Compact Electron Cyclotron Resonance Plasma Research Articles

Characterization of Hydrogen Plasma in an ECR based Large Volume Plasma Chamber

Hydrogen plasma characterization was carried out in a large volume (dia∼ 1.0 m, h∼ 1.0 m) plasma chamber to evaluate the efficacy of production of uniform, large-area H- beam for fusion applications. Up to seven Compact ECR Plasma Sources (CEPS; Indian Patent #301583, Patentee: IIT Delhi) can be mounted on the top dome of the chamber (one in centre and six on a ∼60 cm dia circle). Axially poled permanent ring magnets provide the magnetic field for each CEPS and the total field in the chamber is the combined field of all CEPS. Separate experiments were conducted with: (i) six CEPS with opposite polarity on adjacent magnets, (ii) seven CEPS with identical polarity on all the magnets, and (iii) a single CEPS at the center. In case (ii) the field lines repel while for (i) a cusp field is formed between adjacent sources. For case (iii), one obtains a uniform plasma density over 35 cm radius (n ∼ 4 ϗ 1010 cm-3, Te ∼1-2 eV), ∼ 60 cm downstream (from central source mouth) with 400W power at ∼ 1-3 mTorr pressure, yielding ideal conditions for volume mode H- production over a large area. For case (ii), however, plasma from each source flows along its individual field lines, without forming uniform plasma down-tream. Whereas for case (i), because of cross-talk between adjacent sources, the system became unstable, giving oscillations in plasma formation and microwave reflected power. Hence, it appears that the single CEPS configuration is the most efficacious for large area H- generation.

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.

The first measurement of plasma density by means of an interfero-polarimetric setup in a compact ECR-plasma trap

This paper presents the first measurement of plasma density by a K-band microwave polarimetric setup able to measure the magnetoplasma-induced Faraday rotation in a compact size plasma trap. The polarimeter, based on rotating waveguide OMTs (OrthoModeTransducers), has been proven to provide reliable measurements of the plasma density even in the unfavorable conditions λp≈ Lp≈ Lc (being λp, Lp and Lc the probing signal wavelength, the plasma dimension and the plasma chamber length respectively) that complicates the measurements due to multi-patterns caused by reflections of the probing wave on the metallic walls of the plasma chamber. An analysis method has been developed on purpose in order to discriminate the polarization plane rotation due to magnetoplasma Faraday rotation only, excluding the effects of the cavity resonator. The measured density is consistent with the previous plasma density interferometric estimations. The developed method is a powerful tool for probing plasmas in very compact magnetic traps such as Electron Cyclotron Resonance Ion Sources and for in-plasma β-radionuclides' decay studies.

Characterization of He-Induced Bubble Formation in Tungsten due to Exposure from an Electron Cyclotron Resonance Plasma Source

A compact electron cyclotron resonance plasma source has been utilized at Sandia National Laboratory to expose heated W samples (1270 K) to 50–75 eV He ions at fluxes on the order of 1019 m−2 s−1 and fluences on the order of 1024 m−2. Scanning electron microscopy (SEM) analysis of the surface has indicated bubbles up to 150 nm in diameter that exhibit signs of bursting near the surface. Comparisons have been made between W samples prepared from warm-rolled W sheet stock and ITER-Grade W rod stock. Focused ion beam (FIB) cross sectioning has been used with SEM and transmission electron microscopy (TEM) to identify large sub surface bubbles (100 nm diameter) at depths up to one micron as well as a dense layer of smaller bubbles (<10 nm diameter) within the first 100 nm of the surface, similar to bubble layers observed on higher flux experiments. SEM-Electron Backscatter Diffraction (EBSD) analysis has identified a unique surface morphology feature associated with the exposed ITER-Grade W as well as features similar to previous EBSD studies of rolled W stock. Thermal desorption spectroscopy (TDS) analysis has identified that pre-existing He bubbles found in the Sandia He-ion exposed samples do alter the D trapping and desorbing behavior in W. The findings from these preliminary characterization studies are presented and discussed in context with results from similar plasma exposure stages at other facilities around the world.

Studies on plasma production in a large volume system using multiple compact ECR plasma sources

This paper presents a scheme for large volume plasma production using multiple highly portable compact ECR plasma sources (CEPS) (Ganguli et al 2016 Plasma Source Sci. Technol. 25 025026). The large volume plasma system (LVPS) described in the paper is a scalable, cylindrical vessel of diameter ≈1 m, consisting of source and spacer sections with multiple CEPS mounted symmetrically on the periphery of the source sections. Scaling is achieved by altering the number of source sections/the number of sources in a source section or changing the number of spacer sections for adjusting the spacing between the source sections. A series of plasma characterization experiments using argon gas were conducted on the LVPS under different configurations of CEPS, source and spacer sections, for an operating pressure in the range 0.5–20 mTorr, and a microwave power level in the range 400–500 W per source. Using Langmuir probes (LP), it was possible to show that the plasma density (~1 − 2 × 1011 cm−3) remains fairly uniform inside the system and decreases marginally close to the chamber wall, and this uniformity increases with an increase in the number of sources. It was seen that a warm electron population (60–80 eV) is always present and is about 0.1% of the bulk plasma density. The mechanism of plasma production is discussed in light of the results obtained for a single CEPS (Ganguli et al 2016 Plasma Source Sci. Technol. 25 025026).

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.

