Session NM5: What's Next for Helicon Sources?

Dual-Source Operation of the HelCat (Helicon-Cathode) Device

The HelCat (Helicon-Cathode) device is a dual-source linear plasma device that has recently begun full operation at the University of New Mexico. HelCat is 4 m long, 50 cm diameter, with axial magnetic field $<$ 2.2 kG. An RF helicon source of tunable frequency 10 -- 30 MHz and P $<$ 5 kW, resides at one end of the device, while a thermionic BaO-Ni cathode capable of discharge currents up to 2.5 kA is located at the other end. Nominal parameters are: T$_{e} \quad \sim $ 5 -- 10 eV, n$_{e} \quad \sim $ 10$^{18}$ m$^{-3}$ (cathode), 10$^{19}$-10$^{20}$ m$^{-3}$ (helicon), plasma diameter 15 -- 20 cm. Diagnostics now online include electrostatic and magnetic probes, mm wave interferometry, visible spectroscopy, Mach probes, and LIF. We present first results of operation with both sources simultaneously.   

Development and Characterization of Inverted Helicon Plasma Sources

Helicon plasmas are useful as hot, dense sources requiring low magnetic fields. Since Boswell's$^{1}$ use of helicon waves in low pressure gas, research has strived to determine the wave-plasma coupling mechanism. Trivelpiece-Gould (TG) modes$^{2}$ remain a strong candidate. An inverted helicon plasma source uses a Nagoya Type III dielectric-covered helicon antenna, placed within a vacuum chamber. The antenna is 8.2 cm long, 2.2 cm in radius, using an frequency of 13.56 MHz. Basic dispersion relation theory is developed as an extension of existing helicon theory which includes TG modes and annular helicons$^{3}$. With this arrangement, diagnostic measurements are made inside and outside the antenna volume. To characterize the plasma, an RF-compensated Langmuir probe measures ne and Te, and 3 B-dot probes measure the field shape of the R, Theta and Z components about the antenna region in the R and Z directions. Typical ne and Te in an Ar plasma were found to be 3x10$^{18}$/m$^{3}$ and 3 eV. The goal of this work is to find another configuration to determine the method of efficient plasma heating. [1] R.W. Boswell, Phys. Lett. 33A, 457 (1970) [2] A.W. Trivelpiece and R.W. Gould, Jour. App. Phys. 30 (11) (1959) [3] M. Yano and M.L.R. Walker, Phys. of Plasmas. 13 (063501) (2006)   

The Toroidal Helicon Experiment at IPR

Successful demonstration of plasma current drive by low-frequency bounded whistlers, launched in a toroidal vacuum chamber of small aspect ratio, near lower-hybrid frequency, motivated the novel study of wave induced helicity current drive in helicon wave generated plasma. Helicon discharge, produced in a toroidal vacuum chamber of small aspect ratio, shows strong poloidal asymmetry in the wave magnetic field components. Owing to this strong poloidal asymmetry in the wave magnetic field structures, a nonresonant current is driven in plasma by the dynamo electric field, which arise due to the wave helicity injection by helicon waves. Simultaneous rise in wave helicity is observed when the input RF power is increased. Study of parametric dependence of plasma current in very high frequency operating regime, along with numerical estimations of nonresonant components, has been done. Based on the excitation of toroidal bounded whistlers, which sustain the discharge, a new approach to the problem of current drive is discussed. Close agreement between the numerical estimations and the experimentally obtained plasma current magnitudes clearly delineates the plasma current due to wave-induced helicity from other possible resonant or nonresonant sources at present parameter regime. Preliminary results of helicon current drive experiments and comparison with the numerical estimations are presented.   

Ar + CO$_{2}$ and He + CO$_{2}$ Plasmas in ASTRAL

Spectroscopy study of the ASTRAL helicon plasma source running Ar + CO$_{2}$ and He + CO$_{2}$ gas mixes is presented. ASTRAL produces plasmas with the following parameters: n$_{e}$ = 10$^{10}$ - 10$^{13}$ cm$^{-3}$, T$_{e}$ = 2 - 10 eV and T$_{i}$ = 0.03 - 0.5 eV, B-field $\le $ 1.3 kGauss, rf power $\le $ 2 kWatt. A 0.33 m scanning monochromator is used for this study. Using Ar + CO$_{2}$ gas mixes, very different plasmas are observed as the concentration of CO$_{2}$ is changed. At low CO$_{2}$ concentration, the bluish plasma is essentially atomic and argon transitions dominate the spectra. Weak C I and O I lines are present in the 750 - 1000 nm range. At higher CO$_{2}$ concentration, the plasma becomes essentially molecular and is characterized by intense, white plasma columns. Here, spectra are filled with molecular bands (CO$_{2}$, CO$_{2}^{+}$, CO and CO$^{+})$. Limited molecular dissociative excitation processes associated with the production of C I and O I emission are also observed. On the other hand, He + CO$_{2}$ plasmas are different. Here, rf matches are only possible at low CO$_{2 }$concentration. Under these conditions, the spectra are characterized by strong C I and O I transitions with little or no molecular bands. Strong dissociative processes observed in these plasmas can be link to the high T$_{e}$ associated with He plasmas. An analysis of the spectra with possible scientific and industrial applications will be presented.   

Results from the mini-Helicon Thruster Experiment

A mini-Helicon Thruster Experiment (mHTX) was designed to study possible space applications. High beam and gas utilization efficiencies are of major importance, as well as the compact design and system integration. We target gases with intermediate weight as diatomic nitrogen, monoatomic argon, and mixtures like air, operating at low $<$1kW 13.56MHz RF power. We find that higher magnetic fields $\sim $0.2-0.4T in a non-uniform configuration allow shortening the plasma source and achieving intensive collimated exhaust plume. Applied magnetic field is created by copper electromagnets and/or by permanent rare-earth magnets. Application brings other particularities to the design that will be mentioned. The mHTX gas discharge is characterized with UV-VIS spectroscopy using portable spectroscopic system. It shows high $>$90{\%} gas-to-plasma utilization. High resolution $\sim $0.01A allows measuring Doppler shift of the plume, which appears to be on the order of 10-20km/s. To articulate ionic line shift boron impurity seed is attempted. Emission data are cross-correlated with direct measurements of plasma parameters using various plasma probes and direct thrust-balance data. Additional diagnostics include those for the matching network and wall heat fluxes to analyze the circuit, plasma resistance, RF-to-plasma coupling and power redistribution.   

Power Balance in a Helicon Plasma.

We present results of a series of experiments exploring the mechanisms of power flow in a helicon plasma. The power absorbed by the plasma ultimately flows out in the form of heat through various channels. An infra-red camera records images of the quartz dielectric confinement tube outer surface, from which we calculate the total power lost to the glass as a function of position. Inside the vacuum chamber, Langmuir probes measure plasma density and temperature profiles, and bolometers measure the radial energy flux near the plasma edge. RF current and voltage sensors measure the forward power delivered to the plasma. Using these diagnostics, we can account for most of the input power, and we see that a large portion of the power is lost radially to the walls of the dielectric confinement tube.