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Summary of basic plasma physics sessions at the first Asia Pacific Plasma Conference, 2017

  • Abhijit Sen
Review Paper
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Abstract

This summary article is based on the papers presented in the Basic Plasma Physics Sessions of the First Asia Pacific Plasma Conference held at Chengdu, Peoples Republic of China, during 18–23 September 2017. It highlights the principal advances made in diverse areas of basic plasma physics and their potential impact on progress in various applied areas of the field.

Keywords

Summary Basic plasma physics Strongly coupled plasmas Plasma propulsion Plasma diagnostics 

1 Introduction

The Basic Plasma Physics sessions at this conference had over 56 contributions including five plenary talks, 21 invited talks, ten oral presentations and ten poster papers (see Abstracts of 1st Asia-Pacific Conference on Plasma Physics, (http://aappsdpp.org/DPPProgramlatest/wholeitem/Plenarysessions.html) and (http://aappsdpp.org/DPPProgramlatest/topical_session.html). A wide variety of topics were covered that included complex (dusty) plasmas, quantum plasmas, diagnostic advances (including atomic and molecular processes in plasmas), space propulsion, wave propagation studies, ion sources, basic studies in linear and mirror machines and magnetic reconnection experiments. This summary article covers a large majority of these contributions. I have attempted to group the papers around some common themes and have highlighted the major achievements and important issues discussed in the papers. The following four sections (Sects. 2, 3, 4 and 5) contain summary discussions of these papers followed by a few brief concluding remarks in Sect. 6. The papers are referred to by the last name of the presenting authors and their presentation codes as listed in the conference program.

2 Strongly coupled plasmas

Strong correlations are omnipresent in nature and can be found in a wide variety of systems ranging from electrolytes to dense plasmas to quark-gluon plasmas to ultra-cold ions and dusty plasmas. These systems are characterized by the existence of long range order and display a host of interesting behavior such as crystal formation in dusty plasmas and the emergence of visco-elastic phenomena in strongly coupled liquids (Ichimaru 2017). There has been a surge in theoretical and experimental investigation of strongly correlated plasmas in recent years, particularly in the area of dusty plasmas, that have looked at phase transitions, transport phenomena, collective modes as well as quantum and magnetic field effects. A plenary talk by Bonitz [P19] provided a comprehensive overview of this topic (Bonitz et al. 2010) and also highlighted a few recent novel research findings.

2.1 Dusty plasmas

In general, experimental investigation of strongly correlated systems are often difficult due to the requirement of extreme conditions for the creation of a strongly correlated system. Dusty plasmas are an exception in this regard as the dust component can get strongly coupled at room temperatures and observations are easy to make due to the macroscopic sizes of the dust particles. This feature has been exploited a great deal in recent years and dusty plasma experiments have provided valuable insights for other strongly coupled systems. Most of these studies have however been restricted to unmagnetized dusty plasmas as one requires a very strong magnetic field (of the order of a few Teslas) to magnetize the dust component. A clever technique of using gas flow to rotate the dust particles to thereby create a “quasi-magnetization” of the dust particles in an experiment was reported by Bonitz (Kählert et al. 2012; Bonitz et al. 2013). The idea is to replace (simulate) the Lorentz force effect with a Coriolis force. The gas flow acts only on the dust particles and has no effect on the electrons and ions. For dust particles of mass m \(\approx \) 10\(^{-12}\) kg and Q \(\approx \) 10\(^4\) e a rotation frequency \(\Omega \approx \) 10 Hz was achieved experimentally which is equivalent to an effective magnetic field \(B_{\text {eff}} \approx \) 10\(^4\) T. In another experiment, the magneto-plasmon dispersion was successfully measured in a rotating dusty plasma by using the same method (Hartmann et al. 2013).

