Heavy Ion Beam Probe Measurements Research Articles

Improvement in the spatial resolution of heavy ion beam probe measurements through application of ion optics.

A technique for more accurately modeling and improving the spatial resolution of heavy ion beam probe measurements is described. We use a set of particle trajectories to numerically determine the focusing properties of a complicated three-dimensional magnetic field and characterize these properties with a transfer matrix. We then modify the transfer matrix approach of traditional ion optics to include a parameter that describes the ionization location of the detected ions. The ion optics model calculated using this technique enables a more accurate description of the particle trajectories than previously feasible. The model also allows one to easily determine an initial beam focus that could be used during experimental operation to optimize the spatial resolution of measurements. The technique has been applied to the design of a heavy ion beam probe diagnostic for the Wendelstein 7-X stellarator, and improvements in the modeled spatial resolution by a factor of about 2 over previous estimates are possible. The improved spatial resolution will enable measurements of plasma fluctuations with smaller wavelengths than would otherwise be possible.

Effect of suprathermal electrons flux on the ambipolar electric field in stellarators

Low collisionality plasmas in non-axisymmetric toroidal magnetic devices have significant radial particle fluxes due to trapped particles in the magnetic ripples. The associated ambipolar radial electric field cannot be quantitatively reproduced by standard neoclassical models since the measured values are systematically larger. Here the contribution of suprathermal electrons is included in the radial electric field. The radial flux of suprathermal particles is calculated following a kinetic analysis of the particle transport in which a non-Maxwellian velocity distribution is assumed for the high velocity electrons in the low collisionality regime. The electric field modification produced by the suprathermal electrons is then obtained and an analytical expression for the radial electric field is found which is compared with the one obtained from the thermal particles. An estimate of the needed contribution of the suprathermal electrons is given. Experimental data of the TJ-II Heliac-stellarator are taken for the computations since good Heavy Ion Beam Probe measurements are available.

Fusion Research in Ioffe Institute

In this paper we present the fusion related activities of the Plasma Physics Division at the Ioffe Institute. The first experiments on lower hybrid current drive (LHCD) in a spherical tokamak performed at the Globus-M tokamak (R = 0.36 m, a = 0.24 m, Bt = 0.4 T, Ip = 200 kA) with a novel poloidally oriented grill resulted in an RF driven current of up to 30 kA at (100 kW, 2.5 GHz), exceeding the modelling predictions. At the FT-2 tokamak (R = 0.56 m, a = 0.08 m, Bt = 3 T, Ip = 30 kA) experiments with a traditional toroidally oriented grill revealed no strong dependence of the LHCD density limit on the H/D ratio in spite of LH resonance densities differing by a factor of 3. Microwave Doppler reflectometry (DR) at the Globus-M, and DR and heavy ion beam probe measurements at the tokamak TUMAN-3M (R = 0.53 m, a = 0.24 m, Bt = 1.0 T, Ip = 190 kA) demonstrated geodesic acoustic mode (GAM) suppression at the L to H transition. Observations at FT-2 using Doppler Enhanced Scattering showed that the GAM amplitude is anti-correlated both spatially and temporally to the drift turbulence level and electron thermal diffusivity. For the first time turbulence amplitude modulation at the GAM frequency was found both experimentally and in global gyrokinetic modelling. A model of the L–H transition is proposed based on this effect. The loss mechanisms of energetic ions' (EI) were investigated in the neutral beam injection (NBI) experiments on Globus-M and TUMAN-3M. Empirical scaling of the 2.45 MeV DD neutron rate for the two devices shows a strong dependence on toroidal field and plasma current justifying the Bt and Ip increase by a factor of 2.5 for the proposed upgrade of Globus-M. Bursts of ∼1 MHz Alfvenic type oscillations correlating with sawtooth crashes were observed in ohmic TUMAN-3M discharges. The possibility of low threshold parametric excitation of Bernstein and upper hybrid waves trapped in drift-wave eddies resulting in anomalous absorption in electron cyclotron resonance heating (ECRH) experiments in toroidal plasmas was identified theoretically. A novel method of radial correlation Doppler reflectometry is shown to be capable of measuring the turbulence wave-number spectrum in realistic 2D geometry. On the progress in design and fabrication of three diagnostics for ITER developed in the Ioffe institute is reported: neutral particle analysis, divertor Thomson scattering and gamma spectroscopy.

