Plasma Experimental Device Research Articles

Design of the electron cyclotron resonance (ECR) plasma source for the space plasma environment research facility.

The Space Plasma Environment Research Facility (SPERF) was built in Harbin to study the three-dimensional magnetic reconnection and wave-particle interactions relevant to space physics in laboratory settings. A 2.45GHz Electron Cyclotron Resonance (ECR) plasma source is adopted in the device to simulate the Earth's magnetosphere and achieve the scientific goals. In this paper, the design of the ECR plasma source is presented. The structures of the microwave source, the microwave transfer system, and the antenna are introduced. Additionally, the resonant surfaces are computed to predict the locations of microwave absorption. The absorption mechanisms of the microwave in the SPERF are also discussed. The discharge experiment demonstrates the utility of the ECR source in simulating the Earth's magnetosphere. The successful operation of the source indicates that the ECR discharge is a powerful tool for creating a plasma environment in a large plasma experimental device.

Fluid simulations of resistive drift-wave turbulence with diamagnetic flow in ZPED experiments

We derive a diamagnetic resistive fluid model (DRF) and develop an associated two-dimensional fluid simulation code (DRF-2D) to explore the dynamics of resistive drift modes within the plasmas of the Zheda Plasma Experiment Device (ZPED). The validation of the linear dispersion relation for the DRF-2D code revealed a harmonious agreement between analytical theory and linear numerical simulations. Leveraging plasma parameters obtained from the ZPED experiments, we conducted a comprehensive series of nonlinear simulations using the DRF-2D code. Our simulations successfully replicate the nonlinear trends in turbulent fluctuations and transport observed in the ZPED experiments, particularly demonstrating a remarkably accurate alignment of the turning point in the magnetic field. Notably, the DRF model sheds light on the observed frequency sign reversal from the electron diamagnetic direction to the ion diamagnetic direction in the ZPED experiments. This is demonstrated through well-matched turning points in the confining magnetic field between the nonlinear simulations and ZPED experiments. The fidelity of our model in capturing these phenomena underscores its efficacy in providing valuable insights and predictive capabilities for the intricate dynamics observed in the ZPED plasmas.

Excitation of cylindrical and spherical precursor solitons in a flowing dusty plasma: Experimental and simulation studies

We report the first laboratory observation of precursor cylindrical and spherical solitons excited in a flowing dusty plasma. The experiments are carried out in an inverted Π-shaped dusty plasma experimental device in which a dust cloud is created in a background Argon plasma using micrometer-sized Kaolin particles. Using the single gas injection technique, the dust fluid is made to flow in a controlled manner over a cylindrical (or a spherical) charged object for a range of flow velocities. When the flow velocity exceeds a critical value, cylindrical (or spherical) solitons are excited, which, in the frame of the dust fluid, travel in the upstream direction, while wake structures propagate in the downstream direction. Unlike one-dimensional solitons, the amplitudes of these higher dimensional solitons decrease with time (and hence distance) while maintaining constancy of the product of their amplitudes with the square of their widths. The spherical solitons decay at a rate faster than the cylindrical soliton. It is also found that their amplitudes and velocities increase, and their widths decrease with an increase in the dust fluid velocity. Furthermore, the radii of curvature of the cylindrical and spherical solitons decrease with an increase in the dust fluid flow velocity. A 3D molecular dynamics simulation of the excitation phenomenon provides a good theoretical support to the experimental observations.

Observation of turbulence energy transfer in a cylindrical laboratory plasma device

We report the experimental results on the turbulence energy transfer in a cylindrical laboratory plasma based on the data obtained by a set of quadruple Langmuir probe on Zheda plasma experiment device. The turbulence energy transfer is directly embodied in the alternating change in the fluctuation amplitudes between the low frequency turbulence at ∼0.1–0.2 kHz and the drift wave (DW) turbulence at ∼1–2 kHz. The estimation of the radial electric field and the bispectral analysis of the experiments suggest that the DW turbulence gains the energy from the low frequency turbulence with the magnetic field increase. The energy transport due to the interaction between the low frequency turbulence and the DW turbulence is a possible reason to drive the turbulence energy transfer associated with the magnetic field increase in the laboratory plasma.

