －50 V
where e is the electronic charge, n is the electron density, k
negatively biased circular planar probe 50 mm downstream
is the Boltzmann constant, ϕ
is the potential, B is the
from the open exit. It is found that ion beam current is
magnetic field, and Te is the electron temperature. For an
influenced substantially by the magnetic field strength and
uniform plasma density, the potential difference in the plasma
magnetic gradient. The ion beam current versus discharge
required to balance the magnetic field force on the electrons
current increases with the permanent magnets as illustrated in
can be deduced from Eq.（1）
Fig. 3. When four magnets are used in the Hall ion source, the
ΔVp＝
kTe
B
ln
e
B0
ratio of the ion beam current to discharge current increases
（2）
slightly from 0.3 to 0.35 with increasing of discharge current.
where B and B0 are the magnetic field strengths at the two
However, 12 magnets change the ratio significantly between
locations and Vp is the plasma potential in the discharge
0.38 and 0.6 in the cylindrical Hall ion source when the
channel. According to Eq.（2）, it can be concluded that the
discharge currents are changed from 0.5 to 4 A.
The experimental results indicate that the conversion
plasma potential is more positive in the high magnetic field
region.
ratio of the low discharge current varies slightly when
Axial surveys of plasma potential in end Hall ion source
changing the magnets, that is, the magnetic field strength and
are found to be in approximate agreement with Eq.（2）. Owing
gradient. However, the conversion ratio increases substantially
to the negative magnetic field gradient in the end Hall ion
when the Hall ion source is operated in the high current mode.
source, the axial plasma potential decreases from the gas
It is concluded that the ion beam current increases with higher
distributor to the open end exit. It induces a substantial axial
magnetic field strength and gradient, especially for the high
electron current loss upwards to the gas distributor leading to a
discharge current. This is partly contributed by the saddle field
low ionization efficiency in the discharge channel. In a
profile in the discharge channel induced by the positive-
cylindrical Hall ion source with variable magnetic gradient in
negative variable magnetic gradient. This magnetron glow
the center, the axial plasma potential increases upstream and
discharge constitutes a hybrid ion source having the properties
diminishes downstream. Meanwhile, the radial magnetic field
of both an end Hall ion source and saddle ion source. The
has a negative gradient distribution from the annular anode to
saddle field can reflect axial electrons and effectively enhance
the central axis. Therefore, the location of the maximum axial
the ionization efficiency with a higher azimuthal hall current.
magnetic field strength has a minimum radial magnetic field
3
Conclusion
component. The radial plasma potential decreases from the
The magnetic field effects on the conversion ratio of the
annular anode to the center. The non-dimensional magnetic
ion beam current to discharge current in a cylindrical Hall ion
field distribution also brings forth a saddle profile in the
source are investigated. The positive-negative magnetic
discharge channel.
gradient provides saddle field profile in the plasma discharge
A non-uniform potential distribution has been found for
channel. The ion beam current increases with the magnetic
the highest electric field in the region where the magnetic field
field and a conversion ratio of up to 60% can be achieved 50
is high. The magnetic field gradient produces a non-uniform
mm downstream from the source exit. The strong magnetic
plasma potential distribution. It is consistent with Eq.（2）.
field and variable gradient enhance the efficiency of the ion
Therefore, a saddle plasma potential field may be formed in the
source.
discharge channel of a cylindrical Hall ion source which
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