Compact ECR Plasma Source Research Articles

Thrust evaluation of compact ECR plasma source using 2-zone global model and plasma measurements

Plasma thrusters are required for deep space applications for reducing the fuel payload. This paper evaluates the capability of a compact ECR plasma source (CEPS) as a standalone plasma thruster in space. The unique magnetic field profile of CEPS’ ring magnets enables very efficient plasma production and electron heating within the CEPS plasma source section (ID ≈ 8.5 cm, length ≈ 11.6 cm) resulting in high plasma potentials. Experiments were undertaken with the CEPS attached coaxially to a larger expansion chamber (ID ≈ 48.2 cm, length ≈ 75 cm). Diagnostics included specially designed Langmuir probes (LP) and ion energy analyzers (IEA) that are capable of withstanding the harsh plasma conditions in front of and within the CEPS. LP measurements reveal high-density argon plasma (≈1012 cm−3), with high bulk electron temperatures (≈20 eV) and plasma potentials (≈100 V) at very modest microwave power (≈600 W) over a wide range of pressures (0.3–1 mTorr). An important observation is the fall of almost the full plasma potential inside the CEPS within a short distance close to its exit, giving rise to strong ambipolar electric fields suitable for accelerating ions. IEA measurements confirmed the presence of streaming ions with energies ≈87 eV at ≈2 cm in front of the CEPS. Estimates based on the IEA results yield thrust values ≈50 mN at ≈0.5 mTorr. A 2-zone, global model was developed to determine the steady state plasma parameters (including plasma flow velocity) in the geometry of the experimental system (without magnetic field). The model reproduced the key experimental plasma parameters and was extended to compute plasma thrust under actual space-like conditions. The computed thrust values for xenon are higher (≈80 mN) than that for argon (≈40–45 mN) at ≈600 W of microwave power.

Investigations on argon and hydrogen plasmas produced by compact ECR plasma source

Argon and hydrogen plasmas were produced by a Compact ECR Plasma Source (CEPS) attached coaxially to a large chamber. This paper presents characterization results of the two plasmas using a newly designed Langmuir Probe. Experiments in argon were conducted to benchmark the plasma parameters and to determine the efficacy of the CEPS for thruster applications, recommended by recent results (Ganguli et al 2019 Plasma Sources Sci. Technol. 28 035014), while the hydrogen experiments were undertaken to determine the typical range of plasma parameters for assessing the usefulness of CEPS for different applications. In argon plasma, high densities (≈1012 cm−3), high electron temperatures and plasma potentials are obtained at fairly low pressures ≈0.3–0.5 mTorr. The plasma potential drops substantially (≈65 V) within the CEPS itself, demonstrating its suitability as a plasma thruster. On the other hand, plasma densities are lower for hydrogen in front of the CEPS, and the electron temperatures and plasma potentials, higher. The hydrogen experiments have helped establish an important aspect of such plasmas. At low pressures (≈0.5 mTorr) plasma density is relatively low with a single, high temperature electron population; on the other hand, at higher pressures (≈6 mTorr), the plasma has two electron populations, a low temperature, high density population and another low density, high temperature warm population. For hydrogen this transition from the former (with single electron population) to the latter state (with two electron populations) occurs at a pressure, ≈3 mTorr while similar transition for argon occurs at ≈0.3 mTorr. Apart from the observed two states of plasma, each state with its own distinctive properties, another important feature of the hydrogen plasma is that the axial density profiles obey approximately n/B = constant although no such scaling is observed for argon.

Evaluation of compact ECR plasma source for thruster applications

The performance of a plasma thruster is assessed for efficient propulsion by analyzing the unique downstream characteristics of plasma produced by a Compact ECR Plasma Source (CEPS). For this, data presented in an earlier publication (Ganguli et al 2016 Plasma Sources Sci. Technol. 25 025026), wherein plasma produced in the CEPS is allowed to flow into a small stainless (SS) steel chamber, are revisited so as to throw new light on the nature of the plasma flowing out from the CEPS. It is shown that the normalized axial density profiles of the bulk and warm electrons (on account of their being strongly magnetized) follow closely the normalized profile of CEPS axial magnetic field expanding into the SS chamber, yielding the scaling n/B ≈ constant over a wide range of pressures. In addition, the bulk electrons obey the Boltzmann relation indicating that they are in thermal equilibrium and also satisfy the double adiabatic equation of state. Analysis of the data using a simple model that takes into account plasma flow, ambipolar electric field and collisions is also presented. It is shown that efficient thruster operation is possible only at low pressures (≤0.1 mTorr). Empirical estimates for argon yield peak thrusts of ∼2.5 mN (at 0.5 mTorr) and 7.5 mN at 0.05 mTorr, for moderately low microwave powers of ∼500 W. Physics issues relevant to CEPS are considered for improvements in future thruster designs.

