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

Ion Energy Distributions from a Permanent-Magnet Helicon Thruster

Francis F. Chen, UCLA

Low Temperature Plasma Physics Webinar, January 17, 2014


Ion energy distributions from a permanent magnet helicon thruster

The “New Stubby” helicon source

Note “skirt”

Antenna: 1 turn at 27 MHz, 3 turns at 13 MHz.

Aluminum top plate


Ion energy distributions from a permanent magnet helicon thruster

The top plate reflects the backward wave


Ion energy distributions from a permanent magnet helicon thruster

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.


Ion energy distributions from a permanent magnet helicon thruster

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).


Ion energy distributions from a permanent magnet helicon thruster

Sample loading curves from HELIC

R should be > 1W at operating density


Ion energy distributions from a permanent magnet helicon thruster

Operating point on “Low-field peak”

UCLA


Ion energy distributions from a permanent magnet helicon thruster

Different magnet arrays were calculated


Ion energy distributions from a permanent magnet helicon thruster

Final design: single 3 x 5 x 1” magnet


Ion energy distributions from a permanent magnet helicon thruster

Setting the antenna at 60 G


Ion energy distributions from a permanent magnet helicon thruster

Discharge with the original magnet


Ion energy distributions from a permanent magnet helicon thruster

Downstream density vs B and Prf

This shows that only 30 - 60 G is necessary.


Ion energy distributions from a permanent magnet helicon thruster

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.


Ion energy distributions from a permanent magnet helicon thruster

The experimental chamber


Ion energy distributions from a permanent magnet helicon thruster

Typical density profiles at Ports 1-3


Ion energy distributions from a permanent magnet helicon thruster

The SEMion ion energy analyzer

by Impedans, Ltd., Ireland

4” diam x 1 cm thick


Ion energy distributions from a permanent magnet helicon thruster

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.


Ion energy distributions from a permanent magnet helicon thruster

Gridded and Hall ion thrusters


Ion energy distributions from a permanent magnet helicon thruster

A helicon thruster


Ion energy distributions from a permanent magnet helicon thruster

Double-layer thrusters

A review of recent laboratory double layer experiments

Christine Charles, Plasma Sources Sci. Technol. 16 (2007) R1–R25


Ion energy distributions from a permanent magnet helicon thruster

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 distributions from a permanent magnet helicon thruster

Ion energy distribution functions (IEDF)

Expect about 5 the KTe of 1.5-2 eV


Ion energy distributions from a permanent magnet helicon thruster

Where a diffuse “double layer” would occur

UCLA


Ion energy distributions from a permanent magnet helicon thruster

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.


Ion energy distributions from a permanent magnet helicon thruster

IEDFs vs RF power


Ion energy distributions from a permanent magnet helicon thruster

Evidence of ion beam


Ion energy distributions from a permanent magnet helicon thruster

IEDFs vs. pressure


Ion energy distributions from a permanent magnet helicon thruster

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.


Ion energy distributions from a permanent magnet helicon thruster

Effect of top plate bias


Ion energy distributions from a permanent magnet helicon thruster

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.


Ion energy distributions from a permanent magnet helicon thruster

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