Transition of a 2D crystal to a non-equilibrium two-phase coexistence state

In this paper, we present experimental observation of the transition of a 2D dust crystal to a non-equilibrium solid–liquid phase coexistence state. The experiments have been carried out in an L-shaped dusty plasma experimental device in a DC glow discharge argon plasma environment. Initially, a monolayer crystalline structure is formed, which is later transformed to a two-phase coexistence state using the background neutral pressure as a control parameter. Self-excited horizontal oscillations are found in the center of the monolayer prior to the appearance of the coexistence state. It is observed that a molten center coexists with a solid periphery. Various structural, thermodynamic, and dynamical quantities are used to characterize the phase state. The surface tension at the solid–liquid circular interface is also determined. A detailed parametric study is made to delineate the existence region of such a state. It is found that melting caused at the core is due to the onset of a localized Schweigert instability in the presence of a few stray particles beneath the top layer in that region.

Confinement controlled dynamical structural rearrangement in a quasi-2D dusty plasma crystal

In this work, we present experimental results on the structural transition of a two-dimensional dust crystal through controlled adjustment of its radial confinement while keeping all other discharge parameters constant. The experiments are performed in an L-shaped Dusty Plasma Experimental device in a DC glow discharge argon plasma environment. Initially, a purely 2D dust crystal is formed inside a circular confining ring at the interface of the plasma-cathode sheath region. This monolayer with a hexagonal lattice configuration of the dust particles gets buckled when the sheath thickness around the radial confinement ring is reduced. A bilayer with a square lattice configuration emerges in the dust system due to the onset of a transverse instability. The multiple crystalline domains at this lower confinement show signatures of a constant structural rearrangement in the system. The timescale associated with this rearrangement is quantified from the bond-orientational correlation function. It is found that the heterogeneous cooperative micro-motion of particles in the quasi-layered system is responsible for the rearrangement over the passage of time.

Kelvin-Helmholtz instability in a compressible dust fluid flow

We report the first experimental observations of a single-mode Kelvin-Helmholtz instability in a flowing dusty plasma in which the flow is compressible in nature. The experiments are performed in an inverted Pi-shaped dusty plasma experimental device in a DC glow discharge Argon plasma environment. A gas pulse valve is installed in the experimental chamber to initiate directional motion to a particular dust layer. The shear generated at the interface of the moving and stationary layers leads to the excitation of the Kelvin-Helmholtz instability giving rise to a vortex structure at the interface. The growth rate of the instability is seen to decrease with an increase in the gas flow velocity in the valve and the concomitant increase in the compressibility of the dust flow. The shear velocity is further increased by making the stationary layer to flow in an opposite direction. The magnitude of the vorticity is seen to become stronger while the vortex becomes smaller with such an increase of the shear velocity. A molecular dynamics simulation provides good theoretical support to the experimental findings.

Trapping of waves in a flowing dusty plasma

We report on experimental observations of trapping of waves in a flowing dusty plasma. The experiments are performed in an inverted ∏-shaped dusty plasma experimental device in which the dusty plasma is created in a DC glow discharge argon plasma using micrometer-sized kaolin particles. Two copper wires are installed radially on the cathode, which serve to generate the flow in the dust fluid as well as to confine the waves. The dust fluid is initially made to flow over both the wires by altering the sheath potential of one of these wires, and as a result, the wave gets excited and propagates in the downstream direction. The wave gets trapped in between the wires when their separation is below a critical value of ∼2 cm. For a long time (of the order of a few seconds), the trapped-wave structure retains its identity. The amplitude of the wave crests and the distance between them remain constant with the dust fluid flow velocities. A numerical solution of the forced Korteweg-de Vries equation with two source terms as well as molecular dynamic simulations reproduce our experimental findings in a qualitative manner.

