Langmuir Probe Array Research Articles

Density fluctuation of geodesic acoustic mode on the HL-2A tokamak

The density fluctuations of the geodesic acoustic mode (GAM) have been observed in Ohmic deuterium plasma discharges with a combination of rake-like and three-step Langmuir probe arrays on the HL-2A tokamak. The probe arrays with poloidal and toroidal separations of 36 and 1330 mm are applied to measure the spectral property and intermittency of the GAM density fluctuations. The poloidal and toroidal mode numbers of the fluctuations are simultaneously measured for the first time. The measured fluctuation amplitude is consistent with the theoretical prediction. High coherence and near zero phase shift of the GAM density fluctuations separated toroidally by 37.5° at the same magnetic flux surface was first observed, indicating the symmetric structure of the GAM in the toroidal direction. The peak time delays of the cross-correlation function of the fluctuations above and below the midplane suggest the expected sin θ dependence. The nonlinear three wave coupling between the GAM and the ambient turbulence is shown to be a plausible mechanism for the generation of the GAM density fluctuations. The significant coherence and the corresponding fixed phase shift (∼π) between the original data and the envelope of the high frequency ambient turbulence provide the experimental evidence for the envelope modulation. The GAM amplitude is out of phase with the particle flux. Most of the intermittent frequencies for particle flux are close to the GAM frequency.

Experimental Studies of Low-Pressure Plasma Jet by Means of Langmuir Probes and Emission Spectra

An investigation was made into the argon plasma jet that expanded in a low-pressure vacuum chamber. The spatial distributions of the parameters of the plasma jet with different supplied powers were measured using a ten-channel Langmuir probe array. The chemical species in the plasma jet were identified by emission spectroscopy. The electron excitation temperatures at two positions, 10 cm and 50 cm downstream from the nozzle exit were calculated, respectively, by the Boltzmann plot method.

High heat flux Langmuir probe array for the DIII-D divertor plates

Two modular arrays of Langmuir probes designed to handle a heat flux of up to 25 MW/m(2) for 10 s exposures have been installed in the lower divertor target plates of the DIII-D tokamak. The 20 pyrolytic graphite probe tips have more than three times higher thermal conductivity and 16 times larger mass than the original DIII-D isotropic graphite probes. The probe tips have a fixed 12.5 degree surface angle to distribute the heat flux more uniformly than the previous 6 mm diameter domed collectors and a symmetric "rooftop" design to allow operation with reversed toroidal magnetic field. A large spring-loaded contact area improves heat conduction from each probe tip through a ceramic insulator into a cooled graphite divertor floor tile. The probe tips, brazed to molybdenum foil to ensure good electrical contact, are mounted in a ceramic tray for electrical isolation and reliable cable connections. The new probes are located 1.5 cm radially apart in a staggered arrangement near the entrance to the lower divertor pumping baffle and are linearly spaced 3 cm apart on the shelf above the in-vessel cryopump. Typical target plate profiles of J(sat), T(e), and V(f) with 4 mm spatial resolution are shown.

Characterization of intermittent bursts at the edge of the CASTOR tokamak

Temporal characteristics of density bursts in the scrape-off layer of the CASTOR tokamak were investigated. Intermittent positive bursts of density are observed by means of a radial array of Langmuir probes. Globally (comparing the closest and furthermost radial positions with respect to the center of the discharge chamber) we observe a radial decrease of average burst rate together with a radial increase of the average burst duration. Recently, we observed a monotonic radial decrease of average burst rate together with an increase of the average burst duration in the Tore Supra tokamak [1, 2], but the decay length is significantly shorter than in CASTOR. This is most probably due to radially elongated turbulent events (streamers), which govern the radial transport at the edge of the CASTOR tokamak [3] and are responsible for the appearance of density bursts.

Statistical properties of edge plasma turbulence in the Large Helical Device

Ion saturation current (Isat) measurements made by three tips of a Langmuir probe array in the Large Helical Device are analysed for two plasma discharges. Absolute moment analysis is used to quantify properties on different temporal scales of the measured signals, which are bursty and intermittent. Strong coherent modes in some datasets are found to distort this analysis and are consequently removed from the time series by applying bandstop filters. Absolute moment analysis of the filtered data reveals two regions of power-law scaling, with the temporal scale τ ≈ 40 µs separating the two regimes. A comparison is made with similar results from the Mega-Amp Spherical Tokamak. The probability density function is studied and a monotonic relationship between connection length and skewness is found. Conditional averaging is used to characterize the average temporal shape of the largest intermittent bursts.