A number of papers presented experimental and simulation results on the dynamics and micro-processes governing the behavior of strongly coupled dusty plasmas. Lin I [B-I1] presented experimental results of a 2D cold dusty plasma liquid that was sheared by a laser beam leading to avalanche-like cracking/healing of ordered domains through stick-slip type collective small domain rotations (Chiang and Lin 1996; Chen et al. 2014; Yang et al. 2012, 2016). The dynamics of surface-assisted crystalline domain growth in cooled 3D dusty plasma liquids were discussed by Wang [B-O2] who showed the propagation of crystal like fronts that displayed fluctuations with a power law scaling (Chu and Lin 1994; Arai et al. 2017). The transient dynamics of the melting of a 2D Yukawa crystal was investigated using MD simulations to track the emergence of small scale interacting phonon modes and synchronization sites. The rupture/healing of the crystalline ordered domains through domain rotation was traced to the spatio-temporal propagation of these sites by Hu [BP5] (Yang et al. 2012; Chen et al. 2014). Lin [B-O4] reported on experimental observations of dust acoustic wave turbulence that were characterized using the spatio-temporal empirical mode decomposition method and shown to consist of multi-scaled acoustic vortices with helical waveforms winding around short-lived defect filaments (Huang et al. 1998; Tsai and Lin 2014; Tsai et al. 2017; Lin et al. 2017). Feng [B-I2] presented an analytic formulation of the equation of state for a 2D dusty plasma that depended on the calculation of pressure in Yukawa liquids using MD simulations. This could be a very useful theoretical tool for obtaining exact expressions for material characteristics like bulk modulus of a dusty plasma (Feng et al. 2016a, b).

Wave propagation in a dusty plasma medium has been an active area of research for quite some time now. The basic interest has been to explore the additional features arising in the propagation characteristics of these waves due to the presence of the dust component and strong coupling effects if any. A number of theoretical studies pertaining to the formation and propagation characteristics of nonlinear waves in a dusty plasma medium were presented at the conference. These included a variety of solitonic solutions (e.g. dust ion acoustic solitons, Gardner solitons), shock waves, dust rogue waves and cnoidal waves. Singh [B-01] presented analytic results of phase shifts arising from the head on collision of two dust acoustic wave solitons and also estimated the effect of the polarization force on the structure of rogue waves in dusty plasmas. Kaur [BP12] analyzed the influence of energetic particle beams on the various features of Gardner solitons and other nonlinear structures. Saini [B-I19] provided an overview of some of the nonlinear structures when they were derived using Kappa distributions for the background electrons or ions in order to make the results of practical relevance to space plasma observations (Saini and Singh 2016).

Laboratory studies of dusty plasmas reported two interesting observations on nonlinear wave structures by Jaiswal [B-I3] and Choudhary [BP15]. Jaiswal presented an experimental study of a supersonic flow of a dust liquid over an electrostatic potential that revealed the existence of fore-wake structures in the form of precursor solitons propagating at supersonic speeds and moving in the direction of the flow (Jaiswal et al. 2015, 2016). When the speed was lowered to below the sound speed only wake field structures were observed. The results were well explained by a driven Korteweg–De Vries model equation. In the experiment by Choudhary, co-rotating vortices were observed in an extended dust grain medium with an inhomogeneous plasma background (Choudhary et al. 2017). The results were well explained by a quantitative analysis based on the charge gradient of the dust cloud which led to the vortex motion and its multiplicity.

2.2 Quantum plasmas

The topic of quantum plasmas i.e. plasma systems where quantum effects can be important, has evoked considerable interest in recent years. Typically, quantum effects become relevant when the De Broglie wavelength of a charged species is comparable to or much larger than its inter-particle distance. Such a situation can arise, for example, in the interior of white dwarfs, intense laser plasma interactions, metallic nano-structures and thin metal films. There is considerable interest in exploring the influence of quantum effects on linear and nonlinear wave propagation in such media. A number of papers addressed this topic starting from the basic modification of the “classical” Landau damping phenomenon to modifications in the nonlinear Schrödinger equation describing the propagation of envelope solitons, to the excitation of Alfven waves in a magnetized quantum plasma system. Misra [B-I6] investigated the effects of the quantum parameter H (the ratio of the plasmon energy to the thermal energy density) and showed that they reduced the Landau damping rate while Chatterjee [B-O10] found the decay rate of the solitary wave amplitude to be reduced compared to its classical value (Chatterjee and Misra 2015, 2016). The influence of quantum effects on the collective modes in one component plasma were highlighted by Bonitz in terms of the quantum coupling parameter and the quantum degree of degeneracy (Bonitz 2016).

