Large Volume Plasma Device Research Articles

Observation of turbulence-induced reduced electrostatic particle flux in the presence of QL whistlers in large laboratory plasma

The electrostatic particle flux is measured in the presence of obliquely propagating quasi-longitudinal (QL) whistler turbulence (ωci<ωLH≈ω<ωce) in the large-volume plasma device (LVPD). The QL whistler is observed with frequency band between 40 and 100 kHz, and the characteristic wave numbers k∥≪k⊥ are excited by the reflected energetic electrons via loss cone (localised mirror type magnetic geometry) formation in the presence of a transverse magnetic field [BEEF(x̂)] of electron energy filter and axial magnetic field of LVPD [Bo(ẑ)]. The effect of mirror strength on radial particle flux is explored by changing the BEEF as this variation changes the excited QL-whistler turbulence. We observed that the increase in QL-whistler turbulence level, the radial particle transport, i.e., the radial particle flux subsides, is accompanied by particle flux direction reversal from radially inward to outward.

Development of electronic record-keeping software for remote participation in Large Volume Plasma Device upgrade using Angular 2 and NodeJS web technologies.

In an era of digital transformation and collaborations, the Web Information System (WIS) becomes an essential requirement for the information and data sharing of large experimental facilities among users. With the enhancement in the capabilities and performance of web technologies, sharing of experimental data using a flexible, modular, secure, and robust mechanism is feasible. In this direction, the Large Volume Plasma Device (LVPD), an experimental device dedicated for carrying out investigations for unfolding physical phenomena of relevance to Earth's magnetosphere and fusion plasmas, also adopts web-based electronic record keeping for its operation. The nature of investigations is concerned with plasma turbulence of electron scale, induced anomalous plasma transport and mitigation of energetic electrons by excited whistler turbulence that requires large scale, and simultaneous multiple point measurements from different electrostatic and electro-magnetic diagnostics. This paper discusses the WIS implementation in LVPD for the experimental configuration, information logging, and preliminary data analysis. The architecture of the system is spread over three tiers covering application, data, and presentation layers. The presentation layer is developed using the Angular 2 framework on the progressive web application architecture. The application and data layers are developed using NodeJS and PostgreSQL, respectively. The novelty of this paper lies in the integrated application development approach and applicability of the latest web technologies in the scientific and experimental domains. This paper discusses the literature survey of similar developments at other places, requirements, scopes, development artifacts, adapted tools and technologies, obtained results from actual plasma discharges of LVPD, and future enhancements.

Investigations into growth of whistlers with energy of energetic electrons

Runaway electrons have great significance in fusion plasmas as they are considered to be responsible for the excitation of instabilities and for damage to the first wall components. The threshold for the generation of the runaway electron population in tokamaks strongly depends on the toroidal magnetic field and bulk electron temperature. The presence of these runaway electrons with a critical density and magnetic field value are correlated with the magnetosonic whistler activity. The Rosenbluth condition applies to the destabilization of upper hybrid modes, that remains mostly stable in large-scale tokamaks like Joint European Tokamakand International Thermonuclear Experimental reactor. On the other hand, whistlers with a frequency in the lower hybrid range are more likely to be destabilized in a lower magnetic field and temperature, along with the whistlers’ excitation threshold. The process of destabilization of whistlers by energetic electrons thus has a finite overlap between fusion and basic plasma devices, and the outcome from the latter can be scalable to fusion machines. We report that in the linear large volume plasma device, the destabilization of quasi-longitudinal whistlers, driven by the reflected particles from the mirror, takes place in the bulk plasma region i.e. away from the mirror in the energetic belt region, and follows the frequency ordering of . Our explanation involves an alternate limit of quasi-linear analysis, commonly employed for recovering the instability threshold of runaways scattering off whistlers in fusion plasmas, and the growth rate due to the reflected particle is proportional to the inverse square root of electron temperature, where the reflected particle fraction is (Sharma and Vlahos 1984 Astrophys. J. 280 405). The measurements required to obtain the energy scaling of runaway electrons with whistler instability are critical for large devices but remain scarce. We have emphasized the analytic correlation between the two regimes in these investigations.

