Ion Extraction from a Helicon Plasma Source for an Inertial Electrostatic Confinement Fusion Device G.E

Ion Extraction from a Helicon Plasma Source for an Inertial Electrostatic Confinement Fusion Device G.E. Becerra, G.L. Kulcinski and J.F. Santarius Fusion Technology Institute, University of Wisconsin–Madison

Summary Faraday cup Ion Beam Current Measurement method • The plasma parameters of a helicon ion source coupled to an inertial electrostatic confinement fusion • Total cathode current is mostly device suggest that the extractable ion current should be higher than previously measured by a witness Witness-plate method secondary electrons. plate method. I • Ion current, more relevant to • A Faraday cup has been constructed to have a more reliable method for ion current measurement. meas fusion rates, is calculated from • Double probe measurements may only be valid below an ion-mass-dependent threshold magnetic field e- total measured current by strength. assuming a secondary emission 18 cm e- coefficient of a flat witness plate. • The ion source is likely operating in an inductively-coupled mode rather than a helicon wave mode, + I which would yield higher achievable plasma densities and extractable ion current. He meas IHe+ = tungsten 1 + γ e- • The applicability of γ(E) for this Inertial Electrostatic Confinement measurement can be problematic.

• IEC devices are well suited for • An electrostatic well between • A Faraday cup has been constructed and will be used to obtain a more studying advanced fuel fusion two concentric electrodes or reliable ion current measurement, since it is independent of secondary reactions requiring high ion semi-transparent grids confines emission characteristics. energies. Instead of heating a ions radially. Atomic and • A suppression electrode is impractical given the large observed beam bulk Maxwellian plasma, ions molecular interactions with size; instead, magnets are used to provide a transverse magnetic field are directly accelerated to fusion- background neutral gas greatly (up to ~500 G) to confine the secondary electrons to the cup. relevant energies. affect the ion energy spectrum.

Helicon Plasma Characterization HELIOS Experimental Setup Magnets • Double probe measurements of n0 and Te were made near the ion • HELIOS is an IEC device with an external extraction aperture at Prf = 1 kW, B0 ≤ 1 kG and p = 1–15 mTorr. helicon ion source, allowing for a lower • Plasma densities are low for the helicon wave mode, suggesting High-voltage background pressure to mitigate the ion the source is likely operating in the inductively coupled mode. feed-through energy spectrum softening by charge- • Hydrogen density unexpectedly peaks at an intermediate field IEC exchange reactions with neutrals. strength, possibly due to increased end losses at higher B0. chamber Helicon RF 3 3 • Similar density peaking using deuterium may indicate that probe • He- He fusion reactions in an IEC system plasma antenna were first demonstrated in HELIOS, at measurements are only reliable up to an ion-mass-dependent 3 ~10 reactions/s at Vcath = -134 kV and • A campaign is underway to enhance threshold field strength. Iion = 7 mA [1]. Higher rates are essential the ion current extracted from the 10 cm • Helium-4 densities increase monotonically within B-field range. for diagnostic investigations of IEC physics helicon ion source, as well as the • Previous witness-plate ion current measurements [1,2] are with helium-3 fuel. high-voltage capabilities. Helicon source significantly lower than the predicted Bohm current to the aperture according to previous line-intensity-ratio spectroscopic [1] G.R. Piefer, “Performance of a Low-Pressure, Helicon Driven IEC 3He Fusion Device,” Ph.D. thesis, UW–Madison (2006). measurements with hydrogen [3] and these double probe results. [2] S.J. Zenobia, “Effects of Helium Ion Implantation on the Surface Morphology of Tungsten at High Temperature for the Contact: [email protected] First Wall Armor and Divertor Plates of Fusion Reactors ,” Ph.D. thesis, UW–Madison (2010). Research supported by the Greatbatch Foundation. [3] E.C. Alderson, “Spectroscopic Diagnosis of a Dense Hydrogen Plasma Source,” M.S. thesis, UW–Madison (2008).