Square Lattice Formation in a Monodisperse Complex Plasma.

We present the first observations of a square lattice formation in a monodisperse complex plasma system, a configurational transition phenomenon that has long been an experimental challenge in the field. The experiments are conducted in a tabletop L-shaped dusty plasma experimental device in a dc glow discharge Argon plasma environment. By a careful control of the vertical potential confining the charged particles as well as the strength of the ion wake charge interactions with the dust particles, we are able to steer the system toward a crystalline phase that exhibits a square lattice configuration. The transition occurs when the vertical confinement strength is slightly reduced below a critical value leading to a buckling of the monodisperse hexagonal 2D dust crystal to form a narrowly separated bilayer state (a quasi-2D state). Some theoretical insights into the transition process are provided through molecular dynamics simulations carried out for the parameters relevant to our experiment.

Application of Dual Frequency Comb Method as an Approach to Improve the Performance of Multi-Frequency Simultaneous Radiation Doppler Radar for High Temperature Plasma Diagnostics

A new Doppler radar using millimeter-waves in the Ka-band, named the “dual-comb Doppler reflectometer”, has been developed to measure the turbulence intensity and its velocity in high-temperature plasmas. For the realization of a fusion power generation, it is very important to know the spatial structure of turbulence, which is the cause of plasma confinement degradation. As a non-invasive and high spatial resolution measurement method for this purpose, we apply a multi-frequency Doppler radar especially with simultaneous multi-point measurement using a frequency comb. The newly developed method of synchronizing two frequency combs allows a lower intermediate frequency (IF) than the previously developed frequency comb radar, lowering the bandwidth of the data acquisition system and enabling low-cost, long-duration plasma measurements. In the current dual-comb radar system, IF bandwidth is less than 0.5 GHz; it used to be 8 GHz for the entire Ka-band probing. We applied this system to the high-temperature plasma experimental device, the Large Helical Device (LHD), and, to demonstrate this system, verified that it shows time variation similar to that of the existing Doppler radar measurements.

Experimental observations of multiple modes of filamentary structures in the magnetized dusty plasma experiment (MDPX) device

Due to the small charge-to-mass ratio of dust particles, it is often necessary to use large magnetic fields of B ≥ 1 T, in order to observe the influence of magnetic forces in laboratory dusty plasmas. However, when experiments are performed at high magnetic fields in capacitively coupled, radio frequency discharges used for these dusty plasma experiments, the plasma is often observed to form filamentary structures between the electrodes that are aligned to the external magnetic field which disrupt the uniformity of the plasma and adversely impact some of our dusty plasma experiments. Recent experiments performed in the Magnetized Dusty Plasma Experiment device seek to identify and characterize these filamentary structures. This paper discusses the morphology of several distinct filamentary modes that are formed in low temperature argon plasmas. Three different spatial modes of filaments that appear to be strongly dependent on the magnetic field and neutral pressure are identified. There is strong evidence that each spatial mode has a threshold condition that is dependent on the ion Hall parameter. The criteria for the formation of the filaments are shown to be consistent with predictions of recent numerical simulations.

Thermodynamics and self-organization of strongly coupled Coulomb clusters: An experimental study

In this experimental work, the thermodynamics and self-organization of classical two-dimensional Coulomb clusters are studied as a function of the cluster size. The experiments are carried out in a DC glow discharge argon plasma in the dusty plasma experimental device for clusters with different number of particles. Hexagonal symmetry around each individual particle is quantified using the local orientational order parameter (|ψ6|) for all the configurations. The screened Coulomb coupling parameter, which plays a key role in determining the thermodynamic nature of a Coulomb cluster, is estimated using Langevin dynamics and found to be sensitive to the number of particles present in the cluster. In addition, the process of self-organization and the dynamics of individual particles of the cluster as it changes from a metastable state to the ground state are examined through the estimation of dynamic entropy. Our findings suggest an intimate link between the configurational ordering and the thermodynamics of a strongly coupled Coulomb cluster system—an insight that might be of practical value in analyzing and controlling the microdynamics of a wider class of finite systems.