Charge Fraction Measurements for Heavy Particle Beams Generated by the Tandem Accelerator with MCP System

In order to expand the applied range of the heavy ion beam probe measurement on LHD, it is important to increase the probing beam current. A method of increasing the current is the improvement of charge exchange efficiency of a gas cell. The experiments were carried out on a tandem accelerator at Kobe University, 5SDH-2, because various conditions of experiments can be selected for gas species, gas pressure, and beam energy. Charge fractions of negative ions, neutral atoms and the positive ions were measured by a micro channel plate or Faraday cup. Dependence of the fractions of Au - , Au 0 , Au + and Au 2+ on gas thickness was measured. Results were compared with prior calculations, and a solution of rate equations. The cross sections for ionizing and electron capture were calculated, and some cross sections were obtained from experiments. The dependence of charge fractions for Au 0 , Au + and Au 2+ on the gas

Production of Sub-MeV Positive Gold Ion Beams with Various Gas Targets to Improve the Tandem Accelerator of the LHD-HIBP

To expand the application range of heavy ion beam probe measurement in the Large Helical Device (LHD), it is important to increase the probing beam current. The current can be increased by improving the charge exchange efficiency in a gas cell. Experiments were conducted on a tandem accelerator at Kobe University, 5SDH-2, which allows various experimental conditions such as gas species, gas pressure, and beam energy to be selected. Initially, the gas thickness was determined by a beam attenuation method with a He+ ion beam. Next, the dependence of the positive beam current on the gas thickness was measured. The ionic charge fraction of the accelerated beam was measured and compared with the value calculated using the ionization cross section calculated in [M. Nishiura et al., NIFS Report, NIFS-884 (2008)]. The dependence of the Au+ fraction on the target gas thickness shows good qualitative agreement with the experimental value. However, the cross sections obtained experimentally are several times larger than those calculated.

Heavy ion beam probe coordinate mapping and calibration at WEGA stellarator

The heavy ion beam probe (HIBP) is an established nonperturbing diagnostic for high spatially and temporary resolved measurements of magnetically confined plasma parameters such as potential, density, and temperature. These quantities can be determined from the change in the ion beam parameters (charge, intensity, and trajectory) passing through a plasma volume due to collisions with electrons and interaction with the confining magnetic field. One of the problems that should be solved during HIBP installation and tuning is the coordinate matching. Conventionally the coordinate mapping of the HIBP measurement point is provided by ray tracing calculations of the ion beam in the magnetic field. However, it is very difficult to include all physical effects and uncertainties in the model. Thus, the result of the calculations may differ from the real probing position. In order to improve the mapping precision of the HIBP installed at the WEGA stellarator an additional measurement of the beam position is provided using a primary beam detector array inside the vacuum vessel. This allows comparing the measured and calculated ion beam positions in order to prove the calculated coordinate precision and include adjustments in the calculation code if necessary. The principle and the results of this calibration, which is not specific to WEGA but could be adapted to other experiments as well, are presented in this work.

Measurements of spatial structure of plasma potential and density fluctuations by multichannel heavy ion beam probe on large helical device

Heavy ion beam probe (HIBP) on large helical device is currently equipped with three channel detectors, which can observe three spatial points simultaneously inside the plasma with resolution of approximately 10 mm. The beam trajectories and observation point location are calculated numerically and optimized allowing for the identification of the mode structure in multichannel (up to 9) HIBP measurements. The calculations show that the radial and poloidal wavenumbers can be identified by proper changing and choosing of the beam energy and trajectory.