3 Plasma propulsion systems

For long term space travel the use of plasma propulsion systems is an attractive option that has spurred much R&D activity in this field. Some of the technological and basic physics challenges facing this novel thruster system include limitations arising from damage of electrodes contacting directly with a plasma and thrust efficiency issues related to the behaviour of the ejected plasma jet. A number of papers addressed these and other related issues and provided an update on advances made in this field. A microwave rocket, where the ground launch of a rocket is initiated by the thrust generated from an atmospheric millimetre wave discharge instead of the detonation of liquid or solid chemical fuels, is a promising future concept that has been under study for quite some time. The availability of high power gyrotrons has considerably improved the practical feasibility of this concept and experimental progress in the field of Space Propulsion Powered by Millimeter-Wave Discharge was reported by Komurasaki [B-I11]. Basic studies related to the nature of the millimeter-wave discharge has revealed the existence of unique comb-shaped filament structures with a pitch of 0.85\(\lambda \). These 3D structures have been found to enhance the discharge extension speed by \(50\%\) and thereby improve the thrust characteristics of the engine (Oda et al. 2009; Nakamura et al. 2016) . An overview of the progress made in the development of advanced electrodeless propulsion using a high density Helicon plasma source was reported by Shinohara [P17]. Remarkable progress has been made under the Helicon Electrodeless Advanced Thruster (HEAT) project such as a factor of 10\(^6\) increase in the volume of the helicon source, production of plasma densities of the order of 10\(^{13}\) cm\(^{-3}\) and the development of a host of electromagnetic acceleration schemes e.g. use of a rotating magnetic field and an m = 0 half cycle scheme (Shinohara et al. 2009, 2014; Kuwahara et al. 2017) . The future of advanced electrodeless thrusters with high efficiencies and long life times looks promising. The results of a theoretical study of the helicon discharge based on a self-consistent fluid model were presented by Isayama [B-I12]. The model includes wave excitation, collisional electron heating, and diffusion of plasma and neutrals and delineates the roles of the helicon wave and the Trivelpiece-Gould in the temporal behavior of the helicon discharge (Isayama 2018). The results show remarkable agreement with experimental data.

4 Plasma diagnostics

A variety of diagnostic techniques, some under development and others of an advanced nature, to measure a wide range of plasma parameters were presented at the meeting. The use of optical emission spectroscopy (OES) of atomic and molecular processes as a diagnostic for nitrogen plasmas was discussed by Akatsuka [P32]. Nitrogen plasmas are widely used in the industry for applications like surface hardening of metals or preparing special insulating layers in electronics devices. The knowledge of various plasma parameters is important for critical control of the industrial processes and OES offers a powerful non-intrusive method. The paper reported recent progress in the spectroscopic analysis of various band spectra and actinometry measurements supported by model calculations of the excitation kinetics (Ichikawa 2010; Akatsuka et al. 2016). Another non-intrusive diagnostic, this time to measure fluctuating and static electric fields in a plasma, based on the Lamb shift due to radiative corrections was presented by Doveil [B-I7]. The novel method uses a beam of hydrogen ions or atoms and measures the Lyman-\(\alpha \) light with a spectrometer in a direction perpendicular to the beam (Chérigier-Kovacic et al. 2015). In early experiments the Lamb-shift resonance was used to measure oscillating electric fields around 1 GHz by observing a strong enhancement of the Lyman- signal (Lejeune et al. 2011; Doveil et al. 2013). The method provides excellent sensitivity for measurement of electric fields, namely mV/cm and temporal resolution of a (ns) that are three orders of magnitude higher compared to other current diagnostics (Doveil et al. 2017). A complementary approach for remotely measuring the magnetic field in a plasma is based on using magnetic field induced atomic transitions. Such a method based on a line ratio measurement in the soft X-ray spectrum of Fe\(^{9}\) was presented by Yang [B-I8] and proposed as a possible means of measuring and monitoring the active Solar Corona magnetic field (Li et al. 2015b, 2016). A novel diagnostic to measure plasma flows (or beam velocities) based on topological light (so-called optical vortex) was proposed by Aramaki [B-I10]. Conventional Plane Wave Doppler spectroscopy has no sensitivity to motion across the beam but an optical vortex, due to its three-dimensional phase structure, can induce Doppler shifts in all the three-dimensional directions (Allen et al. 1994). It therefore provides a convenient means of measuring flows perpendicular to the optical beam.

