TJ-II Flexible Heliac Research Articles

Modelling of beam-driven Alfvén modes in TJ-II plasmas

The properties of hydrogen neutral beam injection driven Alfvén eigenmodes (AEs) in the TJ-II flexible heliac are investigated using a reduced magnetohydrodynamic (MHD) model coupled to a realistic MHD equilibrium. The simulation results of AE frequency and radial localization are found to be broadly consistent with experimental observations. The simulation results show that an AE with the same poloidal and toroidal harmonics persists under small variations in on-axis iota values, which is consistent with the experimental observations. The corresponding wave-particle resonance maps suggest, for low bounce harmonics, the possibility of describing nonlinear evolution of the AEs in TJ-II by a sum of two ion populations with different weighting factors, one of which is dominated by drag and the other by diffusion. As the bounce harmonic increases, the resonance region starts to expand and can cover a significant area of the particle phase space until this resonance region vanishes at high bounce harmonics.

Heavy ion beam probing—diagnostics to study potential and turbulence in toroidal plasmas

Heavy ion beam probing (HIBP) is a unique diagnostics to study the core plasma potential and turbulence. Advanced HIBPs operate in the T-10 tokamak and TJ-II flexible heliac with fine focused (<1 cm) and intense (100 µA) beams. They provide measurements in the wide density interval = (0.3–5) × 1019 m−3, in a wide range of Ohmic and electron cyclotron resonance heated (ECRH) discharges with various currents at T-10, and in the wide range of magnetic configurations with ECR and neutral beam injection (NBI) heating at TJ-II. Time evolution of the radial profiles and/or local values of plasma parameters from high field side (HFS) to low field side (LFS), −1 < ρ < 1, is observed in TJ-II by 125 keV Cs+ ions in a single shot, while LFS (+0.2 < ρ < 1) is observed in T-10 by 300 keV Tl+ ions. Multi-slit energy analyzers provide simultaneously data on the plasma potential φ (by the beam extra energy), plasma density ne (by the beam current), poloidal magnetic field Bpol (by the beam toroidal shift), poloidal electric filed Epol that allows one to derive the electrostatic turbulent particle flux ΓE×B. The cross-phase of density oscillations produces the phase velocity of their poloidal propagation or rotation; also it gives the poloidal mode number. Dual HIBP, consisting of two identical HIBPs located ¼ torus apart provide the long-range correlations of core plasma parameters. Low-noise high-gain electronics allows us to study broadband turbulence and quasi-coherent modes like geodesic acoustic modes and Alfvén eigenmodes.

Study of the neoclassical radial electric field of the TJ-II flexible heliac

Calculations of the monoenergetic radial diffusion coefficients are presented for several configurations of the TJ-II stellarator usually explored in operation. The neoclassical radial fluxes and the ambipolar electric field for the standard configuration are then studied for three different collisionality regimes, obtaining precise results in all cases.

Drift Kinetic Equation Solver for Grid (DKEsG)

Neoclassical (NC) transport calculations are necessary for the complete simulation cycle of the behavior of plasmas inside both tokamaks and stellarators, which are more complex to be performed on the latest. In addition, due to the fact that the NC transport is mainly determined by the magnetic properties of the device (mathematically designed by the number of Fourier harmonics needed to describe the confining magnetic field), its study is mandatory to ensure the efficiency of a certain coil configuration before it is implemented in a fusion reactor. Thus, improvements to the already constructed devices and certain design decisions for the new ones, such as W7-X, NCSX, or QPS, have been made based on the estimations of the NC transport, among other criteria. In this sense, a compromise between the number of modes and polynomials to describe the distribution function and the required computing time to obtain reliable estimates is a must. This paper presents the porting process to the Grid and the computational optimization of the Drift Kinetic Equation Solver code, devoted to obtain the monoenergetic diffusion coefficients of the NC transport as well as the first coupled prototype for estimating the transport coefficients with such a new code release. The advantage of this code over the Monte Carlo applications already in production on the Grid is that it allows the estimation of all the transport coefficients, not only the diagonal ones. The tests and results obtained have been applied to the TJ-II Flexible Heliac, a stellarator in operation at National Fusion Laboratory (Spain), by using the EELA-2 Project infrastructure.