Measurement of Radial Correlation Lengths of Electron Density Fluctuations in Heliotron J Using O-Mode Reflectometry

The radial correlation length Lr of electron density fluctuations has been measured using a two-channel Omode reflectometer system in the helical-axis fusion plasma experimental device Heliotron J. The experimental results show that Lr is around 1.4 n 0.7mm in three different magnetic field bumpiness configurations (i.e. low, medium and high) for the low-density electron cyclotron heating (ECH) discharges. In high-density neutral beam injection (NBI) discharges with HIGP and pellet injection, Lr is found to be around 1 n 0.2 mm.

Excitation of dust acoustic shock waves in an inhomogeneous dusty plasma

An experimental investigation of the propagation characteristics of shock waves in an inhomogeneous dusty plasma is carried out in the dusty plasma experimental device. A homogeneous dusty plasma, made up of poly-dispersive kaolin particles, is initially formed in a direct current glow discharge argon plasma by maintaining a dynamic equilibrium of the pumping speed and the gas feeding rate. Later, an equilibrium density inhomogeneity in the dust fluid is created by introducing an imbalance in the original dynamic equilibrium. Non-linear wave structures are then excited in this inhomogeneous dusty plasma by a sudden compression in the dust fluid. These structures are identified as shock waves, and their amplitude and width profiles are measured spatially. The amplitude of a shock structure is seen to increase, whereas the width broadens as it propagates down a decreasing dust density profile. A modified-Korteweg–de Vries–Burger equation is derived and used to provide a theoretical explanation of the results, including the power law scaling of the changes in the amplitude and width as a function of the background density.

Novel HTS coil protection method using secondary windings as a quench heater

In NIFS, research and development of HTS magnets have been begun aiming at the application to a next experimental device of Large Helical Device which is a heliotron-type fusion plasma experimental device. The next experimental device needs large coil current density such as 80 A/mm2, therefore it is important to develop a coil protection method. In this study, we have proposed a novel coil protection technology of HTS magnets using secondary windings. By embedding secondary windings which are electrically isolated in a part of the conductor and using the secondary windings as a quench heater, the occurrence of normal conduction is quickly diffused, then prevented local temperature rise in the HTS coil. We applied this method to the conductor and coil under development, and analysed the thermal behaviour of the secondary windings. As a result, we found that the temperature rise of the entire coil can be suppressed and the hotspot region can be diffused. This shows the applicability of coil protection by using the secondary windings, therefore optimal coil protection is possible by changing the ratio of magnetic stored energy between internal consumption and external recovery depending on the value of external resistance.

Measurement of temperature of a dusty plasma from its configuration

AbstractA new concept called “configurational temperature” is introduced in the context of dusty plasma, where the temperature of the dust particles submerged in the plasma can be measured directly from the positional information of the individual dust particles and the interaction potential between the dust grains. This method does not require the velocity information of individual particles, which is a key parameter to measure the dust temperature in the conventional method. The technique is initially tested using two‐dimensional (2D) OpenMP parallel molecular dynamics and Monte Carlo simulation and then compared with the temperature evaluated from experimental data. The experiment have been carried out in the Dusty Plasma Experimental (DPEx) device, where a 2D stationary plasma crystal of melamine formaldehyde particles is formed in the cathode sheath of a DC glow discharge argon plasma. The kinetic temperature of the dust is calculated using the standard particle image velocimetry technique at different pressures. An extended simulation result for the three‐dimensional case is also presented, which can be employed for the temperature measurement of a three‐dimensional dust crystal in laboratory devices.