Development and studies on a compact electron cyclotron resonance plasma source

It is well known that electron cyclotron resonance (ECR) produced plasmas are efficient, high-density plasma sources and have many industrial applications. The concept of a portable compact ECR plasma source (CEPS) would thus become important from an application point of view. This paper gives details of such a CEPS that is both portable and easily mountable on a chamber of any size. It uses a fully integrated microwave line operating at 2.45 GHz, up to 800 W, cw. The required magnetic field is produced by a set of suitably designed NdFeB ring magnets; the device has an overall length of ≈60 cm and weighs ≈14 kg including the permanent magnets. The CEPS was attached to a small experimental chamber to judge its efficacy for plasma production. In the pressure range of 0.5–10 mTorr and microwave power of ≈400–500 W the experiments indicate that the CEPS is capable of producing high-density plasma (≈9 × 1011–1012 cm−3) with bulk electron temperature in the range ≈2–3 eV. In addition, a warm electron population with density and temperature in the range ≈7 × 108–109 cm−3 and ≈45–80 eV, respectively has been detected. This warm population plays an important role at high pressures in maintaining the high-density plasma, when plasma flow from the CEPS into the test chamber is strongly affected.

Characterization of a compact ECR plasma source and its applications to studies of helium ion damage to tungsten

Exposure of tungsten to low energy (<100 eV) helium plasmas at temperatures between 900–1900 K in both laboratory experiments and tokamaks has been shown to cause severe nanoscale modification of the near surface resulting in the growth of tungsten tendrils. Tendril formation can lead to non-sputtered erosion and dust formation. Here we report on characterization of a compact electron cyclotron resonance (ECR) He plasma source with an ion flux of ∼2.5 × 1019 ions m−2 s−1, average fluence of 3 × 1024 ions m−2, and the surface morphology changes seen on the exposed tungsten surfaces. Exposures of polished tungsten disks at temperatures up to 1270 K have been performed and characterized using scanning electron microscopy and atomic force microscopy (AFM) scans. Bubbles and craters have been seen on the exposed tungsten surface growing to up to 150 nm in diameter. The ECR source has been tested for eventual use on a scanning tunneling microscopy experiment intended to study the early stages of surface morphology change due to He ion exposure.

A Mirror-like ECR Plasma Source for Ionosphere Environment Simulator

A compact mirror-like ECR (electron cyclotron resonance) plasma source for the ionosphere environment simulator was described for the first time in China. The overall sources system was composed of a 200 W 2.45 GHz microwave source, a coaxial 3λo/4 TEM-mode microwave resonance applicator, column and cylindrical Nd-Fe-P magnets, a quartz bell-shaped discharge chamber, a gas inlet system and a plasma-diffusing bore. The preliminary experiment demonstrated that ambi-polar diffusion plasma stream into the simulator (∼500 mm long) formed an environment with following parameters: A plasma density ne of 104 cm-3-106 cm-3, an electron temperature Te < 5 eV at a pressure P of 10-1 Pa-10-3 Pa, a plasma uniformity of > 80% over the experimental target with a 160-mm-in-diameter, satisfying primarily the requirement of simulating in a severe ionosphere environment.

A study of plasma propulsion system with RF heating

Some experiments and computer simulations to develop a plasma propulsion system by using electron and ion cyclotron resonance (EICR) plasma are reported. The EICR thruster is an advanced space propulsion concept that has possible uses in satellite station keeping and interplanetary travel. An experimental apparatus to generate EICR plasma was constructed in 1999. It consists of a cylindrical vacuum chamber (inner diameter of 21 cm and axial length of 150 cm), RF power supply, and eight electromagnets. It is run at a pressure in the 10 −2 Pa range with Ar or He gas as propellant. This experiment aims to generate plasma (without electrode) by ECR and to accelerate ions (without grid) by ICR under a proper magnetic field configuration. We also plan to develop a compact ECR plasma source that uses hot electron ring for ion acceleration.

Energy-selective electron cyclotron resonance plasma for controlled surface reaction processes

A new compact electron cyclotron resonance (ECR) plasma source for future plasma-assisted processing was developed. The source allows for improved control and direct monitoring of the discharge. Control of the ion energy distribution by movement of the magnetic field is demonstrated for the first time in a compact ECR plasma source. The ion energy can be tuned from 10 to 30 eV. An explanation based on different ambipolar diffusion coefficients for different magnetic fields is outlined. Nitrogen-plasma-induced surface reaction processes on sapphire substrates, with a view to achieving growth of a thin A1N buffer layer prior to group III nitride growth, was investigated using this novel energy-selective ECR plasma source.