Heat wave experiments on TJ-II flexible heliac

Heat wave experiments are performed on TJ-II stellarator plasmas to estimate both heat diffusivity and power deposition profiles. High frequency electron cyclotron resonance heating (ECRH) modulation experiments are used to obtain the power deposition profile, which is observed to be wider and duller than that estimated by ray tracing techniques. The causes of this difference are discussed in the paper. Fourier analysis techniques are used to estimate the heat diffusivity in low frequency ECRH modulation experiments. This includes the power deposition profile as a new ingredient. ECRH switch-on/off experiments are exploited to obtain power deposition and heat diffusivities profiles. These quantities are compared with those obtained by modulation experiments and transport analysis, showing good agreement.

Review of confinement and transport studies in the TJ-II flexible heliac

TJ-II is a four period, low magnetic shear stellarator (R = 1.5 m, a < 0.22 m, B0 ⩽ 1.2 T) which was designed to have a high degree of magnetic configuration flexibility. In the most recent experimental campaign, couplingof the full ECRH power (PECRH ⩽ 600 kW) to the plasma has been possible using two ECRH transmission lines which have different power densities. Both helium and hydrogen fuelled plasmas have been investigated. The article reviews the latest physics results in particle control, configuration effects, and transport and fluctuation studies.

The first operation of the advanced heavy ion beam probing diagnostic on the TJ-II flexible heliac

An advanced heavy ion beam probing diagnostic (HIBP) based on simultaneous utilization of two different systems for the secondary ions detection (a multiple cell array detector and 30° Proca-green electrostatic energy analyzer) has been developed for the TJ-II flexible heliac. This innovative design aims at enlarging the HIBP capabilities in order to allow the instantaneous measurements of electron density and plasma potential profiles together with their respective fluctuations. This paper presents the detailed description of the main parts of the HIBP equipment and their characteristics obtained during the first operation on TJ-II. Special attention is employed to the control and data acquisition system built on two VME controllers. The results of the diagnostic beam carrying through the magnetic structure of TJ-II into secondary beam line are presented and compared with the trajectory calculations. The problems of plasma loading of the detection system and HX-ray excitement by the probing beam are considered together with the ways of their solution.

A study of topological structures in TJ-II radiation profiles using anautomated pattern recognition procedure and their correlation with plasmaconfinement

Significant features, consisting of flats and humps, have been observed inimpurity-ion ultraviolet line-emission profiles measured on the TJ-II flexibleheliac, thereby providing evidence for the existence of topological structuresin the plasma interior. The data for this study were collected using a fastspectroscopic scanning system with good spatial resolution capabilities; theywere analysed using an in-house developed pattern recognition algorithm. Thisalgorithm is capable of separating effects due to plasma fluctuations andnoise from those caused by two-dimensional structures. In the paper wedescribe the data collection system and the pattern recognition algorithm andwe analyse plasma radiation profiles from standard TJ-II configurations todemonstrate that this pattern recognition algorithm can identify real featuresin these. Next, we analyse profile data from a configuration scan made on theTJ-II and finally we report on the features found, their typical widths andsymmetry, as well as on the time evolution and correlation with the electrondensity.

A multi-channel spectroscopic system for measuring impurity ion temperatures and poloidal rotation velocities in TJ-II

In this article, we report on the hardware and software of an eight-channel, high-resolution, spectroscopic diagnostic system that has been built for the TJ-II flexible heliac. This system is currently being used on TJ-II to measure impurity ion temperature and poloidal rotation using passive emission spectroscopy. The principal features of the diagnostic include independent focusing of its channels, high sensitivity for performing Doppler measurements in low-density ECR-heated plasmas, as well as a flexible and fast in-house-developed software program for performing integrated data reduction and analysis.

Loss cone structure for ions in the TJ-II helical axis stellarator Part I: Properties without a radial electric field

Losses and trapping regions in real and velocity space have been studied in the TJ-II flexible heliac for collisionless ions of low and intermediate energy (0.1-1 keV, roughly). The complexity of the magnetic field structure is described with emphasis on the ripple, Fourier mode composition, angular dependences and drift velocity behaviour. The resulting strong asymmetries of the loss distributions are discussed and explained. The influence of pitch and position is analysed by means of different complementary diagrams and a strong sensitivity to the angular position of trapping regions is shown. Electric field effects have not been included, this will be done in a companion article, with particular stress on the influence of resonances.

