Filipe da Silva

Reflectometry is anticipated to play a significant role in plasma positioning, shaping, and tracking for DEMOnstration power plant, potentially replacing magnetic diagnostics. We present an overview of the outcomes of the EUROfusion Enabling Research (EnR) project ENR-04-IST-01, a coordinated effort of European experts and developers of reflectometry systems, whose main results were: (i) the proof of principle of a new diagnostic concept able to track and monitor the position and shape of the plasma in the initial stage of the discharge, in the start-up phase, and also at the ramp-down phase; and (ii) the improvement of the capabilities of operation in the stationary phase (flattop) to provide an accurate and precise substitute for the positioning magnetic diagnostics in real-time. Important advances have also been achieved in finite-difference time-domain simulation codes, allowing access to two-dimensional and three-dimensional sophisticated reflectometry simulations; and in the development, manufacturing, and testing of antenna prototypes. An important issue addressed was the synchronisation between different reflectometers with the start of an experimental validation on the Tungsten (W) Environment in Steady-state Tokamak. We also describe the advances in hardware, leading to a redesigned, assembled and tested prototype of a compact coherent fast frequency sweeping radio frequency back-end using commercial monolithic microwave integrated circuits, with the integration of direct digital synthesis as continuing work. Another result was the conception of an integrated design workflow for the design, evaluation and implementation of microwave diagnostic systems and its application in the assessment of plasma position reflectometers for high field and low field sides of the divertor test tokamak.

The Divertor Tokamak Test (DTT) is a compact sized (R = 2.19 m, a = 0.70 m) fully superconducting tokamak presently under construction at the ENEA (Italian National Agency for New Technologies, Energy and Sustainable Economic Development) research center in Frascati, Italy. With its high field (B = 5.85 T) and plasma current (Ip = 5.5 MA), actively cooled in-vessel W components to sustain long pulse duration (tpulse = 100 s), and a significant amount of external heating power (up to P = 45 MW to the plasma), it aims at exploring heat exhaust solutions in fusion-relevant environments by reproducing the level of divertor heat loads expected in ITER and DEMO. Recently, a first version of the DTT research plan was released, defining the project objectives and describing targeted plasma scenarios. The research plan also sets the basis for the development of diagnostic systems, closely following the experimental phases of the machine therein described, with a particular focus toward the divertor region. Since DTT’s first plasma will soon be followed by an experimental phase, operations will start with a full set of systems for machine protection and plasma control, as well as diagnostics to probe initial scenarios. Advanced systems for the investigation of the physics of transport and fast particles will be developed and installed as the machine performance evolves, following its power upgrades. This paper gives an overview of the DTT tokamak targeted diagnostic set, focusing on how these systems are designed to address the scientific goals of the project from first plasma to full power scenarios, and providing context for further publications dedicated to specific diagnostic systems.

Sheaths, Plasma Edge & Kinetic Turbulence Radiofrequency Experiment is a new research platform dedicated to plasma physics and nuclear fusion, designed to investigate magnetised plasmas. The platform will be relevant to edge physics in fusion devices, and will offer extensive diagnostic coverage, enabling comprehensive measurements. One of the diagnostics being designed is the Variable Oblique Probing on O-mode (VOPOO) to study normal to tangential wave propagation at the edge of a plasma and to contribute to the development of novel analysis tools for experimental data, with applications to intensity refractometry and innovative tomographic approaches for next-generation fusion machines such as DEMO (DEMOnstration Power Plant). This study introduces a method for constructing plasma descriptions for a VOPOO synthetic diagnostic. It accommodates a broad range of plasma scenarios, including instabilities and various turbulence regimes, which are evaluated across multiple antenna configurations and injection angles. The implemented synthetic diagnostic is crucial for interpreting the results of the experiment, especially for treating intensity refractometry data, and for driving the development of advanced processing techniques, including machine learning-based methods. This is a first step towards the final goal of having a numerical twin of VOPOO.

