M. E. Manso

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 50–75 GHz frequency range, aiming at measuring the edge density profile in the JET advanced scenarios with an axial magnetic field of 2.4 T. 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...

Global full-wave simulation of the Tore-Supra doppler reflectometer

The Tore-Supra doppler reflectometer (TSDR) aims at measuring the poloidal wave number spectrum (k spectrum) of the density fluctuations. To evaluate the performance and the sensitivity of the TSDR, simulations were made using a global full-wave code. The TSDR modeling with a finite-difference time-domain scheme and the required constraints of the simulation are presented, namely the account of the real size of the system: Distance horn plasma and plasma size. Particular attention is given to the role of the radiation pattern both on the TSDR spatial resolution and the wave number detection.

Simulation of amplitude and phase variations induced by magnetic islands with turbulence on reflectometry signals

Reflectrometry is influenced by plasma perturbations and turbulence, making the understanding of how signals are affected by such phenomena of prime importance. In this work we look at the effect magnetic islands with turbulence have on the amplitude and phase of reflectometry signals. For this purpose we resort to a two-dimensional finite difference time domain code and use a turbulence model based on experimental data. We investigate the possibility of occurrence of destructive interference due to a chain of islands and the role turbulence plays. The simulation of an island with turbulence present at the resonant surface q=2, with dimensions of the same order of magnitude as the phenomena observed on ASDEX Upgrade on Tore Supra, is also done with results which are in agreement with experimental observations.

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.

Upgrade to the control system of the reflectometry diagnostic of ASDEX upgrade

The broadband frequency modulation-continuous wave microwave/millimeter wave reflectometer of ASDEX upgrade tokamak (Institut fur Plasma Physik (IPP), Garching, Germany) developed by Centro de Fusao Nuclear (Lisboa, Portugal) with the collaboration of IPP, is a complex system with 13 channels (O and X modes) and two types of operation modes (swept and fixed frequency). The control system that ensures remote operation of the diagnostic incorporates VME and CAMAC bus based acquisition/timing systems. Microprocessor input/output boards are used to control and monitor the microwave circuitry and associated electronic devices. The implementation of the control system is based on an object-oriented client/server model: a centralized server manages the hardware and receives input from remote clients. Communication is handled through transmission control protocol/internet protocol sockets. Here we describe recent upgrades of the control system aiming to: (i) accommodate new channels; (ii) adapt to the heterogenei...

New frequency translation technique for FM-CW reflectometry.

In broadband microwave reflectometry, coherent detection is widely used to obtain the phase information and to improve the systems sensitivity, both in diagnostics measuring the electronic density profile and plasma fluctuations. Coherent detection uses a translated version of the probing signal to guarantee a stable intermediate frequency. Here, a novel technique to generate the frequency translation by double frequency conversion is presented and its advantages over the commonly used single frequency conversion techniques employing image rejection mixers are discussed. The results obtained with the new frequency translator modules developed for the three JET FM-CW reflectometers, operating successfully at JET since mid-2009, are presented.

Frequency modulation continuous wave reflectometry measurements of plasma position in ASDEX Upgrade ELMy H-mode regimes

Plasma position measurements of frequency modulation continuous wave reflectometry are analyzed in ASDEX Upgrade ELMy H-mode regimes [International Thermonuclear Energy Reactor (ITER) reference scenario], to assess the viability of using reflectometry for position and shape control purposes in ITER. Plasma separatrix position is estimated both at the low and high field sides of the tokamak. A scaling factor applied to the linear average density (an ITER online measurement) is used to estimate the separatrix density, whose position is then tracked using reflectometry profile measurements. Edge localized mode (ELM) activity effects are overcome by disregarding measurements performed during the onset and magnetohydrodynamic phases of the ELMs. Position data is compared, with data derived from the magnetics and its quality is assessed using one dimensional full-wave numerical simulations, taking into account the influence of the nonprobed edge plasma region (O-mode operation) and of the plasma turbulence. Res...

High resolution edge density measurements in ASDEX upgrade H-mode discharges with broadband reflectometry

In high-confinement (H-mode) plasmas with edge localized modes (ELMs), the energy stored at the edge pedestal region directly affects the global plasma confinement. The measurement of the edge pedestal evolution and the study of the collapse mechanism of the pedestal structure due to ELMs is, therefore, of great importance to understand plasma confinement. In Axially Symmetric Divertor Experiment (ASDEX) Upgrade, information about the edge density profile has been obtained in H-mode plasmas both with Thomson scattering and by combining lithium beam and interferometry measurements. However, information around the edge pedestal region is not routinely available. Here we use microwave reflectometry to obtain high resolution measurements of the edge transport barrier and pedestal region, namely the pedestal position and density, the pedestal width, and the edge density gradient. The evolution of the pedestal density is obtained automatically with an algorithm applied directly to the group delay. The other par...

High temporal and spatial resolution reflectometry investigations of type-I ELMs induced edge barrier collapse at ASDEX Upgrade

The edge barrier collapse induced by spontaneous and pellet triggered ELMs has been characterized by means of a new technique based on the reflectometry group delay. The analysis of the behaviour of the density layers at the plasma edge revealed small differences between the two types of ELMs. With this technique we are able to detect profile steepening and fuelling effects due to pellet injection, mainly at the high field side. The presence of asymmetries between high and low field sides in the ELM dynamics has also been studied by evaluating the ELM onset time on both sides. The ELM onset on the HFS is observed to be only slightly delayed compared with the LFS onset?the barrier collapses are nearly coincident within the diagnostic time resolution. In our new findings, which apply to both spontaneous and triggered ELMs, there is no clear evidence of long HFS/LFS delays as previously observed for spontaneous ELMs.

Automatic evaluation of edge profile characteristics in ASDEX Upgrade H-mode density limit discharges with FM-CW reflectometry

The set of data processing techniques used on ASDEX Upgrade to automatically evaluate edge density profile characteristics from broadband data, obtained with the frequency-modulated continuous-wave reflectometer, has recently been upgraded to include (i) an adaptive algorithm that automatically optimizes the length of the window used in the spectrogram analysis of broadband signals and (ii) a new method to estimate the edge pedestal density based on the evolution of the local derivative of the group delay curve, removing the need for user supplied parameters. These tools are especially important in cases where the group delay of the reflected signals varies significantly in a short probing frequency range, such as when the profile exhibits a density shoulder at the edge. The application of these techniques in ASDEX Upgrade H-mode density limit discharges, to automatically obtain the time evolution of the edge pedestal parameters, namely, its density, position, and width, shows a clear improvement both on ...