Jesper Rasmussen

Electron cyclotron heating by high-power microwave beams is a key method for plasma heating in tokamaks and stellarators. Under some conditions, these beams engage in nonlinear three-wave interactions, known as parametric decay instabilities (PDIs), where energy is transferred from the main beam to two downshifted waves. PDIs are well documented to occur during fundamental electron Bernstein wave heating, with observations in multiple devices. However, PDIs were overlooked in the more common second-harmonic electron cyclotron heating until recently, when nonlinear scattering was observed. This has been attributed to a class of absolute PDIs that involves wave trapping in density fluctuations. Here we provide an overview of recent numerical and experimental results in the study of PDIs. We show direct observations of PDIs at the devices ASDEX Upgrade, TCV, Wendelstein 7-X, and NORTH, and show that the occurrence of PDIs is correlated with density fluctuations in the scrape-off layer caused by edge localized modes and blobs as well as density fluctuations in the plasma core due to NTMs. We outline the consequences of PDI occurrence, which could include a decrease in heating efficiency and an associated fast-ion production. In addition, we discuss possible schemes for exploiting PDIs, including controlled energy transfer to selected plasma waves and potential diagnostic techniques for density fluctuations.

Among the feasible options for diagnostics on burning plasma devices, collective Thomson scattering (CTS) is a promising technique for monitoring core ion dynamics. This work aims to identify an optimised CTS configuration for the European DEMOnstration power plant (DEMO) using an accuracy-driven approach. The optimisation procedure combines ray tracing, forward modelling of CTS spectra, and Bayesian inversion to evaluate predicted measurement uncertainties for key plasma parameters. We apply the optimisation methodology to the 2024 low-aspect-ratio DEMO design point. By scanning over the choices of probe frequency, receiver launch angle, and mirror positions, we optimise the CTS system for simultaneous measurements of the ion temperature T _i , the bulk ion rotation velocity v _φ , and the alpha particle density n _α in the centre of the plasma. To inform the optimisation, we estimate the intrinsic toroidal rotation profile, obtaining a core rotation velocity of about 70 km/s. The resulting CTS configuration achieves predicted measurement uncertainties of approximately 4% for T _i , 19% for v _φ , and 5% for n _α at a time resolution of 0.5 s. These results confirm the potential of CTS as a core diagnostic for DEMO and provide a foundation for future work, including sensitivity studies and optimisation for additional parameters such as the fuel-ion ratio.

As we approach the era of burning fusion plasma experiments, the ability to monitor and control the conditions in the plasma core becomes increasingly important. The versatility and reactor relevance of collective Thomson scattering (CTS) makes it an attractive option for diagnosing thermal and energetic confined ions in the core of burning plasmas. Here we discuss the diagnostic design and measurement potential of CTS at ITER, DEMO, SPARC, STEP, and the non-burning but supporting experiment JT-60SA, including novel results for the latter three devices. At ITER, CTS will measure fusion-born alpha particles in seven spatial locations, from their birth energy to thermalization, and the diagnostic design is now in the manufacturing stage. For EU-DEMO, accurate measurements of core ion temperature, toroidal rotation, alpha-particle density, and potentially D/T fuel-ion ratio are also feasible with the considered CTS diagnostic designs. At SPARC, CTS measurements can additionally help to monitor the core concentration of ³ He, used as a minority species for ion heating. In contrast to these cases, which would all operate at frequencies below the fundamental electron cyclotron resonance frequency throughout the plasma, initial studies for STEP suggest challenging conditions for microwave-based CTS, although these might be overcome using a THz-range vertical forward-scattering setup. We also briefly review challenges to CTS operation in the burning-plasma regime, along with ongoing efforts to enable rapid and continuous inference of CTS-relevant plasma parameters for reactor control purposes.

The fast-ion distribution function in fusion plasmas can be inferred by inverting Doppler-shifted measurements from fast-ion diagnostics. The full fast-ion distribution function can be parametrised by three constants of motion with the addition of a binary index. However, with a limited number of measurements, cogent prior information must be added to regularise the inverse problem, enabling the reconstruction of the distribution function. In this paper, we demonstrate how to incorporate wave–particle interactions in the ion cyclotron range of frequencies (ICRFs) as prior information with the future ITER tokamak as a test case. We find that the addition of ICRF physics as prior information improves the reconstruction of a test ICRF-heated fast-ion distribution function in ITER using synthetic data based on the planned collective Thomson scattering sightlines and the planned gamma-ray spectroscopy sightlines. The addition of such prior information is beneficial in the case of a limited phase-space coverage of fast-ion diagnostics.