Electron temperature gradient turbulence induced energy flux in the large volume plasma device

The Large Volume Plasma Device (LVPD) has successfully demonstrated excitation of the Electron Temperature Gradient (ETG) driven turbulence in the finite plasma beta (β∼0.06−0.4) condition, where the threshold condition for ETG turbulence is, ηETG=Ln/LT>2/3 satisfied, where, Ln=1ndndx−1 is the density scale length and LTe=1TedTedx−1 is the temperature scale lengths [Mattoo et al., Phys. Rev. Lett. 108, 255007 (2012)]. The observed mode follows wave vector scaling and frequency ordering as k⊥ρe≤1 ≪ k⊥ρi, Ωi<ω ≪ Ωe, where k⊥ is the perpendicular wave vector, ρe, ρi are Larmor radii of the electron and ion, respectively, and Ωi, Ωe, ω are the ion, electron gyro frequencies and the mode frequency, respectively. Simultaneous measurement of fluctuations in electron temperature, δTe ∼ (10−30) %, plasma density, δne ∼ (5−12) %, and potential δVf ∼ (1−10) % are obtained. A strong negative correlation with correlation coefficients Cδn−δφ ∼−0.8 and CδT−δφ ∼−0.9 is observed between the density and potential and temperature and potential fluctuations, respectively. These correlated density, temperature, and potential fluctuations lead to the generation of turbulent heat flux. The measured heat flux is compared with the theoretically estimated heat flux from ETG model equations. The experimental result shows that the net heat flux is directed radially outward.

Observation of electromagnetic fluctuation induced particle transport in ETG dominated large laboratory plasma

The large volume plasma device (LVPD), a cylindrically shaped, linear plasma device of dimension (Φ = 2 m, L = 3 m) have successfully demonstrated the excitation of electron temperature gradient (ETG) turbulence (Mattoo et al 2012 Phys. Rev. Lett. 108 1–5). The observed ETG turbulence shows significant power between corresponding to the wave number, where, and ion cyclotron frequency and electron Larmor radius, respectively. The observed frequency and wave number matches well with theoretical estimates corresponding to Whistler-ETG mode. We investigated electromagnetic (EM) fluctuations induced plasma transport in high beta ETG mode suitable plasma in LVPD. The radial EM electron (ion) flux results primarily from the correlation between fluctuations of parallel electron current and radial magnetic field The EM particle flux is observed to be much smaller than the electrostatic particle flux, The EM flux is small but finite contrary to the conventional slab ETG model. The EM flux is non-ambipolar. A theoretical model is obtained for the EM particle flux in straight homogeneous magnetic field geometry. The theoretical estimates are seen to compare well with the experimental observations. Sluggish parallel ion response is identified as the key mechanism for generation of small but finite EM flux.

Electro-mechanical probe positioning system for large volume plasma device.

An automated electro-mechanical system for the positioning of plasma diagnostics has been designed and implemented in a Large Volume Plasma Device (LVPD). The system consists of 12 electro-mechanical assemblies, which are orchestrated using the Modbus communication protocol on 4-wire RS485 communications to meet the experimental requirements. Each assembly has a lead screw-based mechanical structure, Wilson feed-through-based vacuum interface, bipolar stepper motor, micro-controller-based stepper drive, and optical encoder for online positioning correction of probes. The novelty of the system lies in the orchestration of multiple drives on a single interface, fabrication and installation of the system for a large experimental device like the LVPD, in-house developed software, and adopted architectural practices. The paper discusses the design, description of hardware and software interfaces, and performance results in LVPD.

Observation of radially inward turbulent particle flux in ETG dominated plasma of LVPD

Radially inward turbulent particle flux is observed in the core region of target plasma of Large Volume Plasma Device where electron temperature gradient (ETG) driven turbulence conditions are satisfied with threshold, ηETG=Ln/LT>2/3 [Mattoo et al., Phys. Rev. Lett. 108, 255007 (2012)]. The observed mode satisfies the scale length and frequency ordering of ETG (k⊥ρe≤1≪k⊥ρi,Ωi<ω≪Ωe), where k⊥ is the perpendicular wave vector, ρe ,ρi are Larmor radii of electron and ion, respectively, and Ωi,Ωe, and ω are the ion, electron gyro frequencies, and the mode frequency, respectively. The measured flux is dominantly electrostatic (Γes≈105Γem), although the nature of the turbulence is electromagnetic (β∼0.1−0.6). Experimental observations of the phase angle between density and potential fluctuations, θñ,ϕ̃, and turbulent particle flux, Γes, shows good agreement with the theoretical estimations derived for ETG turbulence.

