D. Testa

Prototyping a High-Frequency Inductive Magnetic Sensor Using the Nonconventional, Low-Temperature Co-Fired Ceramic Technology for Use in ITER

Abstract The ITER high-frequency (HF) magnetic sensor is currently intended to be a conventional, Mirnov-type, pickup coil, designed to provide measurements of magnetic instabilities with magnitude as low as [vertical bar]δB[vertical bar] [approximately] 10-4 G at the position of the sensors and up to frequencies of at least 300 kHz. Previous prototyping of this sensor has indicated that a number of problems exist with this conventional design that are essentially related to the winding process and the differential thermal expansion between the metallic wire and the ceramic spacers. Hence, a nonconventional HF magnetic sensor has been designed and prototyped in-house in different variants using low-temperature co-fired ceramic (LTCC) technology, which involves a series of stacked ceramic substrates with a circuit board printed on them with a metallic ink (silver in our case). A method has then been developed to characterize the electrical properties of these sensors from the direct-current range up to frequencies in excess of 10 MHz. This method has been successfully benchmarked against the measurements for the built sensors and allows the electrical properties of LTCC prototypes to be predicted with confidence and without the need of actually building them, which therefore significantly simplifies future research and development (R&D) activities. When appropriate design choices are made, LTCC sensors are found to meet in full the volume occupation constraints and the requirements for the sensor’s electrical properties that are set out for the ITER HF magnetic diagnostic system. This nonconventional technology is therefore recommended for further R&D and prototyping work, particularly for a three-dimensional sensor, and possibly using materials more suitable for use in the ITER environment, such as palladium and platinum inks, which could remove the perceived risk of transmutation under the heavy neutron flux that we may have with the Au (to Hg, then to Pb) or the Ag (to Cd) metallic inks currently used in LTCC devices.

Functional Performance Analysis and Optimization for the High-Frequency Magnetic Diagnostic System in ITER - I: Overview of the Results

Abstract The measurement performance of the baseline system design for the ITER high-frequency magnetic diagnostic has been analyzed using an algorithm based on the sparse representation of signals. This algorithm, derived from the SparSpec code [S. Bourguignon et al., Astron. Astrophys., 462, 379 (2007)] has previously been extensively benchmarked on real and simulated JET data. To optimize the system design of the ITER high-frequency magnetic diagnostic, we attempt to reduce false detection of the modes and to minimize the sensitivity of the measurement with respect to noise in the data, loss of faulty sensors, and the displacement of the sensors. Using this approach, the original layout design for the ITER high-frequency magnetic diagnostic system, which uses 168 sensors, is found to be inadequate to meet the ITER measurement requirements. Based on this analysis, and taking into account the guidelines for the risk mitigation strategies that are given in the ITER management plan, various attempts at optimization of this diagnostic system have been performed. A revised proposal for its implementation has been developed, which now meets the ITER requirements for measurement performance and risk management. For toroidal mode number detection, this implementation includes two arrays of 50 to 55 sensors and two arrays of 25 to 35 unevenly spaced sensors each on the low-field side and two arrays of 25 to 35 unevenly spaced sensors each on the high-field side. For poloidal mode number detection, we propose six arrays of 25 to 40 sensors each located in nonequidistant machine sectors, not covering the divertor region and, possibly, poloidal angles in the range 75 < |θ|(deg) < 105, as this region is the most sensitive to the details of the magnetic equilibrium. In this paper we present the general summary results of this work, for which more details and an overview of our test calculations are reported in the companion paper.

Functional Performance Analysis And Optimization For The High-Frequency Magnetic Diagnostic System In Iter-Ii: Detailed Overview Of The Analysis Method And Of The Test Calculations

Abstract The measurement performance of the baseline system design for the ITER high-frequency magnetic diagnostic system and attempts at its optimization have been performed using an innovative method based on the sparse representation of signals and the minimization of the maxima of the spectral window for integer mode numbers. This analysis has led to the conclusion that 350 to 500 sensors are in fact needed to satisfy the ITER requirements for the measurement performance and the risk management over the machine lifetime, instead of the originally foreseen approximately 170 sensors. In the companion paper we have presented the general summary results of our work; here we present a more complete overview of the analysis method and further details of our test calculations.