Plasma Start-Up Measurements in the Toroidal Plasma Experiment TJ-K

The torsatron TJ-K confines a toroidal low-temperature plasma, which is accessible for Langmuir probes throughout the whole confinement region. With the Langmuir-probe array ldquoSPIDERrdquo, spatially and temporally resolved profiles of the plasma density can be measured in the poloidal cross section in order to observe dynamical processes in the plasma. Thus, a complete plasma start-up from neutral gas up to the plasma equilibrium can be visualized, which allows one to draw conclusions on heating processes.

IBW heating experiments in HT-7 tokamak

The Experiments with Ion Berstein wave (IBW) heating has been carried out in HT-7 superconducting tokamak. Core electron heating and peaking profile of electron temperature can be observed in the beginning 40 ms after injection of IBW. The temporary heating efficiency of IBW increases with the toroidal magnetic field (BT). The electron temperature drops gradually with the increasing electron density after 40 ms of IBW. The electron density increases by a factor up to 2.4 and the profile of electron density becomes wide. Improved particle confinement induced by IBW is one reason for the density increase. Impurity release during IBW heating is also a contribution for the density increase. Suppression of MHD by IBW is also observed. The fluctuation and transport in the SOL of HT-7 tokamak have been investigated using a Langmuir probe array. The polodial shear flow induced by IBW maybe the reason for improved particle confinement.

Experimental Study of Drift Wave Turbulence in Linear Plasmas

Linear plasmas allow multi-point, low-temperature measurements with Langmuir probes. We measured ion saturation-current fluctuations of a Large Mirror Device-Upgrade linear plasma using a poloidal Langmuir probe array. By varying the discharge conditions, the spatiotemporal behavior showed a change from a coherent sine wave to a turbulent waveform through a periodic, modulated sine wave. The two-dimensional (poloidal wave number and frequency) power spectrum for each regime showed a single fluctuation peak, a peak and its harmonics, and a number of peaks in the poloidal wave number‐frequency space. Bi-spectral analysis was performed for the turbulent regime, and showed the existence of nonlinear couplings among fluctuation peaks and broadband components.

Spatiotemporal Behavior of Drift Waves in LMD-U

In the LMD-U linear magnetized plasma, fluctuation measurements have been performed with multi-channel poloidal Langmuir probe arrays to investigate the spatiotemporal behaviors of drift wave turbulence. Twodimensional (poloidal mode number‐frequency) power spectrum showed not only fluctuation peaks but also the existence of a broadband fluctuation. The broadband fluctuation developed at high poloidal mode number and high frequency regions and not along the linear dispersion relation curve of drift wave modes. It showed shorter correlation time and poloidal length than the peaked fluctuations. Two-dimensional axial coherence was measured with two poloidal probe arrays separated in the axial direction. The axial coherence was strong for both the broadband fluctuation and peak fluctuations, suggesting the quasi-two-dimensional structure of the drift wave turbulence. c

Characteristics of geodesic acoustic mode zonal flow and ambient turbulence at the edge of the HL-2A tokamak plasmas

The three-dimensional characteristics of the geodesic acoustic mode zonal flows (GAMZFs) and the ambient turbulence (AT) at the edge of the HuanLiuqi-2A tokamak [Y. Liu et al., Nucl. Fusion 45, S203 (2005)] are investigated with Langmuir probe arrays and the results are presented in detail. The toroidal and poloidal symmetries, and the radial scale of the GAMZFs are simultaneously identified. The envelopes of the high frequency components of the AT in the presence of the GAMZFs are analyzed. The GAM frequency components (GAMFCs) of the coherent envelopes are also shown to have poloidal and toroidal symmetries, and similar radial scales as the GAMZF does. The correlation between the GAMFCs of the envelopes and the GAMs is high, with phase shifts between π/2 to π, indicating that the GAMZFs may regulate the AT and the regulation is embodied in the envelopes. Three-wave coupling between GAM and AT is found to be a plausible formation mechanism for the former, which acts on the whole spectra of the latter within its scale length. The temporal evolutions of the total fluctuation power, the GAM and the AT powers show that the AT power decreases when GAM power increases and vice versa, indicating possible regulating effects of the latter on the former.

