Research on Mode Transition of Micro-Newton Cusped Field Hall Thruster

The micro-newton-level cusped field Hall thruster is an electric propulsion device that employs microwave-assisted ionization control. It serves as an actuator in drag-free control systems, ensuring control accuracy and stability by providing continuously adjustable thrust over a wide range. However, a mode transition occurring in the regulation process can lead to a sudden change in anode current, thereby degrading control precision and stability. Therefore, it is necessary to investigate the underlying patterns of mode transition. This study examines the variations in internal plasma parameters and discharge characteristics of the thruster before and after microwave mode transition, primarily through probe diagnostics. Experimental results indicate that prior to mode transition, the plasma luminous region is primarily concentrated within the electron cyclotron resonance (ECR) area, approximately 1–3 mm upstream of the anode. After the transition, the luminous region moves further upstream, and the plasma density near the anode exceeds the cutoff density, dropping sharply along the axial direction. The fundamental cause of the change in electron heating mechanism is the alteration in the propagation characteristics of fundamental waves due to this plasma density variation. When the plasma density rises to the cutoff density, the R-wave and O-wave, which drive ionization, are rapidly attenuated or reflected. At this point, the R-wave cannot reach the resonance layer, causing the dominant ECR ionization to become ineffective. The ionization mechanism shifts from being dominated by the R-wave and O-wave to being dominated primarily by the O-wave. Consequently, the electron heating mechanism shifts from volume heating to surface wave heating. This research will provide a basis for subsequently optimizing microwave transmission in the thruster and for reducing the threshold at which mode transition occurs.

The spectral temperature $T_{s}$ obtained from bremsstrahlung spectra emitted from electron cyclotron resonance (ECR) ion sources (ECRISs), in which the plasma is confined in a minimum-B magnetic field, is used as a relative indication of the temperature of the plasma hot electrons. Past bremsstrahlung measurements taken on ECRISs indicate that $T_{s}$ is strongly dependent on the magnetic field gradient at the resonance zone or $({B_{\mathrm{ min}}}/{B_{\mathrm{ ECR}}})$ . However, this dependence was never fully proven or explained. To further our understanding a more detailed study of the bremsstrahlung radiation for X-rays above 10 keV is underway using VENUS, a third-generation ECRIS at Lawrence Berkeley National Laboratory. Initial analysis of previous and new data has revealed that $T_{s}$ appears to be dependent solely on the minimum magnetic field $B_{\mathrm{ min}}$ rather than $({B_{\mathrm{ min}}}/{B_{\mathrm{ ECR}}})$ and the microwave frequency $\omega$ . Decoupling $T_{s}$ from $({B_{\mathrm{ min}}}/{B_{\mathrm{ ECR}}})$ , mainly $B_{\mathrm{ ECR}}$ , implies that $T_{s}$ does not depend on the magnitude of the heating frequency $\omega$ . While it certainly appears to be true that plasma density $n_{e}(\propto \omega ^{2}\propto B^{2})$ increases with heating frequency, as was postulated by Geller in 1987, a more careful consideration into the heating mechanism of the plasma electrons is warranted. The disassociation of $T_{s}$ from the heating frequency $\omega$ , while an interesting discovery, implies that we must change the way we understand how ECRISs operate. This paper presents new bremsstrahlung measurements, analyses, and discussions of the results.

We present experimental evidence of different behaviors of plasma parameters depending on changes in the electron energy distribution (EED), caused by an electron heating mechanism and electron-neutral collision processes in an Ar/He mixture inductively coupled plasma. At a low gas pressure of 3 mTorr, where the electron neutral collision frequency νm is much smaller than the driving frequency ωRF, the EEDs evolved from a bi-Maxwellian distribution to a Maxwellian distribution, due to the efficient heating of low energy electrons when the He flow rate increased at a fixed total gas pressure. The plasma density slowly decreased with the He flow rate portion ([He]/[Ar] + [He]) in a range of 0%–70%, while the plasma density largely decreased in the He flow rate portion of 70%–100%. On the other hand, at a high gas pressure of 350 mTorr where νm ≫ ωRF, the EEDs evolved from a Druyvesteyn-like distribution to a Maxwellian distribution, due to a cooling of low energy electrons and an increase in the population of high energy electrons, when the mixing ratio of the He gas is increased. In this case, plasma density abruptly decreased for a He flow rate ratio of 0%–30%. This result directly shows that the EEDs significantly affect the different variations of plasma parameters, even in the case of the same mixing ratio of the gases.

