Ion energy distributions from a permanent magnet helicon thruster
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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. 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

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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

Note “skirt”

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”

UCLA


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


Double-layer thrusters

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)

Maxwellian electrons

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

UCLA


IEDFs vs distance from source

close to tube

further downstream

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


Summary

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

spacecraft thruster.


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