Modeling the electron heating in a compact electron cyclotron resonance ion source

An electromagnetic particle-in-cell (PIC) model and a guiding-center particle model are developed and used to model a compact electron cyclotron resonance (ECR) plasma source. The finite-difference time-domain technique is used to model the microwave fields which excite the plasma at 2.45 GHz. The PIC technique is used to model the dynamics of the electrons in the plasma. The electromagnetic fields and the plasma dynamics are solved in a self-consistent manner. The ECR heated electrons are confined to magnetic field lines and subsequently make multiple passes through ECR regions experiencing both increases and decreases in energy. The distribution function of these energy changes is determined from the electromagnetic PIC model and used in a guiding-center particle model. The longer time scale collisional phenomenon in the plasma is modeled using this guiding-center particle model. A compact ECR plasma source used for the generation of ions for materials processing is simulated. This source has a plasma size of 3.6 cm in diameter and 3 cm in height.

Comparison between a two-dimensional simulation and a global conservation model for a compact ECR plasma source

We compare results from a two-dimensional simulation of plasma transport in a compact electron cyclotron resonance (ECR) source with predictions from a spatially averaged global model based on particle and energy conservation. Model predictions include plasma density, plasma potential and electron temperature. In addition, the models predict the fraction of input power lost to various processes in the plasma. The models agree to within about 10% over the conditions examined: 0.5-10 mTorr and 850-1500 W in a model argon gas. The global model can be used to establish scaling laws, and it also provides insight into the overall plasma behaviour in terms of general conservation principles. The two-dimensional simulation is essential when spatial profile information is crucial, such as for predictions of plasma uniformity.

Three-dimensional electromagnetic PIC model of a compact ECR plasma source

A three-dimensional electromagnetic particle-in-cell (PIC) model is developed and used to model a compact ECR plasma source. The finite-difference time-domain (FDTD) technique is used to model the microwave fields which excite the plasma at 2.45 GHz. The PIC technique is used to model the dynamics of both the electrons and ions in the plasma. The grid structure used is constructed using the cylindrical coordinate system. Techniques which permit stable numerical solutions in the cylindrical coordinate system are developed and described. The electromagnetic fields and the plasma dynamics are solved in a self-consistent manner. A compact ECR (electron cyclotron resonance) plasma source used for the generation of ions for materials processing is simulated. This source has a plasma size of 3.6 cm in diameter and 3 cm in height. Simulation results of microwave power absorption, plasma potential, and microwave electric fields are presented. Distributed computing techniques are explored to handle the large computer memory requirements and the long computer simulation times associated with the three-dimensional particle-in-cell plasma model. >

Optimally Stable Electron Cyclotron Resonance Plasma Generation and Essential Points for Compact Plasma Source

This study examines the strong dependence of electron cyclotron resonance (ECR) plasma generation on the microwave conditions and the magnetic field profiles in the ECR plasma. When an introduced microwave frequency oscillates and has a large ripple, the ECR plasma around the ECR region vibrates and regularly turns on and off. The microwave oscillation and the microwave ripple must be eliminated to prevent poor etching anisotropy. Moreover, the nonuniform plasma causes disturbed ion motions due to magnetohydrodynamic (MHD) plasma instability. It is found that uniform and high-density ECR plasma is generated by optimizing the waveguide, the location of the ECR region and the magnetic field profiles for precise ULSI patterning. Based on these results, we determine the essential features for realizing a compact ECR plasma source.

Development and Applications of a Compact Electron Cyclotron Resonance Source

A new compact ECR plasma source has been developed. The characteristics of this source and it's applications are discussed. Irradiation by oxygen radicals O* for the oxidation during deposition process was found to produce high quality superconducting thin films with increased characteristic temperatures. Hydrogen radical H* beam cleaning of GaAs substrate surfaces was achieved at temperatures as low as 100°C.

Potential applications of an electron cyclotron resonance multicusp plasma source

An electron cyclotron resonance (ECR) multicusp plasmatron has been developed by feeding a multicusp bucket arc chamber with a compact ECR plasma source. This novel source produces large (about 25 cm diam), uniform (to within ±10%), dense (&amp;gt;1011 cm−3) plasmas of argon, helium, hydrogen, and oxygen. It has been operated to produce an oxygen plasma for etching 12.7 cm (5 in.) positive photoresist-coated silicon wafers with uniformity within ±8%. Results and potential applications of this new ECR plasma source for plasma processing of thin films are discussed.