Ion Energy Distributions from a Permanent-Magnet Helicon Thruster. Francis F. Chen, UCLA. Low Temperature Plasma Physics Webinar, January 17, 2014. The “New Stubby” helicon source. Note “skirt”. Antenna: 1 turn at 27 MHz, 3 turns at 13 MHz. Aluminum top plate.
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Ion Energy Distributions from a Permanent-Magnet Helicon Thruster
Francis F. Chen, UCLA
Low Temperature Plasma Physics Webinar, January 17, 2014
The “New Stubby” helicon source
Antenna: 1 turn at 27 MHz, 3 turns at 13 MHz.
Aluminum top plate
The top plate reflects the backward wave
The B-field is from a Neodymium magnet
The magnet is 5” OD, 3” ID, and 1” thick. We use the almost uniform field below the stagnation point.
The tube was designed with the HELIC code
D. Arnush, Role of Trivelpiece-Gould Waves in Antenna Helicon Wave Coupling, Phys. Plasmas 7, 3042 (2000).
Sample loading curves from HELIC
R should be > 1W at operating density
Operating point on “Low-field peak”
Different magnet arrays were calculated
Final design: single 3 x 5 x 1” magnet
Setting the antenna at 60 G
Discharge with the original magnet
Downstream density vs B and Prf
This shows that only 30 - 60 G is necessary.
Only an off-the-shelf magnet is needed
The magnet is 4” OD,
2” ID, and 1/2” thick
The plasma potential is set by grounding the top plate.
The experimental chamber
Typical density profiles at Ports 1-3
The SEMion ion energy analyzer
by Impedans, Ltd., Ireland
4” diam x 1 cm thick
The sensor height can be varied continuously
When the sensor is too close to the discharge, it forms an endplate, and the discharge is double-ended.
We know that the discharge is affected because the tuning is changed.
Gridded and Hall ion thrusters
A helicon thruster
A review of recent laboratory double layer experiments
Christine Charles, Plasma Sources Sci. Technol. 16 (2007) R1–R25
Cause and location of the “double layer”
F.F. Chen, Phys. Plasmas 13, 034502 (2006)
Bohm sheath criterion
A sheath must form here
Single layer forms where r has increased 28%
Ion energy distribution functions (IEDF)
Expect about 5 the KTe of 1.5-2 eV
Where a diffuse “double layer” would occur
IEDFs vs distance from source
close to tube
There is no sign of a double layer jump.
This is probably because the sensor changes the effective length of the discharge.
IEDFs vs RF power
Evidence of ion beam
IEDFs vs. pressure
Can we increase the ion drift speed?
Yes! Applying +24V to top plate
increases vi by ~16eV, while
applying -24V reduces vi by ~6eV.
The voltage is applied with a Pb-acid battery from an electric scooter.
Effect of top plate bias
A small helicon discharge was developed
using a permanent magnet for the B-field.
Ions are ejected with a drift velocity of
about 5KTe, measured with a retarding-
field energy analyzer.
The ion drift can be increased by biasing
the top plate of the discharge relative to
nearby grounded surfaces.
This device could be developed into a