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The JET baseline scenario is being developed to achieve high fusion performance and sustained fusion power. However, with higher plasma current and higher input power, an increase in pulse disruptivity is being observed. Although there is a wide range of possible disruption causes, the present disruptions seem to be closely related to radiative phenomena such as impurity accumulation, core radiation, and radiative collapse. In this work, we focus on bolometer tomography to reconstruct the plasma radiation profile and, on top of it, we apply anomaly detection to identify the radiation patterns that precede major disruptions. The approach makes extensive use of machine learning. First, we train a surrogate model for plasma tomography based on matrix multiplication, which provides a fast method to compute the plasma radiation profiles across the full extent of any given pulse. Then, we train a variational autoencoder to reproduce the radiation profiles by encoding them into a latent distribution and subsequently decoding them. As an anomaly detector, the variational autoencoder struggles to reproduce unusual behaviors, which includes not only the actual disruptions but their precursors as well. These precursors are identified based on an analysis of the anomaly score across all baseline pulses in two recent campaigns at JET., (to appear)

Electron cyclotron resonance heating (ECRH) and electron cyclotron current drive systems in fusion- grade devices meet the severe requirements (in terms of high power handling capability, extended steering range, and room availability) that guide the design of complex multiple-mirror quasi-optical launchers. A valuable step in this process is a beam-pattern calculation in vacuum including relevant electromagnetic effects not easily included in analytical evaluations. In fact, the analytical approach is a means to study the design layout at a first order and is able to derive the relevant quantities as a function of the steering angle and of the beam path in a form suitable to interface with most of the currently available beam-tracing codes. On the other hand, electromagnetic calculations using physical optics tools provide a complete description of the resulting full beam pattern, including the effects of aberration, beam truncation, thermal deformation of the mirrors, and the surrounding structures. Moreover, numerical calculation with reliable and benchmarked codes is a very efficient way to test subsequent updates of a given launcher model, once the basic geometry has been implemented. In this paper, we discuss in particular the application of the GRASP® code to the case of the remote steering option for the ITER ECRH upper launcher.

This chapter reports the main physics results obtained with three radio-frequency-injection systems. The frequency of 8 GHz for the lower hybrid (LH) current drive (CD) (LHCD) system was chosen to explore CD at high density: full CD has been demonstrated for central densities up to 1.4 x 10{sup 20} m{sup -3} at 0.5 MA with an applied power up to 2.0 MW. The Frascati Tokamak Upgrade (FTU) database shows CD efficiencies from 0.1 to 0.3 x 10{sup 20} AW{sup -1} m{sup -2}. In combined experiments with electron cyclotron (EC) waves (140 GHz, up to 1.2 MW), a suprathermal absorption by the fast electron tail generated by LHCD has been observed in both downshifted and upshifted interaction regimes, with the resulting electron cyclotron current drive (ECCD) ranging from 20 to 100 kA, depending on experimental conditions. With pure EC resonance heating, the narrowness of the radial power deposition profile has been exploited, resulting in strong local electron heating. Results in high-density regimes are also presented. The third system (433 MHz, 0.5 MW) is the first to test ion Bernstein wave (IBW) coupling with a waveguide antenna. The experiment operates at high frequency, avoiding the occurrence of nonlinear phenomena at the edge.more » Improved confinement regimes resulting in a central peaking of the pressure profiles have been achieved with P{sub IBW} up to 0.4 MW. Modeling and experimental results are summarized.« less

Transport studies are presented in this chapter. Global scaling studies have been performed using several transport codes. Ohmic plasmas are found to follow the ITER97 L-mode scaling. Transport coefficients are discussed for improved confinement scenarios achieved in the Frascati Tokamak Upgrade (FTU): the repetitive pellet enhanced plasma mode, showing neoclassical confinement with H-factors up to 1.6, and the electron internal transport barriers (ITBs) with large transport barriers and H-factors up to 1.3. Heat transport models have been tested using electron cyclotron resonance heating (ECRH), steady or modulated, as a probe. The electron temperature stiffness observed in the main bulk of steady FTU plasmas can be interpreted both with a critical gradient transport model and with a model based on the existence of canonical profiles. ECRH has also been used to benefit from the improved confinement generally associated with low or negative magnetic shear, and large electron temperatures have been achieved in these conditions. Profile resiliency is observed so that heat transport is not consistent with a constant thermal diffusivity. Experimental optimization is discussed together with the analysis of transport coefficients. Thorough discussions of impurity transport are given, both for intrinsic and injected (from laser blow-off) impurities. Code simulation and experimentalmore » data are compared for a series of FTU experiments focusing on the improved confinement modes (pellets and ITBs). A moderate inward pinch velocity is generally required to reproduce the data.« less