The installation of an advanced Plasma Position Reflectometry system, aimed at complementing magnetic sensors for plasma position and shape control, is foreseen on the DEMOnstration Power Plant (DEMO). For such a system, we presented in 2018 the G3C boundary reconstruction algorithm, based on the plasma-wall distance reflectometric measurements and on the the values of the plasma and Poloidal Field Coils (PFC) currents. This work presents the upgrades implemented to the algorithm and its application to both the DEMO baseline and several non-baseline scenarios. The algorithm is further applied to reconstruct the plasma boundary during a Vertical Displacement Event (VDE). Results indicate that the method is able to estimate plasma shape parameters and position with a remarkable accuracy, provided that reliable reflectometric measurements - within 1 cm accuracy - are used as input. Furthermore, it shows significant robustness to higher measurement errors for what concerns most plasma shape parameters estimation. Overall, G3C proves to be an accurate, reliable and flexible reflectometry-based boundary reconstruction tool.

Plasma position reflectometry (PPR) is a leading candidate to complement or replace magnetic diagnostics in DEMO and next-generation fusion devices. The concept of a multi-reflectometer PPR system is currently under development for DEMO, requiring detailed studies of reflectometer performance across different regions of the machine. This work presents the first comprehensive assessment of a multi-reflectometer PPR system using recently developed simulation and data processing automation techniques. A system of reflectometers, located at different poloidal positions around the vessel, is modeled using the 2017 DEMO baseline scenario. The separatrix position error and signal amplitude are evaluated across various plasma and geometry configurations to assess the impact of key system properties, including reflectometer configuration, density curvature, plasma-wall reflections, scrape-off layer decay length, and turbulence-induced errors. Results indicate that while the midplane region meets the 1 cm requirement threshold, further optimization is needed for the divertor and upper poloidal regions. The PPR system’s performance is optimized by adjusting the probing direction and maximizing the average detected signal amplitude across the frequency range. Effective measurement solutions are identified in the upper poloidal and divertor regions. The robustness of the optimized system is assessed by analyzing macroscopic plasma displacements (5–15 cm) and group delay initialization. The results demonstrate that a well-configured PPR system can achieve reliable position measurements across different poloidal regions, confirming its viability for future fusion reactors.

Next-generation fusion reactors, such as DEMOnstration Power Plant (DEMO) [https://doi.org/10.1088/0029-5515/56/2/026009 Nucl. Fusion 56 (2016) 026009], present significant challenges for plasma diagnostics due to limited in-vessel access and harsh operating conditions. Millimeter-wave (MMW) diagnostics and in particular Frequency Modulation of a Continuous Wave (FMCW) reflectometry offer a robust method for measuring electron density, a critical parameter for plasma position and shape control [https://doi.org/10.1088/0029-5515/52/3/032003 Nucl. Fusion 52 (2012) 032003]. However, achieving comprehensive plasma coverage demands numerous probing channels with wide frequency bandwidths and compact, efficient back-ends [https://doi.org/10.1016/j.fusengdes.2023.113512 Fusion Eng. Des. 190 (2023) 113512]. This paper details a redesigned MMW back-end system that overcomes limitations of a previous prototype. The system utilizes readily available Monolithic Microwave Integrated Circuits (MMICs) to establish a base frequency range of 10 GHz to 20 GHz, which is extended to 160 GHz through external frequency multipliers, enabling detailed plasma characterization across an ultra-wideband. To address component obsolescence, the system underwent a complete Printed Circuit Board (PCB) redesign. This work details the new back-end design and its subsequent performance evaluation.

Plasma Position Reflectometry (PPR) is taking an important role in next-generation fusion machines, such as DEMO, as a diagnostic to monitor the position and shape of the plasma, complementing magnetic diagnostics. The Divertor Test Tokamak (DTT) facility is the perfect machine to implement, develop, and test PPR systems. Important work is being done to evaluate the performance of a tentative PPR system in DTT, including assessment efforts integrated into the EUROfusion Enabling Research Project (ENR-TEC.01.IST). These efforts involve designing two- and three-dimensional synthetic diagnostics for the Low (LFS) and High Field Sides (HFS), using Finite-Difference Time-Domain codes. This simulation assessment works in parallel with the design of antennas for the LFS and HFS. Their integration in synthetic diagnostics is done through a CAD conversion pipeline. An overview of the progress achieved, together with future work planned, is provided in the present work devoted to the accuracy of the separatrix localisation at different positions around the vessel.

