L. Cupido

Engineering aspects of the tokamak ISTTOK

AbstractThe first Portuguese magnetic confinement experiment, the tokamak ISTTOK, has been in operation since 1993. This tokamak device is described and the main technological features, as well as the novel techniques of its diagnostics and control and data acquisition system, are reported. A synopsis of the experimental activity is also presented.

Upgrade of the X-mode reflectometry diagnostic for radial correlation measurements in the Joint European Torus

The recent upgrades and the potential of the X-mode reflectometry diagnostic on the Joint European Torus (JET), dedicated to measurement of the turbulence radial correlation length, are presented in this article. The reflectometer system now consists of four independent heterodyne instruments working at different probing frequencies, which in principle enables analysis of the density fluctuations at four distinct radial regions in the plasma. A VERSA Module Eurocard-based computer connected to the JET database computer system is used to control all of the diagnostic in particular the frequency of the tunable sources and frequency of the fast acquisition boards. The diagnostic is fully operated by remote control. A study of the effects of type-I edge localized modes observed at JET illustrates its potential. The main limitation comes from the low quality of the transmission lines, which leads to a strong attenuation of the signal and makes the measurements difficult especially in the plasma core. To reach ...

Multiband reflectometry system for density profile measurement with high temporal resolution on JET tokamak.

A new system has been installed on the JET tokamak consisting of six independent fast-sweeping reflectometers covering four bands between 44 and 150 GHz and using orthogonal polarizations. It has been designed to measure density profiles from the plasma edge to the center, launching microwaves through 40 m of oversized corrugated waveguides. It has routinely produced density profiles with a maximum repetition rate of one profile every 15u2002μs and up to 100,000 profiles per pulse.

Microwave interferometer with a differential quadrature phase detection

A single channel microwave interferometer has been developed for a tokamak with an improved detection that uses an heterodyne configuration and is based on a differential quadrature phase concept. With the new detection very sensitive and accurate direct phase measurements are obtained due to the elimination of the spurious variations of the detected signals resulting from the nonideal behavior of the output mixers. As a result, the line integrated density along the vertical central chord of the vacuum chamber can be determined during the whole plasma discharge. Fast density variations occurring both at the formation phase of the plasma and during sawtooth cycles could also be measured.

First density profile measurements using frequency modulation of the continuous wave reflectometry on JET

We present the main design options and implementation of an X-mode reflectometer developed and successfully installed at JET using an innovative approach. It aims to prove the viability of measuring density profiles with high spatial and temporal resolution using broadband reflectometry operating in long and complex transmission lines. It probes the plasma with magnetic fields between 2.4 and 3.0 T using the V band [approximately (0-1.4)x10(19) m(-3)]. The first experimental results show the high sensitivity of the diagnostic when measuring changes in the plasma density profile occurring ITER relevant regimes, such as ELMy H-modes. The successful demonstration of this concept motivated the upgrade of the JET frequency modulation of the continuous wave (FMCW) reflectometry diagnostic, to probe both the edge and core. This new system is essential to prove the viability of using the FMCW reflectometry technique to probe the plasma in next step devices, such as ITER, since they share the same waveguide complexity.

Two-dimensional full-wave code for reflectometry simulations in TJ-II

A two-dimensional full-wave code in the extraordinary mode has been developed to simulate reflectometry in TJ-II. The code allows us to study the measurement capabilities of the future correlation reflectometer that is being installed in TJ-II. The code uses the finite-difference-time-domain technique to solve Maxwell’s equations in the presence of density fluctuations. Boundary conditions are implemented by a perfectly matched layer to simulate free propagation. To assure the stability of the code, the current equations are solved by a fourth-order Runge–Kutta method. Density fluctuation parameters such as fluctuation level, wave numbers, and correlation lengths are extrapolated from those measured at the plasma edge using Langmuir probes. In addition, realistic plasma shape, density profile, magnetic configuration, and experimental setup of TJ-II are included to determine the plasma regimes in which accurate information may be obtained.

New reflectometer design for density profile measurements on JET

New transmission lines and antennas recently installed at Join European Torus (JET) under the European Fusion Development Agreement enhancement project millimeter wave access meet the performance requirements of broadband reflectometry for density profile measurements. A fast swept frequency modulation continuous wave [H. Bottollier, Curtet-et al., Rev. Sci. Instrum. 58, 539 (1987); F. Simonet et al., ibid. 56, 664 (1985); A. Silva et al., ibid. 70, 664 (1999); S. Hacquin et al., ibid. 75, 3834 (2004)] using coherent detection of the reflected signal was developed to probe the midplane plasma in the X-mode polarization in the 50–75u2002GHz frequency range, aiming at measuring the edge density profile in the JET advanced scenarios with an axial magnetic field of 2.4u2002T. The operating principles of the new system are described and some preliminary results obtained with it are presented. Special attention is given to the capability of operating on bistatic antenna arrangements over long waveguides using a local g...

High resolution fast wave reflectometry: JET design and implications for ITER

The measurement of the fuel mixture remains a very difficult task in thermonuclear plasmas, where the hydrogen isotopes are fully stripped and do not emit line radiation. On the other hand, direct determination of the ion species mix will be essential in the reactor to keep the mixture close to 50/50 and maximize the fusion output. In this paper, the design of fast wave reflectometry for JET is reviewed to show the potential of such a method in the perspective of ITER. The main design elements of the antenna and the detection system, based on vectorial measurements, are reported. The main challenges to such a diagnostic, mainly the intrinsic ion cyclotron emission from the plasma and the extensive use of ion cyclotron radiofrequencies as additional heating, are addressed in detail. The overall design indicates that the proposed system would be able to provide a measurement of the fuel ratio with spatial resolution in the range of few centimeters and temporal resolution in the range of 1 ms in the vast majority of JET scenarios.

X-mode reflectometry measurements in the JET plasma core region

The X-mode radial correlation reflectometry diagnostic on JET has long suffered from strong signal attenuation in the original transmission lines. To reach higher performances, new low-attenuation corrugated circular waveguides and a new cluster of six antennas have been successfully installed in Autumn 2005. The increase of the signal-to-noise ratio using these new transmission lines is found to be on the order of magnitude of 20 dB, thus enabling good quality of reflectometry measurement. With the benefit of this improvement, first X-mode reflectometry measurements of density fluctuations in the core region of the JET plasma are reported in this article. (c) 2006 American Institute of Physics.

Microwave reflectometry for turbulence studies on ASDEX upgrade (abstract)

The reflectometry system installed in the tokamak ASDEX Upgrade has 11 channels primarily aimed at profile measurements and two dedicated channels operating at fixed frequency to study density fluctuations. The channels used for profile measurements can also be operated at fixed frequency. In this article we describe the potential of the system to study important characteristics of the plasma turbulence, from combined fixed frequency and profile measurements (to localize the reflecting layers). The numerical tools used to treat fixed frequency data are briefly described. Using a set of similar discharges and the ability of the microwave reflectometry system to probe simultaneously different layers with several channels, we obtain the time evolution of the integrated power spectra of the different signals versus the minor radius of the plasma. In plasma with H-mode edge and internal transport barrier we could identify a zone at the plasma core and another at the edge with reduced fluctuation levels, coinci...