Enhanced Output from the High Power Helicon Resulting from Modification of the Downstream Magnetic Field with a Magnetic Nozzle Race Roberson, Robert Winglee, Tim Ziemba,

James Prager, University of Washington

The High Power Helicon plasma source developed at the University of Washington is capable of transferring tens of kilowatts of power into the plasma with source plasma densities near 2x1020 m-3.

Plasma density on-axis is increased, along with enhanced optical emission on-axis near the source

Optical emissions from the plasma detected with a high speed camera show an intense, axially peaked central core, typical of a helicon discharge.

The plasma source operates with a neutral pressure of 10-6 Torr downstream of the source. Inside the antenna neutral argon at 20 psi is released by a high speed puff gate.

Figure 1: High Power Helicon Plasma Source

Figure 2: Helicon Antenna, with gas feed and internal langmuir probe.

HPH Plasma Source The Addition of a Magnetic Nozzle

Figure 4: Ar Ion Gyro-radius near source axis.

The addition of the nozzle field keeps the ions magnetized farther downstream and provides a converging field for the plasma to flow into.

Figure 5: Optical emission extends from the source into the face of the nozzle

Figure 6: Density along the axis is measured with a planar probe. Additional nozzle field keeps more of the plasma near the axis

Figure 3: Axial Component of the Base magnetic field with and without the nozzle.

Downstream Plasma Acceleration with Nozzle

Figure 7: A Retarding field energy analyzer is used to determine Ar Ion velocities 67.5 cm downstream of the source and more than one nozzle width beyond the nozzle exit. Without the nozzle (red), we observe a fast ion population followed by a slower population. With the nozzle on (blue) we observe a fast population early in time but the most probable speed of the slow population increases over time.

The addition of the nozzle field results in an acceleration of Ar ions downstream of the source.

Though the exact process is still being investigated, multi-fluid computer models suggest this is the result of plasma self-focusing.

Improved Collimation with a Second Nozzle

Figure 8: Two Nozzle Configuration Figure 9: Optical emission between the source and nozzles is further increased.

Figure 10: Optical emission extends to the input of the second nozzle, nearly 1 meter downstream of the source..

Figure 11: Density along a radial cut upstream of the second nozzle is measured with a symmetric double langmuir probe. An initially broad profile in plasma density becomes narrow collimated beam as the shot progresses.

Further work remains to be done in characterizing the ion velocities downstream of this second nozzle, but optical emissions and langmuir probe measurements suggest an improvement in collimation and likely a further increase in ion velocities.