Three-dimensional features of GAM zonal flows in the HL-2A tokamak

A novel design of the three-step Langmuir probe (TSLP) array has been developed to investigate the zonal flow (ZF) physics in the HL-2A tokamak. Three TSLP arrays are applied to measure the three-dimensional (3D) features of ZFs. They are separated by 65 mm in the poloidal and 800 mm in the toroidal directions, respectively. The 3D properties of the geodesic acoustic mode (GAM) ZFs are presented. The poloidal and toroidal modes of the radial electric fields of the GAM perturbations are simultaneously determined in the HL-2A tokamak for the first time. The modes have narrow radial wave numbers (krρi = 0.03–0.07) and short radial scale lengths (2.4–4.2 cm). High coherence of both the GAM and the ambient turbulence separated by toroidal 22.5° along a magnetic field line is observed, which contrasts with the high coherence of the GAM and the low coherence of the ambient turbulence apart from the field line. The nonlinear three wave coupling between the turbulent fluctuations and the ZFs is a plausible mechanism for flow generation. The skewness and kurtosis spectra of the probability distribution function of the potential perturbations are contrasted with the corresponding bicoherence for the first time, which support the three wave coupling mechanism.

Spatiotemporal temperature fluctuation measurements by means of a fast swept Langmuir probe array

Stationary Langmuir probe measurements of ion saturation current and floating potential in a plasma cannot give direct information on density and plasma potential fluctuations in the presence of temperature fluctuations. This problem can be avoided if the probe bias voltage is continuously swept faster than the fluctuation time scale, recording the current-voltage characteristic. This article reports the development of a spatiotemporal highly resolving Langmuir probe array with 15 fast swept tips, operating in the strongly magnetized, collisionless edge plasma of the Wendelstein 7-AS stellarator [Plasma Phys. Controlled Fusion 31, 1579 (1989)]. The probe tips are aligned in the poloidal direction, the tip spacing is 2 mm, and the sweeping frequency is 1.4 MHz. Current and voltage data are sampled with 50 MHz. The high bandwidth of the measurement is achieved by placing miniaturized differential amplifiers close to the probe tips in order to do an impedance transform. The surface-mounting technology and an additional inverse feedback module are utilized, allowing for an input voltage range of +/-100 V, and a common mode rejection rate of 55 dB at 4 MHz, which is sufficient to resolve the nonlinear probe characteristic. For the evaluation of the data, a fit model for stationary probes is employed and found adequate. Changes of the plasma parameters during one voltage sweep are taken into account by a linear interpolation of the fit parameters. Spatio-temporal fluctuation data gained by a fast swept Langmuir probe array, which can be relevant for the turbulent radial transport of particles and energy, are presented.

Advanced probes for edge plasma diagnostics on the CASTOR tokamak

Understanding of underlying physics in the edge plasma of tokamaks requires knowledge of the plasma density, potential, electron and ion temperature, ion flows and their fluctuations with a high spatial and temporal resolution. A family of electric probes, which have been designed and tested for this purpose in the CASTOR tokamak, is reviewed and examples of their performance are given. In particular, we focus on description of the 1D and 2D arrays of Langmuir probes for spatially resolved measurements of the edge turbulence, the Ball pen and emissive probes for direct measurements of the plasma potential, the optimized Gundestrup probe for measurements of parallel and perpendicular ion flow, and the tunnel probe for fast measurement of electron and ion temperatures. Additional information on individual diagnostics is available in the listed references. PACS 52.70.Ds

Radially propagating turbulent structures in a linear helicon plasma device

The propagation properties of plasma density structures in a turbulent linear cylindrical helicon plasma are investigated. The governing instability is of the drift-wave type. An azimuthal limiter is used to steepen the radial density gradient to achieve a weakly developed turbulent state. The spatiotemporal evolution of turbulent plasma density fluctuations is investigated by Langmuir probe array measurements. In the strong plasma density gradient region the probability distribution function (PDF) of density fluctuations is rather symmetric. Here, the relict of a single m=1 drift mode dominate the correlation of the fluctuations. Radially further outwards the fluctuations are dominated by large positive density bursts, associated with strongly asymmetric PDFs. The spatiotemporal measurements reveal that these intermittent bursts can be ascribed to radially propagating turbulent structures, which originate in the strong plasma gradient region. Their radial velocity reaches 5% of the local ion sound speed.