The Divertor Tokamak Test (DTT) [1], presently under construction in Frascati (Rome), will be equipped with a Plasma Position Reflectometry (PPR) system. It will be installed on a dedicated poloidal sector and consists of a set of antennas placed at different poloidal positions. Featuring a high temporal and spatial resolution and being suitable for harsh fusion reactor experimental conditions (long pulses, high neutron fluxes), for which magnetic probes performance could be severely limited, reflectometry has been proposed as a good candidate for position control in DEMO. Testing and improving a PPR on DTT is then of the utmost importance. The work here presented describes the advances in the High Field Side (HFS) PPR subsystem design. Given the combination of the diagnostics and installation environment complexity, such a design required an intense simulation and optimization work, started with a feasibility study and a definition of the appropriate antennas to be used [2]. The numerical studies needed to validate the diagnostic performance and to guide the design process are presented and discussed. Results allowed to integrate the diagnostic in the extremely demanding, in terms of space constraints, DTT HFS environment. Moreover, the diagnostic's performance has been assessed for the midplane Line Of Sight (LOS), showing that it is possible to accurately reconstruct the inner midplane density profile of a reference DTT Single Null (SN) scenario.

Providing energy from fusion and finding ways to scale up the fusion process to commercial proportions in an efficient, economical, and environmentally benign way is one of the grand challenges for engineering. Controlling the burning plasma in real-time is one of the critical issues that need to be addressed. Plasma Position Reflectometry (PPR) is expected to have an important role in next-generation fusion machines, such as DEMO, as a diagnostic to monitor the position and shape of the plasma continuously, complementing magnetic diagnostics. The reflectometry diagnostic uses radar science methods in the microwave and millimetre wave frequency ranges and is envisaged to measure the radial edge density profile at several poloidal angles providing data for the feedback control of the plasma position and shape. While significant steps have already been given to accomplish that goal, with proof of concept tested first in ASDEX-Upgrade and afterward in COMPASS, important, ground-breaking work is still ongoing. The Divertor Test Tokamak (DTT) facility presents itself as the appropriate future fusion device to implement, develop, and test a PPR system, thus contributing to building a knowledge database in plasma position reflectometry required for its application in DEMO. At DEMO, the PPR diagnostic’s in-vessel antennas and waveguides, as well as the magnetic diagnostics, may be exposed to neutron irradiation fluences 5 to 50 times greater than those experienced by ITER. In the event of failure of either the magnetic or microwave diagnostics, the equilibrium control of the DEMO plasma may be jeopardized. It is, therefore, imperative to ensure that these systems are designed in such a way that they can be replaced if necessary. To perform reflectometry measurements at the 16 envisaged poloidal locations in DEMO, plasma-facing antennas and waveguides are needed to route the microwaves between the plasma through the DEMO upper ports (UPs) to the diagnostic hall. The main integration approach for this diagnostic is to incorporate these groups of antennas and waveguides into a diagnostics slim cassette (DSC), which is a dedicated complete poloidal segment specifically designed to be integrated with the water-cooled lithium lead (WCLL) breeding blanket system. This contribution presents the multiple engineering and physics challenges addressed while designing reflectometry diagnostics using radio science techniques. Namely, short-range dedicated radars for plasma position and shape control in future fusion experiments, the advances enabled by the designs for ITER and DEMO, and the future perspectives. One key development is in electronics, aiming at an advanced compact coherent fast frequency sweeping RF back-end [23–100 GHz in few μs] that is being developed at IPFN-IST using commercial Monolithic Microwave Integrated Circuits (MMIC). The compactness of this back-end design is crucial for the successful integration of many measurement channels in the reduced space available in future fusion machines. Prototype tests of these devices are foreseen to be performed in current nuclear fusion machines.