Isolating a signal of interest from multivariate time-series is of interest for many scientific purposes. An example is the monitoring of thermal- and fast-ion dynamics with fusion plasma diagnostics such as collective Thomson scattering (CTS), which requires careful characterization of an often highly dynamic diagnostic background. Here, we develop and evaluate new CTS background estimation methods inspired by both physics-based approaches and electroencephalogram signal processing. To apply these, we present, for the first time, CTS data from the ASDEX Upgrade (AUG) fusion device taken using rapid (∼5 kHz) on-pulse power modulation of the CTS probe gyrotron. We find that the best performing method is a physics-informed principal component analysis, yielding a typical error on the background estimate of ∼0.5 eV, a factor of ∼3 improvement compared to simpler techniques. We show that this enables CTS-based thermal- and fast-ion monitoring at AUG on the basis of individual 2-ms acquisition pulses, even in the presence of otherwise deleterious edge-localized modes. It also allows accurate tracking of the evolution of CTS spectra across changes in fast-ion heating schemes, in excellent agreement with theoretical expectations. Furthermore, we apply synthetic CTS signals to demonstrate the benefits of rapidly modulating the CTS probe gyrotron power for improved background subtraction. This predicts more accurate estimates for higher modulation amplitude, frequency, and on-pulse duration and may serve as a basis for novel recommendations for the operation of existing and future CTS diagnostics.

The SPARC tokamak is a compact high-field device that will operate at high plasma density with the aim to demonstrate net fusion energy. The experimentally unexplored plasma conditions in SPARC will require a carefully selected set of diagnostics for plasma monitoring and control. Here we explore conceptual design options and potential measurement capabilities of a collective Thomson scattering diagnostic at SPARC. We show that a 140 GHz X-mode CTS system is the most attractive option in terms of optimizing the signal-to-noise ratio and limiting sensitivity to refraction, as well as from a technological readiness perspective. Such a setup can provide core-localized measurements of the fusion alpha distribution function, main-ion temperature and toroidal rotation, fuel-ion ratio, and ³He content with relevant spatio-temporal resolution. Our proposed diagnostic layout can in principle be integrated into SPARC and could provide a valuable addition to its diagnostic suite at limited development costs and time.

We investigate parametric decay instabilities (PDIs) of electron cyclotron waves due to a rotating neoclassical tearing mode (NTM) in ASDEX Upgrade. Strong scattering characteristic of PDIs is observed in a discharge where a (2,1) NTM has been identified. By mapping out the structure of the NTM toroidally, it is possible to determine the phases of the NTM which enable the decay to occur. Signatures of PDIs are seen when the edges of the magnetic island intersect the gyrotron beam path, but not when the O-point of the magnetic island is located directly in front of the gyrotron launcher. We propose an explanation using a simplified model, which reproduces features of the scattering in 1D particle-in-cell (PIC) simulations. The simulations show that a density perturbation caused by an NTM can give rise to a lowered PDI power threshold. The threshold is lowest in an intermediate density perturbation region where certain waves excited in PDIs can become trapped. The PIC simulations show that several decay and combination events involving approximately half frequency waves produce waves slightly downshifted from the main pump frequency of 140 GHz. This is the first time PIC simulations based on experimental profiles reproduce signals close to the pump frequency, which result from interactions with half frequency waves. The numerical results support previous observations from the tokamak TEXTOR.

DEMO burn control will require measurements of a range of plasma parameters, but the suite of feasible diagnostics for this purpose is limited. Here we assess the accuracy with which a collective Thomson scattering (CTS) diagnostic can provide key measurements for burn control in the planned European DEMO (EU-DEMO). This is based on estimated signal-to-noise ratios for a conceptual diagnostic design and trial fits to synthetic DEMO CTS spectra. We show that a diagnostic with a single probe- and receiver beam setup will be able to provide simultaneous measurements of core fusion alpha density and ion temperature to mean accuracies better than 5% and 10%, respectively, along with detecting intrinsic toroidal rotation velocities down to within ∼ 5 km s⁻¹. Adding a second CTS receiver view furthermore enables inference of the core fuel-ion ratio, allowing discrimination between, e.g. a 50%/50% and 55%/45% D/T mixture, while also providing useful information on the thermalized He content. A DEMO CTS diagnostic would thus be able to monitor fusion alpha densities as well as anomalous transport of fast alphas and heat from the plasma core, quantify plasma rotation for confinement enhancement, and track the core isotope mix for optimum fusion performance. This versatility makes such a diagnostic a potentially valuable tool for real-time burn control on DEMO.