Reconstruction method of local density fluctuation for heavy ion beam probe measurements

Heavy ion beam probe (HIBP) is an excellent diagnostic to measure the density and potential fluctuations simultaneously in magnetically confined plasmas. However, it has been well known that the density fluctuation measured with the HIBP is not local but contains the fluctuations along the beam orbits. In this article, a method is proposed to evaluate local density fluctuation in the HIBP measurements by removing the well-known path integral effects. The reconstructed density fluctuation amplitude and power spectrum are shown, for example, by applying the proposed method on the density fluctuation measurement data obtained in a toroidal helical plasma, Compact Helical System.

Recent Experimental Studies of Zonal Flow: Zonal Flow Study with Heavy Ion Beam Probes I

This report presents the recent identification of stationary zonal flow in CHS plasma using twin heavy ion beam probes and describes the general diagnostics principle used. The heavy ion beam probe measurement reveals that electric field fluctuation at low frequencies less than ˜1 kHz exhibits the characteristics of zonal flow, i.e., toroidal symmetry and a finite radial wavelength of the fluctuation.

Turbulent transport reduction by E×B velocity shear during edge plasma biasing: recent experimental results

Experiments in the tokamaks TEXTOR, CASTOR, T-10 and ISTTOK, as well as in the reversed field pinch RFX have provided new and complementary evidence on the physics of the universal mechanism of E×B velocity shear stabilization of turbulence, concomitant transport barrier formation and radial conductivity by using various edge biasing techniques.In TEXTOR the causality between transport reduction and induced electric fields in the edge has been for the first time clearly demonstrated. The high electric field gradients have been identified as the cause for the quenching of turbulent cells. A quantitative analysis of the measured transport reduction is in good agreement with theoretical predictions. The scaling of plasma turbulence suppression with velocity shear has been established, revealing the density-potential cross-phase as a key element. Reduction in poloidal electric field, temperature, and density fluctuations across the shear layer lead to a reduction of the anomalous conducted and convected heat fluxes resulting in an energy transport barrier that is measured directly.In CASTOR the biasing electrode is placed at the separatrix in a non-intrusive configuration which has demonstrated strongly sheared electric fields and consequent improvement of the global particle confinement, as predicted by theory. The impact of sheared E×B flow on edge turbulent structures has been measured directly using a comprehensive set of electrostatic probe arrays as well as emissive probes. Measurements with a full poloidal Langmuir probe array have revealed quasi-coherent electrostatic waves in the SOL with a dominant mode number equal to the edge safety factor.In T-10 edge biasing is clearly improving the global performance of ECR heated discharges. Reflectometry and heavy ion beam probe measurements show the existence of a narrow plasma layer with strong suppression of turbulence.On ISTTOK, the influence of alternating positive and negative electrode and (non-intrusive) limiter biasing has been compared. Electrode biasing is found to be more efficient in modifying the radial electric field Er and confinement, limiter biasing acting mainly on the SOL.In the RFX reversed field pinch it has been demonstrated that also in RFPs biasing can increase the local E×B velocity shear in the edge region, and hence substantially reduce the local turbulence driven particle flux mainly due to a change in the relative phase between potential and density fluctuations.

Preliminary design of a heavy ion beam probe diagnostic for the National Spherical Torus Experiment

The preliminary design of a heavy ion beam probe (HIBP) for the National Spherical Torus Experiment (NSTX) is presented. The primary purpose of the diagnostic would be to measure density and electric potential fluctuations and steady state potential profiles. A secondary purpose is magnetic equilibrium and fluctuation measurements. HIBP particle trajectories have been calculated for a double null discharge case. The key HIBP measurements can be made with the injection point—detection point combination which is presented, including a radial scan line at a single energy (for potential profile measurements), poloidally aligned sample volumes (for poloidal wavenumber measurements), and coverage of a large fraction of the upper low field quadrant of the plasma. In addition, the injection point can be combined with new detectors for radial array measurements (for density fluctuation and/or magnetic measurements). Due to a large range of angles at the detector, up to 23° poloidal angle range and a 6° toroidal angle range, a secondary sweep system will be necessary for NSTX.

