EHT scientist have developed helicon-based plasma sources for customers for material science and fusion science applications based on previous experience with high power helicons through previous work at the University of Washington developing the High Power Helicon Experiment (HPHX) for in-space thruster applications. This work also led to the DOE funded SBIR development of the Electrode-less Plasma Source for startup of fusion science experiments. EHT is prepared to help customers with the development of high density plasma source for a wide range of applications.

HPHX was designed to produce a high density plasma with a high bulk velocity to explore magnetic nozzle physics and helicon physics in the high pulsed power regime. HPHX used a half-wave helical antenna and operated using a pulsed power supply that could source up to 1 MW to the antenna with typical absorbed plasma powers near 50 to 60 kW. In this regime, HPHX could rapidly ionize a high density puff of gas (~90% ionization), which could be expanded into a larger vacuum vessel. Operational frequencies vary from 100 kHz to 2 MHz, with 1 MHz being typical for most work. This frequency range is between the ion cyclotron and the electron cyclotron frequencies. The wave oscillates too rapidly to affect the ion motion; however, the electrons can respond to the wave.

When operated in this manner, HPH produces high-density plasma (> 10²⁰ m^-3 in argon) in the source region under the antenna. The figure shows a peak plasma density greater than 8×10¹⁹ m^-3 in hydrogen. The plasma is subsequently accelerated downstream axially away from the gun’s source region. The mechanism for the plasma acceleration is an important feature of the gun and is discussed in detail below. Plasma acceleration allows for plasma to be created away from the main experiment, thus keeping the gun’s physical and magnetic surfaces remote, limiting negative confinement effects. Figure 3 shows the experimental configuration with an individual HPH and an end on view of its placement in the solenoid B₀ magnets used to generate the gun’s local magnetic field.

An interesting consequence of operating HPH in high power pulsed mode is that the plasma produced by the gun is seen to be highly collimated with a directed bulk flow away from the antenna. The ratio of the axial plasma velocity to the sound speed or the ion sonic Mach number has been measured to be at least 3 to 6 using argon and possibly higher when hydrogen is used. The plasma continues to be accelerated as it transits downstream away from the source region. The mechanism for this acceleration may in part be a consequence of the expanding magnetic field geometry, which could produce a magnetic nozzle effect.

Another possible reason for the high Mach number plasma could be a direct consequence of the whistler wave that is created by the high power antenna and prorogates downstream of the source region. Experimental evidence from HPH suggests that significant azimuthal plasma currents are produced and maintained during the discharge and that these currents are well correlated with antenna turn on/off suggesting they are directly produced by the gun antenna. The plasma currents are seen to extend tens of centimeters downstream of the antenna and are possibly produced and sustained in a similar way that a rotating magnetic field (RMF) antenna builds and sustains the azimuthal currents used for a FRC.

Figure 2 shows the measured magnetic component of the propagating whistler wave field as a function of axial position downstream of the source. The measurements were taken from 20 cm to 60 cm downstream of the source using an on axis B-dot probe. This observation demonstrates that the gun produces a circularly polarized wave that propagates downstream. The measured dispersion relationship is consistent with a whistler mode. The wave structure rotates in time azimuthally. As with an RMF current drive scheme, electrons can respond to this wave and can be driven in the azimuthal direction, which will in turn produce a magnetic perturbation to the initial B₀ magnetic field.

The change to the B₀ magnetic field due to the presence of the plasma currents has been measured experimentally downstream of the gun. The measurements of ΔB_z were taken from 8 cm to 58 cm downstream of the source on axis (here z is the axial coordinate). The perturbation to the field (ΔB_z) points in the opposite direction of the B₀ magnetic field. The plasma is initially created in a region of high magnetic field strength (~400 G) and propagates downstream. As the plasma moves downstream, the B₀ field is getting weaker due to the dipole-like expansion of the B₀ field. The perturbation to this field due to the plasma is shown in the firgure along with Langmuir probe current at the location of B-dot probe and the antenna current profile. Here the B-dot and Langmuir probe are located 60 cm downstream of the antenna. This magnetic field perturbation is produced by an azimuthal current. From the B-dot magnetic field data we can estimate the size of this current density which is approximately 2.2 kA/m² at 20 cm downstream of HPHX. A consequence of this plasma current and the magnetic geometry creates an optimum condition, which can explain the high directed velocity seen with HPHX and the basis for an efficient electrode-less plasma gun.

The figure shows the plasma current and magnetic geometry of HPHX. Here the B₀ dipole field is shown in black and the magnetic field perturbation in magenta. This perturbation is produced by the azimuthal plasma current (red), which is diamagnetic with respect B₀ and corresponds to the direction of whistler wave coupling with the plasma electrons in a similar manner of the RMF current drive. The interaction of the azimuthal current with the base magnetic field produces a j x B force that is in the direction shown. There are two components of this force. The first component is the result of the interaction of azimuthal current with the radial component of the magnetic field, which produces a force on the plasma that point downstream of the source. This force results in acceleration in the direction that is observed experimentally downstream of HPH. The second component is the result of the interaction of the azimuthal current with the axial component of the magnetic field, which produces a force that is radially inward. This force acts to confine the plasma on the axis of the field. The vector sum of these two components is in the direction that is shown in the figure (Blue).

The ion velocity distribution function for a hydrogen plasma has also been measured downstream of HPHX. The distribution shows that the ions have a bulk velocity of 60 km/s and a spread of 20 km/s. Measurements of the bulk magnetic field of the hydrogen plasma also indicate that a j´B may be responsible for the acceleration of the plasma downstream. Again the acceleration is a direct consequence of the current produced and the geometry of the base magnetic field.

 

References

J. Prager, R. Winglee, T. Ziemba, R. Roberson, G. Quetin, “Ion energy characteristics downstream of a high power helicon” Plasma Sources, Sci, Technol., 7, (2007).

R. Winglee, T. Ziemba, L. Giesch, J. Prager, R. Roberson, “Simulation and laboratory validation of magnetic nozzle effects for the high power helicon thruster,” Physics of Plasmas, 14, 063501, (2007)

J. Prager, T. Ziemba, R. Winglee, and B. R. Roberson, “Wave propagation downstream of a high power helicon in a dipolelike magnetic field,” Physics of Plasmas, 17, 013504 (2010).

B. R. Roberson, R. Winglee, J. Prager, “Enhanced diamagnetic perturbations and electric currents observed downstream of the high power helicon,” Physics of Plasmas, 18, 053505 (2011).

A. C. Hossack, B. S. Victor, T. Firman, J. S. Wrobel, T. R. Jarboe, T. Ziemba, J. R. Prager, “Reduction of plasma density in the HIT-SI experiment by using a helicon pre-ionization source,” Rev. Sci. Instrum. 84, 103506 (2013).