Turning to fusion relevant diagnostic techniques, a Laser Induced Breakdown Spectroscopic (LIBS) method for characterization of impurity deposits and deuterium retention in the first wall of EAST tokamak was presented by Hongbin Ding [B-I18]. An in-situ wall diagnosis system based on this method has been operational in EAST since 2014 and provides information on the H/D co-deposited layer on the first wall of the device as well as the chemical composition and depth profile in the interface between the co-deposited layer and the substrate of the first wall (citealtCLi1). Luhmann [B-O5] presented two technological innovations for improving the performance and resolution of the Electron Cyclotron Emission Imaging (ECEI) and the Microwave Imaging Reflectometry (MIR) systems that are standard diagnostics on many tokamaks. By the addition of a V-Band (50 75 GHz) MMIC LNA (monolithic microwave integrated circuit low noise amplifier) (gain   15 dB and noise temperature   500 K) before the mixer diode, the overall noise temperature can be reduced from as much as 250,000 K to as little as 2000 K (Lai et al. 2014). A new system-on-chip (SoC) approach is capable of not only incorporating improvement in system noise temperature, but also provides (a) shielding against interference from RF heating and strong bursts of millimeter wave radiation that sometimes originate in the plasma, and (b) the ability to amplify and multiply low-frequency sources to generate high-frequency local oscillation signals directly on the individual chips (Tobias et al. 2016). Improved noise temperature, due to these innovations, facilitates absolute temperature calibration, which is of vital importance for obtaining electron temperature profiles. The chip greatly enhances the efficiency, reliability and resolution of these diagnostic systems as demonstrated in a recent implementation on the tokamak DIII-D (Wang et al. 2017).

Highlights of the research carried out at the electron beam ion trap facility, Shanghai EBIT, were presented in talks by Yao [B-I9] and Xu [B-O6]. The facility has a number of energetic beams ranging in electron energy from 30 ev to 150 kev thereby covering the main electron energy range relevant for fusion plasmas. Among the important investigations carried out at the facility are detailed spectroscopic studies of Tungsten (important for ITER) (Fei et al. 2014; Qiu et al. 2015) and a host of dielectronic recombination lines relevant to high temperature plasmas in general (Yao et al. 2010; Tu et al. 2016).

5 Linear devices

While the mainstream fusion research is dominated by tokamaks and other toroidal devices, experiments on linear devices like mirror machines continue to provide useful insights into important phenomena relevant for fusion applications and some of these were highlighted at the conference. Results from divertor simulation and hydrogen recycling studies utilizing the end region of the Tandem Mirror GAMMA 10/PDX were reported by Sakamoto [B-I16]. It was shown that the ion temperature of the end-loss flux in GAMMA 10 is much higher than that of other divertor simulators. Furthermore, it was found that the heat-flux density could be controlled within the range of 0.4–0.8 MW/m\(^{2}\) by changing the ICRF power and that it had a strong dependence on the time integrated diamagnetism in the central cell. The particle-flux density, on the other hand, was found to be proportional to the electron line-density in the central cell. These results and the future planned experiments constitute a systematic approach to understand the basic physical mechanisms underlying heat flux generated in fusion devices and will help not only in designing future divertor systems but also to devise ways of keeping the plasma detached from the divertor plates (Nakashima et al. 2016; Sakamoto et al. 2017).