Atomic-Oxygen Beam Source with Compact ECR Plasma

An atomic-oxygen beam source with compact ECR plasma was successfully investigated. The microwave was produced and transmitted in a coaxial mode, and coupled with the loop. The plasma was produced at a higher asymmetry magnetic mirror field, and neutralized with the molybdenum target at a lower asymmetry magnetic mirror field. The magnetic field was constituted with permanent magnets. This source has a higher flux density of atom beam, a lower operating pressure, a smaller power consumption and low-cost. When it was installed at the equipment to study the interaction of the beam with the surface, the operation was carried out very easily and with a good stability.

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.

Studies on enhancement of x-ray flux in the compact electron cyclotron resonance plasma x-ray source

The electron cyclotron resonance (ECR) plasma x-ray source has a potential application for medical imaging. The plasma and x-ray characterization studies on the ECR x-ray source based on cylindrical cavity operated in TE111 mode were reported earlier by us. In order to enhance the x-ray flux and the effective energy of the x-ray spectrum, the use of rectangular cavity in ECR x-ray source is studied. In this article, the theoretical analysis of electron acceleration in TE101 rectangular cavity used as a cyclotron resonance accelerator is presented. The electron orbital values of rectangular cavity are compared with that of cylindrical cavity (TE111 mode). It is found that there is an increase in final energy of electron (170 keV), reduction in electron transit time, and increase in distance between the successive orbits in the rectangular cavity than the cylindrical cavity. An ECR x-ray source based on rectangular cavity has been designed and constructed and the experimental system is described.

Experimental studies on a compact electron cyclotron resonance plasma x-ray source

A compact electron cyclotron resonance plasma x-ray source, which has a potential use for medical imaging is presented in this article. In this article, the experimental system and the characterization studies on plasma and x ray are presented. Using a Langmuir probe, the plasma parameters are measured for different magnetic field profiles and gas pressures. The x-ray spectrum is obtained for various gas pressures and magnetic field profiles. In the x-ray spectrum, the Bremstrahlung radiation, peaking at 20–60 keV is observed and the final energy of the x ray is extended up to ∼200 keV. Thermo luminescence dosimeter (CaSO4 sample) is used for estimating the dose at the port and these results are presented for typical x-ray spectra. Using a teletector, the dose at the port for various coil current is measured and these are compared with the estimated dose obtained from the x-ray spectrum.

Compact electron cyclotron resonance plasma source

A compact electron cyclotron resonance plasma source has been built and is being investigated in view of ion and x-ray production. A method recently developed to deduce the energetic electron temperature parameter from the bremsstrahlung spectra emitted by the plasma is adapted to the present plasma case and shows that this temperature parameter can reach a dozen keV. We show that the high-energy electrons are localized on a ring in the mirror midplane by obtaining the image of the x-ray emitters on the chamber walls with a pinhole camera. It indeed appears that the emitters are two rings, due to the hot electrons leaking towards the magnets from the midplane ring where they are located inside the plasma.

Compact Electron Cyclotron Resonance Plasma Source Optimization for Ion Beam Applications

A prototype electron cyclotron resonance (ECR) plasma source has been designed and developed to deliver a maintenance-free, stable and controllable current ion beam. The plasma source can accommodate both low and high power microwave applicators for different applications. Selection of the optimum magnetic field arrangement and position with respect to the microwave applicator ensures a high density plasma operational at pressures below 5.0×10-2 Pa. From initial tests a plasma density of 5.0×1010 cm-3, electron temperature of 1.5 eV and a plasma potential of 10 V were measured 10 cm downstream for an input microwave power of 100 W at an Ar pressure of 0.1 Pa. A saturation ion current density of 2 mA/cm2 was measured independently using a biased Langmuir probe and results matched well the calculated values. Ion beams with non-uniformity of ±10% have been used to sputter with control a diamond-like-carbon film with rates of up to 30 nm/h at room temperature.

Two-dimensional simulation of compact ECR plasma sources

In the present paper, argon plasma behaviour in a compact electron cyclotron resonance (ECR) plasma reactor is systematically studied by means of 2D simulation. Using a hybrid electron fluid - article ion plasma and particle neutral model, we have obtained the plasma potential, density, electron temperature, ionization rate, ion kinetic energy and velocity distributions and have found that the neutral gas pressure plays an important role in affecting all these distributions. In addition, non-uniform neutral gas depletion effects are examined by using a 2D Monte Carlo method. The depletion can obviously improve the plasma parameter uniformity. The average ion kinetic energy impacting on the substrate increases while the neutral gas pressure decreases. It is found that the plasma density is decreased by a factor of two and the profile becomes more uniform if non-uniform gas density is considered. Meanwhile, the ionization rate is sensitive to the non-uniform neutral density. The ion random kinetic energies of both the parallel and the perpendicular components are obtained and the ion rotating directed movement is observed. The perpendicular random energy near the top of the reactor can be much larger than that near the bottom.

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.

Compact electron cyclotron resonance plasma source for molecular beam epitaxy applications

We describe a compact, versatile electron cyclotron resonance (ECR) nitrogen plasma source specially designed to be mounted on a 35 conflat flange. The operation frequency is 2.45 GHz. The magnetic field is produced by permanent magnets. In order to eliminate ions in the plasma stream, a nonaxial geometry of the magnetic field was chosen. Two applications are discussed, namely, the p-type doping of II–VI tellurides and the growth of III–V nitrides. In both cases, use of the ECR plasma source results in high quality material.

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.