To investigate the transition from the low density mode to the high density mode in an electron cyclotron resonance (ECR) discharge, a Langmuir probe and an E field probe were used to measure ion density and E field intensity as functions of axial position and power. The experiments were performed in argon at 0.13 Pa in a 7.9 cm diam cyclindrical source chamber propagating TE11 mode 2.45 GHz microwave power. Low mode was characterized by a standing wave throughout the plasma chamber and minimal power absorption. High mode exhibited nearly complete power absorption and no standing wave past the ECR zone. A sliding short (ss) was used to determine if the position of an E field null in the source chamber affected the transition between these two modes for various magnetic field configurations. The ss position had little effect on mode transition, relative power absorption or ion density when positioned downstream from a broad, large volume resonance zone (resonance near the mirror midplane). However, the plasma could not be ignited if the short was placed at or upstream from the large volume resonance zone. If the magnetic field was adjusted to yield a sharp, small volume resonance zone (resonance midway between the midplane and the throat), then positioning the ss to force an E field null at the resonance zone would prevent plasma ignition, even at 800 W forward power. The ion density exhibited a hysteresis (i.e., a direct jump from no plasma to a plasma density observed at other sliding short positions for that same forward power, thereafter following the ion density versus forward power dependence observed at nearby ss positions) on power cycling when the sliding short was within ∼1 cm of the broad resonance zone or within ∼1 cm of forcing a null in the sharp resonance zone caes. The ion density versus power curve for the broad volume case exhibited a change in slope over the transition region (Pforward ≊40 W) when the ion density at the resonance zone was ≊1×1011 cm−3. For the narrow zone cases, a region of bistability (i.e., rapid plasma density fluctuations between low and high mode with little or no change in external input power) was observed for which the ion density fluctuated between a value near 5×1010 cm−3 in low mode and 5×1011 cm−3 in high mode. Similar transitions were observed in a 14.6 cm diam ECR source with a TM01 mode, indicating that the effect is not solely dependent on the microwave field structure in the ECR chamber.

The plasma parameters in the 2.45 GHz electron cyclotron resonance (ECR) proton ion source have been measured and optimized at low pressures. These parameters are important as they are related to the beam parameters of the ion source. A detailed plasma characterization study has been conducted using hydrogen as a working gas in the operating pressure and power range 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> -10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> mbar, 400-1000 W of the ion source, respectively. The plasma parameters have been measured in the ECR zone of the plasma chamber. An automatic Langmuir probe and data acquisition system has been used to measure these parameters. In the above-mentioned power and pressure range, the plasma density, plasma electron temperature, and plasma potential measured are in the range 4 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sup> - 6.5 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> , 4-11 eV, and 30-40 V, respectively. The variation of plasma parameters with neutral gas pressure is studied for different microwave powers. A 0-D (global) model has been developed to compare the measured plasma parameter data with the calculated ones. The ion species content in the plasma was measured after extraction using a magnetic mass analyzer and a Faraday cup. A plasma-based model has been developed to compare the measured ion species fraction data with the calculated ones by taking the measured ECR plasma parameters as input.

By using an improved hybrid Monte Carlo/fluid model with the metastable solver and power deposition scheme, we investigate the dynamic characteristics of metastable atoms and their influences on plasma conditions during mode transition, and moreover explore its role in hysteresis by searching the nonlinear mechanism. The evolution behaviours of metastable atoms with power deposition at different pressures are traced. Besides, the effects of metastable atoms and multistep ionization on the variation of plasma parameters, e.g. electron density, temperature and energy distribution function, etc, during the transition are systematically examined. When cycling the inputted electrical parameters, coil current and voltages, hysteresis does not appear. The basic characteristic of plasma dynamics during mode transition is not significantly influenced by the presence of metastable atoms. Moreover, a linearly increasing slope of plasma density with the deposited power is observed and no evidence of nonlinear mechanisms is detected.