A heavy ion beam probe for the Madison Symmetric Torus

A heavy ion beam probe (HIBP) is being developed for application on the Madison Symmetric Torus (MST). It will be used to make measurements of the plasma space potential Φ(r), fluctuations in potential Φ̃(r), the electron density ne(r), fluctuating electron density ñe(r), and radial electric field Er(r) from the core to the edge region of the plasma in MST. While information on these quantities can and has been obtained with probes inserted in the surface region, none of the above measurements have been made in the core of a hot reversed field pinch. Measurements of Φ(r), Er(r), and ne(r) have been well established in previous HIBP systems on tokamaks such as Impurities Studies Experiment, Texas Experimental Tokamak (TEXT), and TEXT Upgrade, stellarators such as Advanced Toroidal Facility and Compact Helical System and Bumpy Tori such as Elmo Bumpy Torus and Nagoya Bumpy Torus. Less well developed in terms of HIBP measurements are equilibrium and fluctuating magnetic fields. Because the confining field on MST is determined by plasma conditions, some effort has been made in the design of the MST beam probe to make it possible to characterize B before, during, and after the plasma discharge.

Consideration of fluctuation in secondary beam intensity of heavy ion beam probe measurements

Heavy ion beam probes have the capability to detect the local fluctuation of density in the interior of plasmas through the fluctuation of the intensity of the detected beam. However, this intensity fluctuation may suffer from a certain degree of distortion from fluctuations of electron density and temperature along the beam trajectory; as a result, the fluctuation signal may be quite different from the local density fluctuation. This article presents the conditions where the intensity fluctuation can be regarded as being local fluctuation of electron density, where it cannot, and discusses the contamination of the fluctuations by fluctuations along the trajectory, particularly in the case of magnetohydrodynamics fluctuations.

Comparison of drift wave models with fluctuation data from the interior of the TEXT tokamak

Experimental evidence for and against drift waves as the origin of the observed fluctuations and anomalous transport in the plasma interior is reviewed. Fluctuation spectra observed by far-infrared (FIR) scattering and a heavy ion beam probe (HIBP) are compared. The FIR system observes broad S(k,ω), which are spatially resolved at large k and readily identified with electron drift waves. At higher densities a clear ion feature, which may be associated with ion temperature gradient (ITG) modes, appears as well. A quasicoherent feature in the drift wave range of phase velocities is also found at the inner midplane. But interior HIBP measurements using a thallium beam have, by a two-point correlation method, measured wave numbers far too small (i.e., phase velocities far too high) to be those of drift waves of either variety. Some recent measurements with a cesium beam have produced phase velocities more closely in accord with drift waves at low frequencies. It is suggested that a new mode may be present, which is not drift wavelike. However, the phase velocity and coherence of the HIBP data exhibit qualitative features that suggest possible instrumental effects. Several of these are investigated, which individually do not appear to reconcile the data with a pure drift wave model. They are (1) sample volume size, (2) common mode effects such as cross-talk or path integrals of beam attenuation, and (3) two-stream instabilities. Reconciliation of these measurements is important to transport studies. That is, calculated particle and energy fluxes depend sensitively on the frequency and wave-number spectrum employed. The theoretical considerations about two-point correlations and possible instrumental effects are also relevant to other diagnostics using this technique.

Heavy ion beam probe energy analyzer for measurements of plasma potential fluctuations

The operation of a Proca and Green type 30° parallel plate electrostatic energy analyzer is modeled in a new manner that permits high-resolution heavy ion beam probe measurements of fluctuating plasma potential. Systematic calibration procedures permit detection of potential changes smaller than 0.01% of the probing beam energy at frequencies up to a megahertz. Most recent applications of beam probes have made use of this new capability.