The upgraded JET toroidal Alfvén eigenmode diagnostic system

The main characteristics of toroidal Alfven eigenmodes (TAEs) have been successfully investigated in JET (Joint European Torus) using the scheme of sweeping-frequency external excitation with tracking of the synchronously-detected resonances. However, due to technical limitations, only modes with low values of the toroidal mode number n <= 7 could be effectively excited and unambiguously identified by the Alfven Eigenmode Active Diagnostic (AEAD) system. This represents a serious restriction because theoretical models indicate that medium-n Alfven eigenmodes (AEs) are the most prone to be destabilized by energetic particles in ignited plasmas and, therefore, reliable measurement of their damping rates remains a relevant issue to properly access their effect in ignited plasmas. For this reason, a major upgrade of the AEAD system has been carried out aiming at providing a state-of-the-art excitation and real-time detection system for the planned DT campaign in JET. This required the development of a new type of radio frequency amplifier and filter, not commercially available, and also a control system. In this paper, details of the concepts that are relevant to understand the operation of the new system in the next experimental campaigns are presented, as are the results of numerical simulations to model its performance.

Measurements of the radial profile of the plasma isotopic composition in JET plasmas using Alfvén eigenmodes

The measurement of the plasma isotopic composition is necessary in future burning plasma devices such as ITER and DEMO as a tool for optimizing the DT fusion performance. This paper reports on the results of experiments performed on the JET tokamak where Toroidal Alfven Eigenmodes (TAEs) with toroidal mode number (N) up to |N|=12 were actively driven with a set of in-vessel antennas, and were then used to infer the value of the plasma isotopic composition at different radial positions. A novel and important result with respect to previous work on JET is that by correctly including the effect of plasma impurities in the calculation of the Alfven frequency, through its dependence on the plasma mass, it has become now possible to distinguish plasmas with different majority ion species but with the same charge-to-mass ratio, notably majority Deuterium and Helium4 plasmas. Furthermore, and combined with modelling of AEs in JET discharges, these experimental results indicate that a diagnostic system based on the detection of AEs with different toroidal mode numbers and at different frequencies, could provide profile measurements of the plasma isotopic composition in future burning plasma devices such as ITER and DEMO.

Sparse Representation of Signals: from astrophysics to real-time data analysis for fusion plasmas and system optimization analysis for ITER and TCV

Efficient, real-time and automated data analysis is one of the key elements for achieving scientific success in complex engineering and physical systems, two examples of which include the JET and I ...

PROTOTYPING CONVENTIONALLY WOUND HIGH-FREQUENCY MAGNETIC SENSORS FOR ITER

Abstract The high-frequency (HF) magnetic sensors for ITER are currently based on a conventional, Mirnov-type pickup coil, with an effective area in the range 0.03 < (NA)EFF (m2) < 0.1; the sensor is required to provide measurements of magnetic instabilities with magnitude around |δB/Bθ| ˜ 10−4 in the 10-kHz to 2-MHz frequency range. The physical, mechanical, and electrical properties of one representative ITER HF pickup coil design have been analyzed with particular attention to the manufacturing and assembly process for the winding pack, as its integrity was found to be of concern when performing a coupled electromagnetic, structural, and thermal analysis of the sensor. Three different options for the guiding grooves in that design have been tested, using copper and tungsten for the winding pack, but none of them has been convincing enough due to the likelihood of breakages of the thin grooving and of the tungsten wire itself. Hence, alternative designs still based on a conventional Mirnov-type pickup coil have been explored, and a nonconventional Mirnov-type pickup coil was produced using direct laser cutting of a Type 316 stainless steel hollow tube, avoiding the difficulties encountered during the winding operations for conventional Mirnov-type sensors. This process of manufacturing appears to be acceptable for HF magnetic sensors of Mirnov-type design in ITER, and it is recommended for future prototyping studies, as the effective area of our first prototype, (NA)EFF ˜ 0.01 m2, was well below the ITER requirement. The electrical characteristics and the frequency response of all these prototypes were evaluated up to 8 MHz, with the results in good agreement with model calculations. The conventional Mirnov-type prototypes behave as expected in terms of their main electrical properties and should satisfy the present measurement performance requirements. Finally, a direct measurement of the effective area of these sensors has shown that the geometrical value is a sufficiently correct estimate of its actual value at low frequencies (<10 kHz) when the winding pack closely follows the nominal shape of the coil itself.