It is essential to investigate the electron dynamics, particularly electron heating mechanisms in order to elucidate the bidirectional E ⇌ H transition in a low-temperature inductively coupled plasma (ICP), sustained by a radiofrequency (RF) power. E ⇌ H transitions are fully 2D or 3D phenomena, and non-invasive optical spectroscopy is appropriate for such studies. One example is a 1D-t image during one RF period. Another example is a 2D or 3D snapshot at each phase of the transition, captured using in computerized emission tomography. Most studies have been performed in argon, oxygen, or their mixture in an ICP driven at 13.56 MHz. In the present paper we review and explore the past three decades of research into the electron dynamics related to plasma sustenance at each phase of the E–H and H–E transitions, including our new results. The hysteresis of the internal plasma parameters as a function of external parameter is discussed in terms of a change of species density, particularly long-lived metastables in plasma in the bidirectional E–H transition.

The dominant losses from hot electron plasmas in the Princeton linear quadrupole device have been found to be radially outward. When this nonclassical radial loss was studied in detail, a surprising amount of variation was found in the amplitude of the loss rate as seen moving parallel to the current-carrying conductors, which led to the further discovery of spatial variations of internal plasma parameters, particulary plasma density. Efforts to correlate the anomalously large loss zones with local perturbations, obstructions, fluctuations, or variations in plasma heating and ionization were only partially successful. The periodic structure in plasma density and charged-particle loss rates apparently reflects stable, internally supported modes that have lifetimes longer than the plasma lifetime presently being considered; i.e., &amp;gt; 10τBohm.

A Langmuir probe was used to measure various electron plasma parameters in O2 capacitively coupled plasma. It was shown that the variation in these plasma parameters was due to changes in the electron heating mechanisms as the discharge conditions varied. The so called ‘α–γ’ mode transition in O2 plasma (100 mTorr) was identified from the power evolution (30–600 W) of the electron energy probability function (EEPF), electron density (ne) and effective electron temperature (Teff). The EEPF evolved from Druyvesteyn to bi-Maxwellian with increasing applied power which resulted in a rapid decrease and an abrupt increase in Teff and ne respectively. Comparisons were made to the same mode transition for similar conditions in Ar plasma. The EEPFs were Druyvesteyn in the α mode and evolved into a Maxwellian like EEPF in the γ mode of an Ar plasma. Two distinct trends of ne versus power was observed, it was shown that the measured rf current and rf voltage had a similar behavior. The pressure evolution of the EEPF, ne, and Teff was also investigated in O2 plasma operated at both 30 and 200 W. At 30 W the number of high energy electrons decreased and flattening of the low energy portion of the EEPF occurred with increasing gas pressure (10–100 mTorr) which indicates a collisionless to collisional heating transition. However, at 200 W the right combination of rf voltage and pressure was met for the discharge to evolve into the γ mode as the pressure increased. This was evident from significant narrowing of the EEPF as the pressure increased.

A high current Electron Cyclotron Resonance (ECR) proton ion source has been developed for low energy high intensity proton accelerator at Bhabha Atomic Research Centre. Langmuir probe diagnostics of the plasma generated in this proton ion source is performed using Langmuir probe. The diagnostics of plasma in the ion source is important as it determines beam parameters of the ion source, i.e., beam current, emittance, and available species. The plasma parameter measurement in the ion source is performed in continuously working and pulsed mode using hydrogen as plasma generation gas. The measurement is performed in the ECR zone for operating pressure and microwave power range of 10(-4)-10(-3) mbar and 400-1000 W. An automated Langmuir probe diagnostics unit with data acquisition system is developed to measure these parameters. The diagnostics studies indicate that the plasma density and plasma electron temperature measured are in the range 5.6 × 10(10) cm(-3) to 3.8 × 10(11) cm(-3) and 4-14 eV, respectively. Using this plasma, ion beam current of tens of mA is extracted. The variations of plasma parameters with microwave power, gas pressure, and radial location of the probe have been studied.