Runaway electrons control during ramp-up and ramp-down of the magnetic fields in the coils of the TJ-II flexible heliac (abstract)

The main accelerating electric field is induced in TJ-II flexible heliac (Ref. Reference 1) by currents ramping in the central coils, while the current in the vertical coils is kept constant. With this operational protocol in first TJ-II discharges, hard x rays (HXR) measured using NaI detectors during ramp-up seem to be generated by electrons confined about 40 ms. This time is coherent with the existence of closed magnetic surfaces before the steady-state phase. To diminish this runaway electron generation in the large configurational diagram of the device, independently of gas puffing control, two methods acting before the steady-state phase are investigated. One has been designed as a mechanical system interfering poloidally with the magnetic surfaces and the other consists in the study of the magnetic field structure needed to avoid the closed magnetic surface formation and in its possible application to the TJ-II operation.

A toroidal focusing mirror based vacuum ultraviolet diagnostic for TJ-II

We describe a vacuum ultraviolet spectrometer having spatial, spectral, and temporal resolution capabilities that is being built as a diagnostic for the TJ-II flexible heliac device. The system is based around a renovated 0.5 m radius Rowland circle grazing incidence duochromator and will cover the wavelength range from 300 to 2000 Å with good spatial resolution. A toroidal mirror used at grazing incidence, mounted on a specially designed mounting positioned between the plasma and spectrometer, will permit ∼80% of the bean shaped plasma cross section to be scanned with spatial resolution on the order of 10 mm. In addition, filters and slits are incorporated to tailor the incident light beam. The duochromator was coupled directly to the TJ-II during the machine start up phase to check the good operation of the duochromator. These data have been fed back into the design program in order to optimize the instrument design.

TJ-II data retrieving by means of a client/server model

The database of the TJ-II flexible heliac is centralized in a Unix server. This computer also commands the on-line processes related to data acquisition during TJ-II discharges: programming of measurement systems, connectivity with control systems, data visualization, and computations. The server has to provide access to the data so that signal analysis can be performed by local users or even from remote hosts. Data retrieving is accomplished by means of a client/server architecture in which two data servers are permanently running in the background of the Unix computer. One of them serves data requests from local clients and the other one sends data to remote clients. The communication protocol in both cases has been developed by using TCP/IP and Berkeley sockets. The client part consists of a set of routines (FORTRAN and C callable), which, in a transparent way, provide connectivity with the servers. This structure allows access to TJ-II data exactly in the same way from any computer, hiding not only specific aspects of the database, but hardware architecture of the server computer as well. In addition, the remote access makes it possible to distribute computations and to reduce the load on the Unix server from analysis and visualization tasks. At present, this software is running in four different environments: the Unix server itself, various types of Unix workstations, a CRAY J90 and a CRAY T3E. Finally, due to the fact that visualization is essential for TJ-II data analysis, a powerful and a very flexible visualization tool has been developed. It is a point and click application based on X Window/Motif. Data access is carried out through the client/server processes mentioned above and the software runs in the client computer.

A vacuum ultraviolet study of rare-earth phosphor screens with application as diagnostics for hot plasmas (abstract)

In a previous work [Zurro et al., Rev. Sci. Instrum. 68, 680 (1997)], we outlined designs for several detector systems, based on rare-earth phosphor powders, with application as broadband VUV plasma radiation monitors in the TJ-II flexible heliac device. In addition to their immunity to microwave power, to ground loops, and to any kind of EMI, the phosphor powders considered as suitable candidates can withstand the high temperatures of the TJ-II vacuum chamber (150 °C), while possessing good sensitivity across the x-ray to UV range and fast response times. Among those selected was Y3Al5O15:Ce (P-46), which emits at 530 nm and when examined, it was found to be particularly suitable for our application due to its very fast response time (∼0.1 μs). We report on absolute efficiency measurements performed between 40 and 200 nm using a calibrated VUV source, and made on thin P-46 phosphor screens, having thicknesses between 1 and 20 mg/cm2. We combine these results with measurements made at visible wavelengths and in the x ray [A. Baciero et al., in Diagnostics for Experimental Thermonuclear Fusion Reactors 2, edited by P. E. Stott et al. (Plenum, New York, 1998)] to obtain fitting parameters for models used to predict phosphor response efficiencies in transmission and reflection modes. These models can be used to tailor the phosphor screen thickness to the radiation range of interest while reducing background noise. Such screens are currently used on the TJ-II.

