Yung-Ta Sung

Experimental observation of ion beams in the Madison Helicon eXperiment

Argon ion beams up to Eb = 165 eV at Prf = 500 W are observed in the Madison Helicon eXperiment (MadHeX) helicon source with a magnetic nozzle. A two-grid retarding potential analyzer (RPA) is used to measure the ion energy distribution, and emissive and rf-filtered Langmuir probes measure the plasma potential, electron density, and temperature. The supersonic ion beam (M = vi/cs up to 5) forms over tens of Debye lengths and extends spatially for a few ion-neutral charge-exchange mean free paths. The parametric variation of the ion beam energy is explored, including flow rate, rf power, and magnetic field dependence. The beam energy is equal to the difference in plasma potentials in the Pyrex chamber and the grounded expansion chamber. The plasma potential in the expansion chamber remains near the predicted eVp ∼ 5kTe for argon, but the upstream potential is much higher, likely due to wall charging, resulting in accelerated ion beam energies Eb = e[Vbeam − Vplasma] > 10kTe.

Ion acceleration in a helicon source due to the self-bias effect

Time-averaged plasma potential differences up to 165 V over several hundred Debye lengths are observed in low pressure (pn < 1 mTorr) expanding argon plasmas in the Madison Helicon eXperiment (MadHeX). The potential gradient leads to ion acceleration greater than that predicted by ambipolar expansion, exceeding Ei ≈ 7 kTe in some cases. RF power up to 500 W at 13.56 MHz is supplied to a half-turn, double-helix antenna in the presence of a nozzle magnetic field, adjustable up to 1 kG. A retarding potential analyzer (RPA) measures the ion energy distribution function (IEDF) and a swept emissive probe measures the plasma potential. Single and double probes measure the electron density and temperature. Two distinct mode hops, the capacitive-inductive (E-H) and inductive-helicon (H-W) transitions, are identified by jumps in density as RF power is increased. In the capacitive (E) mode, large fluctuations of the plasma potential (Vp-p≳140V, Vp-p/Vp¯≈150%) exist at the RF frequency and its harmonics. The more mob...

Fast, hot electron production and ion acceleration in a helicon inductive plasma

A large, time-averaged, double layer-like plasma potential drop of 80 V over several hundred Debye lengths has been observed in the magnetic expansion region on the Madison Helicon eXperiment. It is operated in an inductive mode at 900 W and low argon operating pressures (0.12–0.20 mTorr) in the collisionless regime. The plasma space potential drop is due to the formation of a double layer-like structure in the magnetic expansion region and is much higher than the potential drop caused by a Boltzmann expansion. With the plasma potential drop, a locally negative potential ion hole region at lower pressures with a higher electron density than ion density has been observed just the downstream of the potential drop region. Two-temperature Maxwellian electron distributions with a warm ( Te≈15 eV) and bulk ( Te≈5 eV) components are observed just upstream of the double layer validated through a RF compensated Langmuir probe and an optical emission spectroscopy (OES) diagnostics. In the expansion chamber downstre...

Plasma self-bias and ion acceleration in the madhex helicon source

Summary form only given. Helicon and magnetized radio-frequency (RF) plasmas have been studied for some time. It is often claimed that the ion acceleration observed in these sources is due to double layers. Recently strong (160 V) time averaged self-bias has been observed in MadHeX1, resulting in a large potential difference between the plasma source and expansion regions2, has piqued our interest to further examine and fully realize the ion acceleration process. The modified MadHeX experimental facility consists of a 120 cm long, 10 cm inner diameter Pyrex tube attached to a stainless steel expansion chamber, which is 60 cm long and 45 cm in diameter (expansion ratio RE = 4.5) with an axial magnetic field, variable up to 1.2 kG at the source region that can be operated in flat or nozzle field configurations. An 18 cm long, 12 cm diameter half-turn double-helix antenna is used to excite helicon waves in the source. A new double magnetic mirror and an additional magnetic coil placed between the transition region between plasma source and expansion chamber are used to increase plasma ionization rate and reduce ion-electron recombination and neutral reflux in the expansion region. The effect of RF power, magnetic field strength and gas flow rate on the plasma parameters including electron temperature, density, plasma potential and ion beam acceleration are explored by probe diagnostics (Langmuir probe, emissive probe and retarding potential analyzer) and non-invasive optical techniques (laser induced fluorescence and optical emission spectroscopy). The role of substantial RF fluctuations in the plasma potential and the upstream end-plate boundary condition are addressed. The effect of the electron energy distribution that may include substantial tails on plasma self-bias and the ion beam formation and acceleration is examined by optical emission spectroscopy and cross-checked with the results via using a retarding potential analyzer. Also, its effect on the ion energy distribution is verified by using argon 668 nm laser induced fluorescence.

