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 Note “skirt”

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 > 1  at operating density

UCLA 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 P rf This shows that only 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 4” diam x 1 cm thick by Impedans, Ltd., Ireland

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, (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 KT e of eV

UCLA Where a diffuse “double layer” would occur

IEDFs vs distance from source close to tubefurther 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 v i by ~16eV, while applying -24V reduces v i 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 5KT e, 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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