Experimental results on colliding reversed field pinches (RFPs) and ICRH heating carried out in another tandem mirror machine (KMAX) were reported by Munan [B-I24] and Ming[B-I23]. A large-size field-reversed configuration (FRC) plasmoid was produced by the collision-merging of two high- compact toroids in order to explore the physics of the colliding and merging process (Lin 2017). A multi-channel magnetic probe measurement established the nature of the FRC internal magnetic field while other diagnostics showed that the total temperature of the KMAX-FRC plasma was of the order of 100 eV and the plasma life time was about 300 \(\upmu s\). The plasmoid velocity was found to be about 10 km/s. In a set of heating experiments using ICRH power in the range of 23–50 kW it was shown that the diamagnetism of the central cell plasma increased linearly with the radiated RF power (Liu et al. 2017).

Neutral beam injection (NBI) systems have been the most successful and reliable means of heating fusion plasmas and are widely used on various tokamaks. However at higher energies the neutralization efficiency of positive ions of hydrogen is extremely low and negative ion-based NBI systems are a better option for future machines like ITER and fusion reactors. There is therefore a significant world-wide developmental effort on negative NBI systems. An effort in this direction was reported by Ando [B-I22] with particular focus on the development of a large negative hydrogen ion source operated with radio frequency power and model calculations to assess the effectiveness of a photo-neutralizer. The ion source is based on the excitation of a helicon wave in a hydrogen plasma to create a high-density hydrogen plasma (n > 10\(^{18}\) m\(^{-3}\)) at the driver region in a large RF ion source (Sasaki et al. 2016). The negative ions are then extracted in the surface conversion mode through Cesium (Cs) vapor injection into the source (Ando et al. 2012). In addition to the source experiments, the authors also reported on the results of a PIC/MC simulation to assess the neutralization efficiency of a photo-neutralizer for a 1 MeV D\(^{-}\) beam. The efficiency was found to be enhanced to 95\(\%\) with the use of the photo-neutralizer. A combination of neutral gas and a photo-neutralizer was found to work best in that it reduced the beam divergence and also decreased the amount of laser power for efficient neutralization.

6 Novel areas

The Basic Plasma Physics Sessions also saw a diverse range of papers exploring new ideas aiming to explain unusual natural phenomena that do not receive adequate attention in the main stream research programs or even taking a fresh look at some old but not fully understood problems. Some highlights of these papers are discussed in this section.

The Large Plasma Device (LAPD) is a linear device in the US that serves as a national facility for the exploration of frontier problems in basic plasma physics (Gekelman et al. 2016). Results of some recent investigations carried out in this device were presented by Vincena [B-I20]. The excitation of shear Alfven wave eigenmodes in plasmas with multiple ion species was studied and the concept of an ion-ion Alfven wave resonator was experimentally demonstrated (Vincena et al. 2004, 2013). The results lend strength and credibility to past theoretical models that had been developed for the interpretation of wave phenomena observed in the magnetospheric regions where such conditions can exist (Rauch and Roux 1982).

Streamers play a key role in the early stages of atmospheric discharges like lightnings and sprites. They are essentially rapidly growing plasma filaments that penetrate into non-ionized regions due to the electric field enhancement at their tips and grow as long ionized channels. Detailed simulation results from an advanced 3D particle model describing the inception and development processes of streamers from a positive needle electrode were presented by Sun [B-O8] (Sun et al. 2014; Teunissen et al. 2014; Sun et al. 2013). The model successfully accounts for the effect of background ionization and photo-ionization on the evolution and nature of streamers and gives results that are in qualitative agreement with experimental observations.