High-frequency wave systems with high-power density launching capability have been the preferred choice to heat the Frascati Tokamak Upgrade (FTU) because of physics arguments (electron heating at ...

Strong electron internal transport barriers (ITBs) are obtained in the Frascati Tokamak Upgrade (FTU) with the combined injection of lower hybrid (LH) (up to 1.9 MW) and electron cyclotron (EC) (up to 0.8 MW) radio-frequency waves. ITBs occur during either the current plateau or the ramp-up phase, both in full and partial current drive (CD) regimes, up to n e0 > 1.4 × 10 20 m -3 , relevant to ITER operation. Central electron temperatures T e0 > 8 keV, at n e0 0.8 X 10 20 m -3 , are sustained for up to 36 confinement times. The ITB extends over a region where a slightly reversed magnetic shear is established by off-axis LHCD and can be even larger than r/a = 0.5. EC power is used either to benefit from this improved confinement by heating inside the ITB or to enhance the peripheral LH power deposition and CD with off-axis resonance. Collisional ion heating is also observed, but thermal equilibrium with the electrons is not attained since the electron-ion equipartition time is always 4 to 5 times longer than the energy confinement time. An extensive transport modeling of these discharges, performed by means of the ASTRA code, is also presented. During the ITB phase, the ion diffusivity is close to the neoclassical value while the electron shear-dependent Bohm-gyro-Bohm model accounts quite well for T e (r, t), The Ray Tracing Fokker-Planck model, used to describe the LHCD physics, appears satisfactory to analyze and interpret the experimental results. It turns out that the barrier radius is mainly influenced by the LHCD deposition. In particular, a wider barrier is obtained the lower q a is and the larger the plasma density is.

The main physics results achieved in the recent years in the Frascati Tokamak Upgrade (FTU) are reviewed. The main focus of research has been the development of performance plasmas at high densities (up to 4 x 10{sup 20} m{sup -3}), high magnetic field (up to 8 T) and plasma current (up to 1.6 MA), that are therefore in a domain of relevance for burning physics experiments such as ITER. The main tools consist in the development of plasma conditioning techniques and the use of various electron heating and current drive systems. Improved confinement regimes have been developed, including (a) the production of steady electron internal transport barriers at high density and electron temperature (up to central electron temperature of 11 keV at a central density of 0.9 x 10{sup 20} m{sup 3}), (b) the production of repetitive pellet enhanced plasma modes with deep pellet deposition leading to a substantial increase of the neutron yield (and a record FTU value of the fusion product n{sub i}T{sub i}{tau}{sub E} up to 0.8 x 10{sup 20} m{sup -3} keV.s), and (c) the production of radiation improved modes at high magnetic field. Main results on the supporting physics program will also be given inmore » the domain of plasma wave physics (lower hybrid current drive, electron cyclotron resonance frequency, ion Bernstein waves), heat and impurities transport, and magnetohydrodynamic studies.« less

Compact directional couplers-polarimeters were developed for the circular corrugated waveguide transmission lines of the joint IFP-ENEA Electron Cyclotron Resonance Heating experiment at 140 GHz, 2 MW on the Frascati Tokamak Upgrade in Frascati.A linear array of cutoff holes was drilled in the mirrors of the quasi-optical miter bends. The radiated pattern preserves the symmetry and polarization of the waveguide mode in the plane of the array. The direction of propagation is preserved too.Two pairs of standard gain horns with detectors are placed in the plane of the array along the propagation axes of incident and reflected radiation to detect both linear components. The whole assembly is enclosed in a shielded anechoic box of suitable geometry.