Observation of reflected electrons driven quasi- longitudinal (QL) whistlers in large laboratory plasma

This paper reports experimental and theoretical investigations on plasma turbulence in the source plasma of a Large Volume Plasma Device. It is shown that a highly asymmetrical localized thin rectangular slab of strong plasma turbulence is excited by loss cone instability. The position of the slab coincides with the injection line of the primary ionizing energetic electrons. Outside the slab, in the core, the turbulence is weaker by a factor of 30. The plasma turbulence consists of oblique [θ=tan−1(k⊥/k||)≈87°] Quasi-Longitudinal (QL) electromagnetic whistlers in a broad band of 40kHz<f≤80 kHz with k⊥∼1.2 cm−1 and k||∼0.06cm−1. Experimental observations suggest that the primary agent for the turbulence is not driven by primary ionizing energetic electrons but by the loss cone feature in the velocity distribution of reflected energetic electrons. A magnetic mirror is formed in the Electron Energy Filter when it is energized. It is shown that it is this mirror which is responsible for both reflection of the energetic electrons and imposing loss cone feature on it. Theoretical framework is based upon Oblique whistler approximation by Sharma and Vlahos [Astrophys. J. 280, 405 (1984)] and Verkhoglyadova et al. [J. Geophys. Res. 115, A00F19 (2010)] and Quasi Longitudinal (QL) whistlers by Booker and Dyce [Radio Sci. J. Res 69D (1965)] for excitation of the plasma turbulence in the magnetosphere.

Plasma potential measurement using centre tapped emissive probe (CTEP) in laboratory plasma

Plasma potential measurements using a compensated center tapped emissive probe (CTEP) in quiescent plasma () of large volume plasma device (LVPD) are presented. The CTEP shows distinct advantage over conventional emissive probe (CEP) because of measurement capability, independent of electronics and operating conditions of CEP. Its ability of measuring continuous, uninterrupted plasma potential (DC and fluctuations) measurements for pulsed and DC plasma discharges gives it advantage over other configurations. Also, its push fit design allows easy replacement without realizing frequent vacuum breaks. In small sized plasma devices, CTEP design may introduce some spatial resolution error but for large plasma devices, this is negligible as voluminous data is required over large scale lengths (~few tens of cm).

Open loop control of filament heating power supply for large volume plasma device

A power supply (20V, 10kA) for powering the filamentary cathode has been procured, interfaced and integrated with the centralized control system of Large Volume Plasma Device (LVPD). Software interface has been developed on the standard Modbus RTU communication protocol. It facilitates the dashboard for configuration, on line status monitoring, alarm management, data acquisition, synchronization and controls. It has been tested for stable operation of the power supply for the operational capabilities. The paper highlights the motivation, interface description, implementation and results obtained.

A 5 kA pulsed power supply for inductive and plasma loads in large volume plasma device.

This paper describes 5 kA, 12 ms pulsed power supply for inductive load of Electron Energy Filter (EEF) in large volume plasma device. The power supply is based upon the principle of rapid sourcing of energy from the capacitor bank (2.8 F/200 V) by using a static switch, comprising of ten Insulated Gate Bipolar Transistors (IGBTs). A suitable mechanism is developed to ensure equal sharing of current and uniform power distribution during the operation of these IGBTs. Safe commutation of power to the EEF is ensured by the proper optimization of its components and by the introduction of over voltage protection (>6 kV) using an indigenously designed snubber circuit. Various time sequences relevant to different actions of power supply, viz., pulse width control and repetition rate, are realized through optically isolated computer controlled interface.