ION beam observation in the MadHex helicon source

The MadHex experimental system consists of a 150 cm long, 10 cm inner diameter Pyrex tube connected to a stainless steel expansion chamber 60 cm long and 45 cm in diameter (expansion ratio RE = 4.5) with an axial magnetic field, variable up to 1 kG at the source region that can be operated in nozzle or flat field configurations. An 18 cm long, 12 cm diameter half-turn double-helix antenna is used to excite helicon waves in the source. Ion beams of energy up to E = 160 eV at 500 W RF power have been observed in a flowing argon helicon plasma formed in the expanding region of the MadHex helicon source using a magnetic nozzle (RM = 1.44). The effect of flow rate/pressure, RF power and magnetic field strength on the ion beam acceleration, plasma potential, electron density and temperature are explored. The ion energy distribution function (IEDF) is obtained by a two-grid Retarding Potential Analyzer (RPA). The plasma potential is determined by a floating emissive probe and the electron density and temperature are measured by both single and double probes. The measured density decrease of ∼20 across the double layer in the magnetic expansion region does not fit a Boltzmann expansion but does agree with a conservation-of-flux calculation using the measured beam energy. Additionally, the axial ion velocities and temperatures are observed via argon 668 nm Laser Induced Fluorescence (LIF). The results of fast and slow argon ion beams at lower flow rates (∼1.3 sccm) with different RF powers are reported in the transition between Pyrex and expanding exhaust regions. The variation of the IEDF in the expansion chamber is also confirmed with RPA results.

Thruster evaluation in the MadHex helicon source

Summary form only given. The MadHex experimental thruster consists of a 150 cm long, 10 cm inner diameter Pyrex tube connected to a stainless steel expansion chamber 60 cm long and 45 cm in diameter (expansion ratio RE = 4.5) with an axial magnetic field, variable up to 1 kG at the source region that can be operated in nozzle or flat field configurations. An 18 cm long, 12 cm diameter half-turn double-helix antenna is used to excite helicon waves in the source. Ion beams of have been observed at RF power levels of 100-1,000 W in a flowing argon helicon plasma formed in the expanding region of the MadHex helicon source using a magnetic nozzle (RM = 1.44). The effect of flow rate/pressure, RF power and magnetic field strength dependence on the ion beam acceleration, plasma potential, electron density and temperature are explored. The ion energy distribution function (IEDF) is obtained by a two-grid Retarding Potential Analyzer (RPA). The plasma potential is determined by a floating emissive probe. The electron and ion beam distributions and plasma potential variation correspond to a plasma double layer. Additionally, the axial ion velocities and temperatures are observed via argon 668 nm Laser Induced Fluorescence (LIF) and density via a mm wave interferometer at higher RF power levels. The results of fast and slow argon ion beams at lower flow rates (~1.3-2 sccm) with different RF powers are reported in the transition between Pyrex and expanding exhaust regions. The variation of the IEDF in the expansion chamber is also confirmed with RPA results. The specific impulse, Isp(s), force/watt (mN/kW) and efficiency of the thruster will be discussed for the range of RF power, flow rate and magnetic field examined utilizing collisionless plasma theoretical models.

Ion acceleration in the MadHex helicon source

Summary form only given. The MadHex experimental system consists of a 150 cm long, 10 cm inner diameter Pyrex tube connected to a stainless steel expansion chamber 60 cm long and 45 cm in diameter (expansion ratio RE = 4.5) with an axial magnetic field, variable up to 1 kG at the source region that is operated in a nozzle configuration. A half-turn double helix antenna, 18 cm long and 12 cm diameter, is used to excite helicon waves in the source. Ion beams of energy up to E = 160 eV at 500 W RF power have been observed in a flowing argon helicon plasma formed in the expanding region of the helicon source using a magnetic nozzle (RM = 1.44). The effect of flow rate/pressure, RF power and magnetic field strength on the ion beam acceleration, plasma potential, electron density and temperature are explored. The ion energy distribution function (IEDF) is obtained by a two-grid Retarding Potential Analyzer (RPA). The plasma potential is determined by a floating emissive probe and the electron density and temperature are measured by both single and double probes. Additionally, the axial ion velocities and temperatures are observed via argon 668 nm Laser Induced Fluorescence (LIF). The measured density decrease of ~20 across the double layer in the magnetic expansion region does not fit a Boltzmann expansion but does agree with a conservation-of-flux calculation using the measured beam energy. The results of fast and slow argon ion beams at lower flow rates (~1.3 sccm) with different RF powers are reported in the transition between Pyrex and expanding exhaust regions.