Ball lightning, a luminous sphere sometimes observed on the ground after normal lightning, is a puzzling natural phenomenon that is not well understood yet. A novel theoretical model was proposed by Hui Chun Wu [P23] that described it as a self-trapping of microwaves in a spherical plasma cavity. The basic model, developed from the original suggestion of Kapitza (1955), is that a relativistic electron bunch can be produced by the stepped leader of lightning which can coherently emit high-power microwave when it strikes the ground. The intense microwave then ionizes the local air and evacuates the resulting plasma by its radiation pressure, thereby forming a spherical plasma cavity that traps the microwave. The theoretical results were further supported by PIC simulations to describe the time evolution of such structures (Wu 2016, 2017).

Another puzzling question in plasma physics, posed by Himura [B-I21], is the lack of an experimental validation of the so called “two-fluid” effects—that arise in theoretical descriptions of a plasma where the electron and ion fluids are assumed to move independently. The ostensible reason for this lacuna is due to the experimental difficulty of probing the short-scale length of the ion skin depth at which the two-fluid plasma state or the two-fluid effects are expected to appear. However the skin depth can be much larger in non-neutral plasmas because of the low ion density. To carry out such novel experiments using non-neutral plasmas, a linear device BX-U has been developed (Himura 2016) in which pure lithium ion and electron plasmas are not only produced independently but also trapped simultaneously. The idea is then to mix the two in order to observe the two-fluid effects. Preliminary results presented at the conference reported successful commissioning of the experimental device and independent creation and confinement of the two-nonneutral component plasmas (Kawai et al. 2016).

There is growing interest these days in the experimental study of confined electron–positron plasmas—so called pair plasmas. The interest comes from both basic plasma science to explore novel wave and turbulence phenomena and from astrophysics because pair plasmas are believed to exist in the vicinity of quasars and galactic accretion disks. A scheme for such an experimental study of pair plasmas was described by Saitoh [B-I15] using a positron source, an accumulator and a dipole magnetic trap. Initial results obtained using a prototype dipole trap show good confinement of the positrons and successful radial compression of the positron orbit with rotating wall electric fields (Saitoh et al. 2015; Stanja et al. 2016).

The phenomenon of magnetic reconnection, ubiquitous in space and laboratory experiments, continues to remain an active field of research with many outstanding challenges. Results from spherical tokamak plasma merging experiments on MAST, TS-3 and TS-4, reported by Ono [P12], showed significant ion/electron heating from magnetic reconnection up to 1.2 keV (Ono et al. 2012, 2015, 2016). A huge outflow heating of ions in the downstream region and localized heating of electrons at the X point were also observed. The ions were found to be accelerated up to the poloidal Alfven speed and subsequently thermalized by fast shocks in the down stream region. These observations are in agreement with solar satellite observations and PIC simulations.

7 Concluding remarks

The Basic Plasma Physics Sessions of the First AAPPS-DPP Conference offered a rich variety of very interesting and high quality talks/presentations covering a wide range of topics from exotic plasma systems to exploration of fundamental issues in fusion devices to astrophysical and space plasma observations and spectacular plasma phenomena in the earth’s atmosphere. It was heartening to see that a large number of these contributions came from research laboratories and university departments in the Asia Pacific countries which is indicative of the strong research base in basic plasma physics in this region. The large participation and excellent contributions of scientists from other parts of the world gave the conference a truly international flavor and provided an excellent forum for exposing the latest scientific results and exchanging new ideas. The contents of the Basic Plasma Physics sessions also made it clear that we need to continue to make significant investment in basic research in order to ensure the growth of the field as well as to achieve success in major endeavors like fusion and space sciences.

Notes

Acknowledgements

I would like to thank Dr. M. Kikuchi and Dr. Xuru Duan for inviting me to this conference and for their support and encouragement in this task. I am also grateful to all the contributors of the Basic Sessions who sent me valuable inputs for my summary talk at the conference and which has proved very useful in writing this article.

Compliance with ethical standards

Conflict of interest

As a sole author of this article, I state that there is no conflict of interest.

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Copyright information

© Division of Plasma Physics, Association of Asia Pacific Physical Societies 2018

Authors and Affiliations

  1. 1.Institute for Plasma ResearchGandhinagarIndia

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