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We report the experimental observation of fully developed Kolmogorov turbulence originating from self-excited vortex flows in a three-dimensional (3D) dust cloud. The characteristic −5/3 scaling of 3D Kolmogorov turbulence is consistent in both the spatial and temporal energy spectra within a statistical variation of experimental data. Additionally, the 2/3 scaling in the second-order structure function further supports the presence of Kolmogorov turbulence. We also identified a slight deviation in the tails of the probability distribution functions for velocity gradients, a reflection of intermittency. The experiment showed the formation of a dust cloud in the diffused plasma region away from the electrodes. The dust rotation was observed in multiple experimental campaigns under different discharge conditions at different spatial locations and background plasma environments. [ABSTRACT FROM AUTHOR]

The electrostatic particle flux is measured in the presence of obliquely propagating quasi-longitudinal (QL) whistler turbulence ( ω c i < ω L H ≈ ω < ω c e ) 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 [ B EEF (x ̂) ] of electron energy filter and axial magnetic field of LVPD [ B o (z ̂) ]. The effect of mirror strength on radial particle flux is explored by changing the B EEF 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. [ABSTRACT FROM AUTHOR]

Using a uniquely configured glow discharge-based Shivalik Plasma Device-I, we demonstrate a variety of collective phenomena in dusty plasma away from the glow discharge region. The cylindrical glass device produces plasma using parallel disc-shaped electrodes with a smaller anode size than the cathode. The dust microparticles are initially sprinkled over the grounded cathode. These particles acquire a significant negative charge upon plasma formation, resulting in their levitation due to the balance between the Coulomb force and gravity. The new device supports the levitation of a big-sized (10 × 8 × 5 cm3) three-dimensional dust cloud over the glass surface. It contrasts the dusty plasma formations in-between electrodes reported earlier. As the discharge voltage varies from high to low, the dust cloud travels from over the glass surface to between the electrodes. A complex interplay of dust void over the cathode, a sharp density gradient, and gravity lead to self-excitation of collective dust phenomena. It includes dust density waves (phase velocity, vph ∼ 4 cm/s), dust cloud oscillation (frequency, f = 5 Hz), sheared flow (flow velocity, vf ∼ 1 cm/s), and multiple-sized dust vortices. These dust vortices provided an excellent platform for studying turbulent mixing phenomena. The power spectrum analysis agreed with two-dimensional Kolmogorov power-law scaling. This is an ideal dusty plasma apparatus where we can create or move the dust cloud to a location of choice from the glass surface to in-between the electrodes and excite one among many collective dust dynamics. [ABSTRACT FROM AUTHOR]

Using incoherent Thomson scattering, electron heating and acceleration at the electron velocity distribution function (EVDF) level are investigated during electron-only reconnection in the PHAse Space MApping (PHASMA) facility. Reconnection arises during the merger of two kink-free flux ropes. Both push and pull type reconnection occur in a single discharge. Electron heating is localized around the separatrix, and the electron temperature increases continuously along the separatrix with distance from the X-line. The local measured gain in enthalpy flux is up to 70% of the incoming Poynting flux. Notably, non-Maxwellian EVDFs comprised of a warm bulk population and a cold beam are directly measured during the electron-only reconnection. The electron beam velocity is comparable to, and scales with, electron Alfvén speed, revealing the signature of electron acceleration caused by electron-only reconnection. The observation of oppositely directed electron beams on either side of the X-point provides "smoking-gun" evidence of the occurrence of electron-only reconnection in PHASMA. 2D particle-in-cell simulations agree well with the laboratory measurements. The measured conversion of Poynting flux into electron enthalpy is consistent with recent observations of electron-only reconnection in the magnetosheath [Phan et al., Nature 557, 202 (2018)] at similar dimensionless parameters as in the experiments. The laboratory measurements go beyond the magnetosheath observations by directly resolving the electron temperature gain. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Magnetic flux ropes have been successfully created with plasma guns in the newly commissioned PHAse Space MApping (PHASMA) experiment. The flux ropes exhibit the expected m = 1 kink instability. The observed threshold current for the onset of this kink instability is half of the Kruskal–Shafranov current limit, consistent with predictions for the non-line tied boundary condition of PHASMA. The helicity, paramagnetism, and growth rate of the observed magnetic fluctuations are also consistent with kink instability predictions. The observed fluctuation frequency appears to be a superposition of a real frequency due to a Doppler shift of the kink mode arising from plasma flow (∼ 2 kHz) and a contribution from a wave mode (∼ 5 kHz). The dispersion of the wave mode is consistent with an Alfvén wave. Distinct from most previous laboratory studies of flux ropes, the working gas in PHASMA is argon. Thus, the ion cyclotron frequency in PHASMA is quite low and the frequency of the Alfvénic mode plateaus at ∼ 0.5 of the ion gyro frequency with increasing background magnetic field strength. [ABSTRACT FROM AUTHOR]