Process automation system for integration and operation of Large Volume Plasma Device

Large Volume Plasma Device (LVPD) has been successfully contributing towards understanding of the plasma turbulence driven by Electron Temperature Gradient (ETG), considered as a major contributor for the plasma loss in the fusion devices. Large size of the device imposes certain difficulties in the operation, such as access of the diagnostics, manual control of subsystems and large number of signals monitoring etc. To achieve integrated operation of the machine, automation is essential for the enhanced performance and operational efficiency. Recently, the machine is undergoing major upgradation for the new physics experiments. The new operation and control system consists of following: (1) PXIe based fast data acquisition system for the equipped diagnostics; (2) Modbus based Process Automation System (PAS) for the subsystem controls and (3) Data Utilization System (DUS) for efficient storage, processing and retrieval of the acquired data. In the ongoing development, data flow model of the machine’s operation has been developed. As a proof of concept, following two subsystems have been successfully integrated: (1) Filament Power Supply (FPS) for the heating of W- filaments based plasma source and (2) Probe Positioning System (PPS) for control of 12 number of linear probe drives for a travel length of 100cm. The process model of the vacuum production system has been prepared and validated against acquired pressure data. In the next upgrade, all the subsystems of the machine will be integrated in a systematic manner. The automation backbone is based on 4-wire multi-drop serial interface (RS485) using Modbus communication protocol. Software is developed on LabVIEW platform using NI-Modbus library and tested for the standalone and integrated operation mode of integrated subsystems. The paper discusses the schema of automation, development of software, comparison of modeling results of vacuum production process with actual pump down characteristic and integration results of the FPS and PPS.

A dipole probe for electric field measurements in the LVPD

This paper describes the design, construction, and calibration of an electric dipole probe and demonstrates its capability by presenting results on the measurement of electric field excited by a ring electrode in the Large Volume Plasma Device (LVPD). It measures the electric field in vacuum and plasma conditions in a frequency range lying between . The results show that it measures electric field 2 mV cm−1 for frequency . The developed dipole probe works on the principle of amplitude modulation. The probe signal is transmitted through a carrier of 418 MHz, a much higher frequency than the available sources of noise present in the surrounding environment. The amplitude modulation concept of signal transmission is used to make the measurement; it is qualitatively better and less corrupted as it is not affected by the errors introduced by ac pickups. The probe is capable of measuring a variety of electric fields, namely (1) space charge field, (2) time varying field, (3) inductive field and (4) a mixed field containing both space charge and inductive fields. This makes it a useful tool for measuring electric fields in laboratory plasma devices.

Large-Volume Plasma Device with Internally Mounted Face-Type Planar Microwave Launchers for Low-Temperature Sterilization

Large-Volume Plasma Device with Internally Mounted Face-Type Planar Microwave Launchers for Low-Temperature Sterilization

Performance of large electron energy filter in large volume plasma device

This paper describes an in-house designed large Electron Energy Filter (EEF) utilized in the Large Volume Plasma Device (LVPD) [S. K. Mattoo, V. P. Anita, L. M. Awasthi, and G. Ravi, Rev. Sci. Instrum. 72, 3864 (2001)] to secure objectives of (a) removing the presence of remnant primary ionizing energetic electrons and the non-thermal electrons, (b) introducing a radial gradient in plasma electron temperature without greatly affecting the radial profile of plasma density, and (c) providing a control on the scale length of gradient in electron temperature. A set of 19 independent coils of EEF make a variable aspect ratio, rectangular solenoid producing a magnetic field (B(x)) of 100 G along its axis and transverse to the ambient axial field (B(z) ~ 6.2 G) of LVPD, when all its coils are used. Outside the EEF, magnetic field reduces rapidly to 1 G at a distance of 20 cm from the center of the solenoid on either side of target and source plasma. The EEF divides LVPD plasma into three distinct regions of source, EEF and target plasma. We report that the target plasma (n(e) ~ 2 × 10(11) cm(-3) and T(e) ~ 2 eV) has no detectable energetic electrons and the radial gradients in its electron temperature can be established with scale length between 50 and 600 cm by controlling EEF magnetic field. Our observations reveal that the role of the EEF magnetic field is manifested by the energy dependence of transverse electron transport and enhanced transport caused by the plasma turbulence in the EEF plasma.

Plasma response to electron energy filter in large volume plasma device

An electron energy filter (EEF) is embedded in the Large Volume Plasma Device plasma for carrying out studies on excitation of plasma turbulence by a gradient in electron temperature (ETG) described in the paper of Mattoo et al. [S. K. Mattoo et al., Phys. Rev. Lett. 108, 255007 (2012)]. In this paper, we report results on the response of the plasma to the EEF. It is shown that inhomogeneity in the magnetic field of the EEF switches on several physical phenomena resulting in plasma regions with different characteristics, including a plasma region free from energetic electrons, suitable for the study of ETG turbulence. Specifically, we report that localized structures of plasma density, potential, electron temperature, and plasma turbulence are excited in the EEF plasma. It is shown that structures of electron temperature and potential are created due to energy dependence of the electron transport in the filter region. On the other hand, although structure of plasma density has origin in the particle transport but two distinct steps of the density structure emerge from dominance of collisionality in the source-EEF region and of the Bohm diffusion in the EEF-target region. It is argued and experimental evidence is provided for existence of drift like flute Rayleigh-Taylor in the EEF plasma.