Pattern formation in strongly magnetized plasmas: observations from the magnetized dusty plasma experiment (MDPX) device

The last decade has seen the development of new experimental devices to explore the physics of magnetized dusty plasmas. Because of the small charge-to-mass ratio of the charged microparticles, it is necessary to operate these experiments at high magnetic fields of several Tesla in order to observe the direct effect of the magnetic forces on the transport properties of the charged microparticles. While the study of magnetized dusty plasmas is still the ultimate goal, these experiments have also provided new opportunities to studies regimes of strongly magnetized, low temperature, laboratory plasmas that have not been extensively explored. Experiments show the formation of new types of self- and imposed-ordered structures that form in both the plasma and among the microparticles. This paper summarizes recent experimental observations of plasma filamentation (in the plasma) and will discuss possible connections to ‘dust gridding’ phenomena that are observed in the magnetized dusty plasma experiment device.

Effect of size and shape of a moving charged object on the propagation characteristics of precursor solitons

We report on experimental observations of the modifications in the propagation characteristics of precursor solitons due to the different shapes and sizes of the object over which the dust fluid flows. The experiments have been performed in a Π shaped Dusty Plasma Experimental device where dusty plasma is created in a DC glow discharge Ar plasma using kaolin particles. A floating copper wire installed radially on the cathode acts as a charged object in the plasma environment. The flow on the dust fluid is initiated by suddenly lowering the potential of the charged object from grounded potential to close to floating potential. The size (height and width) of the potential hill is then varied by drawing current from the wire through a variable resistance. With a decrease in the height of the potential hill, the amplitude, velocity, and number of excited precursor solitons are found to decrease, whereas the widths of the solitons are seen to increase. It is found that below a threshold value, these solitary waves are not excited, and the dust fluid simply flows over the hill. To examine the effect due to the shape of the potential profiles, the wire is replaced by a triangular object. Only trailing wakes are seen to be excited when the dust fluid faces the linearly increasing slope of the potential profile, whereas both solitons and wakes get excited when the object is placed with the sharp edge facing the flow. All the experimental findings qualitatively agree with numerical solutions obtained with different source terms in the forced-Korteweg–de Vries model equation.

Observation of a nonlinear phenomenon of the density fluctuations on zheda plasma experiment device (ZPED)

An O-mode microwave reflectometry system has been developed to measure the density fluctuation on Zheda Plasma Experiment Device (ZPED). The microwave frequency range of this diagnostic system is from 10 GHz to 18 GHz, corresponding to the cutoff densities from 0.13×1019m-3 to 0.4×1019m-3. The density fluctuations are measured with a fixed microwave frequency for plasma in different magnetic field. It has been observed that the density fluctuation power changes with the magnetic field nonlinearly: the density fluctuations increase linearly with the magnetic field when the magnetic field is less than the critical magnetic field, while almost no change when the magnetic field is larger than the critical magnetic field.

Melting transition of two-dimensional complex plasma crystal in the DC glow discharge

The formation of self-consistent dust crystal and its melting is a well- known phenomenon in rf generated plasmas but remains challenging in DC glow discharge plasmas. Here, we report the melting of a two- dimensional dusty plasma crystal, suspended in the cathode sheath of a DC glow discharge plasma. The experiments are carried out in a Dusty Plasma Experimental (DPEx) device which consists of a circular powered electrode and an extended grounded cathode tray. A stationary crystal of melamine formaldehyde particles is formed in a background of Argon plasma between a pair of confining strips that are placed on the cathode. The stable structure breaks and transition to a fluid state when the neutral pressure is reduced. The neutral pressure range where this melting transition is observed is an order of magnitude less than what is typically reported in rf discharge plasma. The transition is confirmed by evaluating the variation in different characteristic parameters such as the pair correlation function, Voronoi diagram, local bond order parameter, defect fraction and dust kinetic temperature as a function of background neutral pressure. For instance, the average bond order parameter (Ψ6) reduces from 0.87 to 0.4 when the neutral pressure is reduced from 15 to 11 Pa. The asymmetry of the electrode is a special feature that is believed to enable the formation of the plasma crystal.