Applying object oriented concepts to on-line data acquisition

Concepts of object oriented programming have been applied to the on-line part of the data acquisition system of the TJ-II flexible heliac. The programs were written in C++ for a VxWorks operating system which uses general purpose interface bus (GPIB) connections to control computer automated measurement and control (CAMAC) crates with various types of digitizer modules. Since the digitizers have similar functionality, the application of object oriented methods is particularly efficient. Unfortunately, the control code of the CAMAC had to be built up in layers since the CAMAC/GPIB system is not well suited for good object oriented organization.

Bolometry systems for the TJ-II flexible heliac

Local plasma emissivity of TJ-II plasmas will be determined using pyroelectric detectors. For tomographic reconstructions three fan-like arrays, 20 channels each, will be installed at the top, side, and bottom ports of the same sector. The designed compact in-vessel systems will use folding support structures to introduce the arrays through small diameter flanges and translation and rotary drives to position them at their measuring sites. Special individual “honeycomb-like” collimators (transparency of around 50% and spatial resolution of about 2 cm at the plasma center) have been designed to optimize the signal level and to act as a microwave filter. In the first stage, radial profiles of plasma emissivity will be obtained with a single detector, on the basis of frozen plasma parameters during a few ms. This detector is equipped with a rotary slit that scans the whole plasma cross section from 10–20 times per shot. To perform power balances, global plasma emission is monitored with three pyroelectric detectors. Collimators are movable slits that enable probing either the full plasma cross section or narrow central chords; when fully closed they protect detectors during wall conditioning and permit background noise characterization.

Feasibility of local power losses determination in the TJ-II heliac using movable bolometers

A method to fully diagnose energy losses in plasma devices with a reduced experimental setup is presented. This technique is based on the use of single movable detectors instead of full detector arrays, and it is especially suitable for devices with large toroidal and/or poloidal asymmetries, such as the TJ-II flexible heliac, that would require a large number of measurements at different positions. Local distributions of the plasma losses are deduced from the bolometer signals by using general tomography algorithms. The number of lines of sight is only limited by the sensitivity of the detector and the number of reproducible shots in the device, and, as a consequence, the possibility of obtaining a well-defined mapping of the plasma energy losses can be even better than in the standard case based on the use of detector arrays with a fixed number of detectors. The method has been tested in the TJ-I tokamak and the results obtained are presented, together with a simulation of its application to the TJ-II heliac.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Variational bounds for transport coefficients in three-dimensional toroidal plasmas

A variational principle is developed for the linearized drift-kinetic, Fokker–Planck equation, from which both upper and lower bounds for neoclassical transport coefficients can be calculated for plasmas in three-dimensional toroidal confinement geometries. These bounds converge monotonically with the increasing phase-space dimensionality of the assumed trial function. This property may be used to identify those portions of phase space that make dominant contributions to the transport process. A computer code based on this principle has been developed that uses Fourier–Legendre expansions for the poloidal, toroidal, and pitch-angle dependences of the distribution function. Numerical calculations of transport coefficients for a plasma in the TJ-II flexible heliac [Nucl. Fusion 28, 157 (1988)] are used to demonstrate the application of this procedure.

Influence of a superthermal tail upon ECRH in the TJ-II flexible heliac – a theoretical study

A superthermal tail is simulated by adding a drifted Maxwellian distribution to a bulk Maxwellian distribution, fa = (l−μ)fb + μft. The values of the tail temperature (Tt), drift velocity (v0) and weight coefficient (μ) are chosen so as to reproduce the experimental residual current observed in the Wendelstein VII-A stellarator. An analytical expression for the resulting dielectric tensor coefficients is obtained. It is shown that, even for small tails, the global absorption coefficient increases for both the first harmonic (ω = 28 GHz) and the second harmonic (ω = 56 GHz) propagation. The effect is more dramatic for wave propagation in the first harmonic. The spatial bandwidth of the absorption region increases sensibly with respect to the pure Maxwellian distribution in both harmonics.