Heat- and Particle-Deposition Distribution on Helical Divertor Plates in LHD During Real-Time Magnetic-Axis Swing Operations

Investigations of heat- and particle-deposition distributions on helical divertor plates were performed both experimentally and numerically with the Large Helical Device (LHD). The distributions were measured by thermocouples and Langmuir probe arrays embedded in the divertor plates. They are similar to the distribution of field lines with long connection lengths, which are estimated numerically by field-line tracing calculations. Localized heat- and particle-deposition distributions were observed, and were found to be determined primarily by the magnetic-field line structure in the divertor region. They vary depending on the configuration − the intensively loaded areas move with a change of the magnetic configuration. Using this property, a swing of the magnetic axis successfully dispersed the heat and particle loads on the divertor plate during long-pulse discharges. The magnetic-axis swing around a certain magnetic-axis position is found to be very effective in changing the field-line distribution pattern drastically, and thus dispersing the heat load.

Overview of HIT-SI Diagnostic Systems

The Helicity Injected Torus with Steady Inductive current drive (HIT-SI) device is a spheromak which uses two inductively driven helicity injectors to provide constant helicity injection. The HIT-SI diagnostic set includes 96 3-D magnetic surface probes, 20 toroidal flux loops, an internal array of three stems of 3-D magnetic probes, a far infrared interferometer, a SPRED diagnostic, two tangentially viewing vacuum ultraviolet (VUV) spectrometers, a bolometer, a Zeff diagnostic, H-alpha detectors, a time integrated CCD camera, and Ion Doppler Spectroscopy (IDS). In addition, a multi point Thomson Scattering system, a microwave interferometer/polarimeter, injector voltage probes, an edge Langmuir probe array, and soft X-ray camera are under development.

Novel design for zonal flow probe arrays in the HL-2A tokamak

A novel design of three-step Langmuir probe (TSLP) array has been developed for zonal flow (ZF) studies in the HL-2A tokamak. Three TSLP arrays are used to determine three-dimensional (3D) features of the ZFs. Two TSLP arrays are located in the same poloidal plane at a 6.5cm separation, while the third TSLP array is separated toroidally by 80cm. A fourth TSLP array driven by pneumatically reciprocating system is applied for boundary parameter profile measurements within 8cm. The TSLP structure is described in detail. The measured 3D properties of the geodesic acoustic mode (GAM) ZFs are described and the poloidal and toroidal mode numbers (m<2,n∼0) are simultaneously determined in the HL-2A tokamak for the first time. The radial wave vector at the GAM frequency is estimated as 2.6±0.2cm−1, corresponding to radial scale length of 2.4cm.

Influence of the Static Dynamic Ergodic Divertor on Edge Turbulence Properties in TEXTOR

Systematic measurements on the edge turbulence and turbulent transport have been made by Langmuir probe arrays on TEXTOR under various static Dynamic Ergodic Divertor (DED) configurations. Common features are observed. With the DED, in the ergodic zone the local turbulent flux reverses sign from radially outwards to inwards. The turbulence properties are profoundly modified by energy redistribution in frequency spectra and suppression of large scale eddies. The fluctuation poloidal phase velocity changes direction from electron to ion diamagnetic drift, consistent with the observed reversal of the Er x B flow. In the laminar region, the turbulence is found to react to an observed reduced flow shear.

Analysis on Relation Between Magnetic Structure and Bursty Fluctuation in SOL/Divertor Plasmas of LHD

We have investigated a relation between the edge magnetic structure of the Large Helical Device (LHD) and the fluctuation properties of the ion saturation currents I sat measured by a Langmuir probe array (16 channels) embedded in an outboard divertor plate. The edge magnetic structure is calculated by KMAG and Watanabe codes. The fluctuation properties are analyzed based on the probability distribution function (p.d.f.) and complex Morlet wavelet decomposition. Large positive skewness was obtained from I sat measured at a low field side of a divertor leg and negative skewness was observed at striking point of the divertor leg, where the connection length of magnetic line of force is long. The fluctuation characteristics has very weak dependence of plasma density and its gradient. Waiting time statistics was analyzed to indicate that the waiting time statistics is not a Poisson process and there is no clear signature of SOC paradigm. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Bursty Behaviour of Turbulent Particle and Energy Fluxes in Edge Plasma of HT-7 Tokamak

High time resolution measurements of the electrostatic fluctuations and the turbulent particle and energy fluxes have been performed with a Langmuir probe array in the edge plasma in HT-7 tokamak. Bursty behaviour was observed in the time resolved turbulent fluxes with positive skewness and large kurtosis. The contribution of the large sporadic bursts to the transport losses were estimated. The analysis shows that the turbulent fluxes have different behaviour in different frequency domains and the probability distribution functions (PDFs) of the particle and energy fluxes present two distinct scaling ranges. All these are essentially consistent with the predictions of the self-organized criticality (SOC) model, though further studies are needed.