A new incoherent Thomson scattering system measures the evolution of electron velocity distribution functions perpendicular and parallel to the ambient magnetic field during kinking of a single flux rope and merging of two flux ropes through magnetic reconnection. The Thomson scattering system provides sub-millimeter spatial resolution, sufficient to diagnose the several millimeters sized magnetic reconnection electron diffusion region in the PHAse Space MAppgin experiment. Due to the relatively modest plasma density ∼1019 m−3 and electron temperature ∼1 eV, stray light suppression is critical for these measurements. Two volume Bragg gratings are used in series as a notch filter with a spectral bandwidth <0.1 nm in the collection branch. A CCD with a Gen III intensifier with peak quantum efficiency >47% is used as the detector in a 1.3 m spectrometer. Preliminary results of gun plasma electron temperature will be reported and compared with measurements obtained from a triple Langmuir probe. [ABSTRACT FROM AUTHOR]

Radial control of the electron temperature gradient is demonstrated in a double plasma device by making use of segmented grid biasing. The plasma produced in the source region is allowed to diffuse into the target region through a single grid as well as through the cassette of multiple grid assembly, under different grid bias conditions. Both electron heating and cooling are observed radially at one location in the target region when a single grid is used. The electrons are cooled down to a temperature of 3.3 eV from 5.1 eV when the grid bias is raised from −25 to 0 V. Similarly, during heating, the electron temperature increases from 4.8 to 7.3 eV when the grid bias is varied between 0 and +20 V. Two different transparencies of grids, 45% transparency (mess-size, m = 0.8 mm ∼ λDe) and 75% transparency (mess-size, m = 2.4 mm > λDe), are used, where the value of λDe ≈ 0.8 mm. The obtained electron energy distribution function suggests that a grid with less transparency is more effective in cooling the electrons because of insignificant energetic electron–neutral collisions in the target region as a sheath in the close vicinity of grid allows only the high energetic electrons to pass through it. The higher transparent grid, on the other hand, produces electron heating as it exerts a negligible influence on the free flow of accelerated high energy electrons to target plasma due to insignificant thermalization. We expanded this concept and, for novelty, applied it to a radially segmented grid assembly of electrically isolated grids, for effectively charging different plasma regions with differently various potentials for exerting a radial control on electron temperature. The results obtained show that a significantly sharp electron temperature gradient is obtained with a typical gradient scale length of L T e ∼ 10 cm in the target plasma region. The outcome of this study may be useful both in plasma processing applications and for studying plasma turbulence in unmagnetized plasmas. [ABSTRACT FROM AUTHOR]

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, η ETG = L n / L T > 2 / 3 satisfied, where, L n = 1 n dn dx − 1 is the density scale length and L T e = 1 T e d T e dx − 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 ⊥ ρ e ≤ 1 ≪ k ⊥ ρ i , Ω i < ω ≪ Ω e , where k ⊥ is the perpendicular wave vector, ρ e , ρ i are Larmor radii of the electron and ion, respectively, and Ω i , Ω e , ω are the ion, electron gyro frequencies and the mode frequency, respectively. Simultaneous measurement of fluctuations in electron temperature, δ T e ∼ (10 − 30) % , plasma density, δ n e ∼ (5 − 12) % , and potential δ V f ∼ (1 − 10) % are obtained. A strong negative correlation with correlation coefficients C δ n − δ φ ∼ − 0.8 and C δ T − δ φ ∼ − 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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, ɳETG =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˔pe ⩽1⪡ k˔pi Ωi < ω Ωe), where k˔ is the perpendicular wave vector, pe ; pi 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 (Γ es ≈ 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, θ n ϕ and turbulent particle flux, Γes, shows good agreement with the theoretical estimations derived for ETG turbulence. [ABSTRACT FROM AUTHOR]

Non-Maxwellian electron velocity distribution functions composed of a warm bulk population and a cold beam are directly measured during electron-only reconnection with a strong out-of-plane (guide) magnetic field in a laboratory plasma. Electron heating is localized to the separatrix, and the electron temperature increases continuously along the separatrix. The measured gain in enthalpy flux is 70% of the incoming Poynting flux. The electron beams are oppositely directed on either side of the X point, and their velocities are comparable to, and scale with, the electron Alfvén speed. Particle-in-cell simulations are consistent with the measurements. The experimental results are consistent with, and go beyond, recent observations in the magnetosheath. [ABSTRACT FROM AUTHOR]