Investigations on ETG turbulence in finite beta plasmas of LVPD

This paper presents the first controlled observations on electron temperature gradient (ETG) driven turbulence in finite beta (β ∼ 0.6) plasmas of the Large Volume Plasma Device (LVPD). The observed instability is investigated in the core region of the target plasma when a ∼2 m diameter magnetic electron energy filter is used. The observed instability in the lower hybrid range of frequencies has electromagnetic fluctuations associated with it and is characterized by broadband spectra with central frequency, ν ⩽ 10 kHz, and wave number, k⊥ = (0.1–0.2) cm−1, which satisfies the condition k⊥ρe ⩽ 1 where ρe is the electron Larmor radius. It is also observed that the observed mode satisfies the condition, k∥/k⊥ ≪ 1. A confirmation of it as ETG turbulence is demonstrated by its absence when ∇Te is made vanishingly small.

Revisiting plasma hysteresis with an electronically compensated Langmuir probe

The measurement of electron temperature in plasma by Langmuir probes, using ramped bias voltage, is seriously affected by the capacitive current of capacitance of the cable between the probe tip and data acquisition system. In earlier works a dummy cable was used to balance the capacitive currents. Under these conditions, the measured capacitive current was kept less than a few mA. Such probes are suitable for measurements in plasma where measured ion saturation current is of the order of hundreds of mA. This paper reports that controlled balancing of capacitive current can be minimized to less than 20 μA, allowing plasma measurements to be done with ion saturation current of the order of hundreds of μA. The electron temperature measurement made by using probe compensation technique becomes independent of sweep frequency. A correction of ≤45% is observed in measured electron temperature values when compared with uncompensated probe. This also enhances accuracy in the measurement of fluctuation in electron temperature as δT(pk-pk) changes by ~30%. The developed technique with swept rate ≤100 kHz is found accurate enough to measure both the electron temperature and its fluctuating counterpart. This shows its usefulness in measuring accurately the temperature fluctuations because of electron temperature gradient in large volume plasma device plasma with frequency ordering ≤50 kHz.

Experimental Observation of Electron-Temperature-Gradient Turbulence in a Laboratory Plasma

We report the observation of electron-temperature-gradient (ETG) driven turbulence in the laboratory plasma of a large volume plasma device. The removal of unutilized primary ionizing and nonthermal electrons from uniform density plasma and the imposition and control of the gradient in the electron temperature (T[Symbol: see text] T(e)) are all achieved by placing a large (2 m diameter) magnetic electron energy filter in the middle of the device. In the dressed plasma, the observed ETG turbulence in the lower hybrid range of frequencies ν = (1-80 kHz) is characterized by a broadband with a power law. The mean wave number k perpendicular ρ(e) = (0.1-0.2) satisfies the condition k perpendicular ρ(e) ≤ 1, where ρ(e) is the electron Larmor radius.

Theory of coupled whistler-electron temperature gradient mode in high beta plasma: Application to linear plasma device

This paper presents a theory of coupled whistler (W) and electron temperature gradient (ETG) mode using two-fluid model in high beta plasma. Non-adiabatic ion response, parallel magnetic field perturbation (δBz), perpendicular magnetic flutter (δB⊥), and electron collisions are included in the treatment of theory. A linear dispersion relation for whistler-electron temperature gradient (W-ETG) mode is derived. The numerical results obtained from this relation are compared with the experimental results observed in large volume plasma device (LVPD) [Awasthi et al., Phys. Plasma 17, 42109 (2010)]. The theory predicts that the instability grows only where the temperature gradient is finite and the density gradient flat. For the parameters of the experiment, theoretically estimated frequency and wave number of W-ETG mode match with the values corresponding to the peak in the power spectrum observed in LVPD. By using simple mixing length argument, estimated level of fluctuations of W-ETG mode is in the range of fluctuation level observed in LVPD.