In this work, the effects of stringing termination capacitors on the external circuit parameters, plasma parameters, and mode transition in radio frequency (RF) inductively coupled Ar discharges are investigated. It has been demonstrated that at low pressure (1 Pa), in the absence of termination capacitors, the plasma parameters and external circuit parameters exhibit a continuous variation with increasing RF power. The plasma density is observed to decrease with decreasing capacitance value in the E mode when the termination capacitor is inserted, while the plasma density is increased with decreasing capacitance value in the H mode. During the E–H mode transition process, both the plasma parameters and the external circuit parameters undergo a discontinuous change characterized by a distinct “jump” in each parameter. By increasing and then decreasing RF power, the evolution of each parameter creates a significant hysteresis. As the termination capacitance decreases, the power threshold of the H–E mode transition decreases, resulting in a larger hysteresis loop. The termination capacitor, which is connected in series at the end of the coil, can alter the voltage distribution on the RF antenna. This alteration results in a reduction in the potential difference between the coil and the “common ground,” which effectively diminishes the electrostatic field. Furthermore, the electron energy probability function indicates that the addition of the termination capacitor results in a reduction in the proportion of energetic electrons in the E mode, accompanied by a reduction in the plasma potential.

The characteristics of the blue core phenomenon observed in a divergent magnetic field helicon plasma are investigated using two different helical antennas, namely right-handed and left-handed helical antennas. The mode transition, discharge image, spatial profiles of plasma density and electron temperature are diagnosed using a Langmuir probe, a Nikon D90 camera, an intensified charge-coupled device camera and an optical emission spectrometer, respectively. The results demonstrated that the blue core phenomenon appeared in the upstream region of the discharge tube at a fixed magnetic field under both helical antennas. However, it is more likely to appear in a right-handed helical antenna, in which the plasma density and ionization rate of the helicon plasma are higher. The spatial profiles of the plasma density and electron temperature are also different in both axial and radial directions for these two kinds of helical antenna. The wavelength calculated based on the dispersion relation of the bounded whistler wave is consistent with the order of magnitude of plasma length. It is proved that the helicon plasma is part of the wave mode discharge mechanism.

Sheet-shaped ECR plasma generation using permanent magnets for material processing

An electron cyclotron resonance (ECR) ion source 8 cm in diameter using a reentrant coaxial cavity and a ring cusp magnetic field was developed to improve plasma uniformity and operational reliability. The reentrant coaxial cavity is utilized to achieve electrical breakdown easily, to tune in variable electromagnetic fields and to introduce microwaves into plasma from an annular window of a discharge chamber sidewall. Microwaves are resonantly absorbed by the ECR magnetic field. Above 10 sccm flow rate for Ar, uniform and overdense plasmas were obtained in an area 4 cm in diameter from the center axis. The maximum plasma density of 1.26×1011 cm-3 was achieved on the center axis at 20 sccm. However, plasma densities for N2 and O2 were lower than the cutoff density. The maximum ion beam current density of 1.26 mA/cm2 was obtained at a forward power of 304 W for Ar.

In this study, a diagnostic investigation of plasma density and neutral gas temperature during mode transition in radio frequency (RF) inductively coupled Cl2 plasma is conducted using cutoff probes and fiber Bragg grating. A coupled electromagnetic and global model is used to analyze the underlying physical mechanisms. Results show that the threshold coil current for E–H transition increases with pressure (10–80 mTorr), accompanied by enhanced hysteresis. Hysteresis is also observed in the coil current response to RF power, with opposite trends exhibited during increasing and decreasing power near the mode transition. Capacitive coupling dominates at low plasma densities, while inductive coupling predominates at higher densities. The magnitudes of capacitive and inductive power are separately calculated to quantify their respective contributions during the mode transition, using an analytical electromagnetic model. It is indicated that the dominance alternates between capacitive and inductive coupling mechanisms as the electron density varies and that the mode transition occurs when their contributions become comparable. The distinct gas heating mechanisms in E- and H-modes account for the observed differences in neutral gas temperature. Experimental and simulation results both indicate significantly different power coupling efficiencies between the two modes. Notably, hysteresis emerges in simulations only when the actual gas temperature is considered. Simulations reveal that elevated neutral gas temperatures reduce the power required to sustain H-mode, suggesting that gas heating contributes to hysteresis behavior.