Manufacturing, installation, commissioning, and first results with the 3D low-temperature co-fired ceramic high-frequency magnetic sensors on the Tokamak à Configuration Variable

Abstract

Innovative high-frequency magnetic sensors have been designed and manufactured in-house for installation on the Tokamak à Configuration Variable (TCV), which are now routinely operational during the TCV experimental campaigns. These sensors combine the Low Temperature Co-fired Ceramic (LTCC) and the classical thick-film technologies and are in various aspects similar to the majority of the in-vessel inductive magnetic sensors foreseen for ITER (around 450 out of the 505 currently being procured are of the LTCC-1D type). The TCV LTCC-3D magnetic sensors provide measurements in the frequency range up to 1 MHz of the perturbations to the wall-aligned toroidal (δBTOR), vertical (δBVER), and radial (δBRAD) magnetic field components. Knowledge of the equilibrium at the last closed flux-surface allows us to then obtain the field-aligned parallel (δBPAR ∼ δBTOR), poloidal (δBPOL), and normal (δBNOR) components, the latter being in most cases rather different from the vertical and radial components, respectively. The main design principles were aimed at increasing the effective area and reducing the self-inductance of the sensor in each of the three measurement axes, which are centered at the same position on each sensor, while reducing the mutual and parasitic coupling between them by optimizing the on-board wiring. The physics requirements are set by the installation of two high-power/high-energy neutral beam injection systems on TCV, i.e., studying fast ions physics, coherent instabilities, and turbulence in the (super-)Alfvénic frequency range. In this paper, we report the manufacturing, installation, and commissioning work for these high-frequency LTCC-3D magnetic sensors and conclude with an overview of illustrative experimental results obtained with this system. The LTCC-3D data provide new insights into the δBPOL coherent (eigenmodes, up to ∼400 kHz) and in-coherent background turbulent fluctuations in the higher frequency range up to ∼1 MHz, which were not previously available with the TCV Mirnov sensors. Furthermore, the LTCC-3D δBPOL measurements allow us to cross-check the data obtained with the standard Mirnov coils and have led to the identification of largeelectromagnetic (EM) noise pick-up for the Mirnov data acquisition (DAQ). When the sources of EM noise pick-up on the Mirnov DAQ are removed, the LTCC-3D data for δBPOL are in good overall agreement, i.e., within the expected measurement uncertainties, with those obtained with the standard Mirnov sensors located at the same poloidal position in the frequency range where the respective data acquisition overlap, routinely up to 125 kHz and up to 250 kHz in some discharges. The LTCC-3D δBPAR measurements (not previously available in TCV or elsewhere) provide evidence that certain instabilities have a finite parallel δB at the wall, hence at the LCFS, consistent with the recent theoretical results for pressure-driven modes. The LTCC-3D δBNOR measurements improve significantly on the corresponding measurements with the saddle loops, which are mounted onto the wall and have a bandwidth of ∼3 kHz (due to the wall penetration time). A detailed end-to-end system modeling tool has been developed and applied to test on the simulated data the actual measurement capabilities of this new diagnostic system and obtain the ensuing estimates of the intrinsic measurement uncertainties. A detailed error analysis is then performed so that, finally, fully calibrated, absolute measurements of the frequency-dependent amplitude and spectral breaks of coherent eigenmodes and in-coherent broadband magnetic fluctuations are provided for the first time in physical units with quantitative uncertainties.

First developments towards producing inductive magnetic sensors using state-of-the-art photolithography techniques

“…To estimate the electrical properties of the PL inductive magnetic sensors, we have further developed the 1D variant of the algorithm used to model the LTCC sensors [2,9]. This algorithm calculates the nominal electrical characteristics of the sensor given its outer dimensions, the track specifications (width, thickness, separation) and the materials: effective area 𝑁 𝐴 EFF , self-inductance 𝐿 SELF , selfresistance 𝑅 SELF , resonant frequency 𝑓 RES , effective bandwidth γ RES .…”

“…Knowing this information, 𝑅 SELF , 𝐿 SELF , 𝐶 SELF and 𝑅 ISOL , all as defined earlier, can be found using the machine own software by imposing the circuit model shown in figure 11. Based on the equivalent circuit of figure 11 and by trivial computation [2,9], 𝑍 (ω) is given by:…”

“…The LTCC processing and results obtained with these inductive magnetic sensors are presented in refs. [2][3][4][5][6][7][8][9]. Figure 1 shows a 3D design of an LTCC magnetic sensor: multiple planar turns (m) are screen-printed on a ceramic substrate and then multiple layers (n) are electrically connected in series using vertical tracks (the vias).…”

The SparSpec algorithm and the application to the detection of spatial periodicities in tokamaks: from a 1D to a 2D analysis

“…As it will transpire later, this is a major drawback for 2D spatial analyses, for which the only effective solution is to combine subsets of toroidal and poloidal sensors from different arrays, with the problem that these sensors then might have a different intrinsic measurement error 6 or be at a very different distance from the LCFS. The latter is a problem particularly important in TCV, where the wall is almost never conformal to the LCFS due to the large variety of possible plasma shapes, and which is usually further compounded by the strong ballooning physics (that is: a very strong variation of the poloidal mode amplitude on the LFS, see figure 20 in [29] for a specific example of this). In TCV we also have 3x LTCC-3D magnetic sensors [29], which sit at Z = −115mm in three not equi-spaced toroidal sectors, thus somewhat in between some of the Mirnov sensors on the LFS.…”

Study of Helium Swelling and Embrittlement Mechanisms in SiC Ceramics

“…First, with the adoption of the EuroFusion concept in the development of nuclear and fusion power, the world pays greater attention to studying the properties of new types of structural materials included in the list of candidates for new generation reactors, including fusion reactors [4,5]. It should be noted that ceramics based on oxides (MgO, Al 2 O 3 , ZrO 2 , BeO), carbides (SiC, TaC, TiC), and nitrides (AlN, BN, Si 3 N 4 ) play an important role in this list [6][7][8][9][10]. Interest in this class of materials is due to the fact that they have a high melting point, strength, and mechanical resistance to external influences, which makes them one of the promising materials for high-temperature nuclear reactors, as well as thermonuclear reactors.…”