M. Brombin

Diagnostics of the ITER neutral beam test facility.

The ITER heating neutral beam (HNB) injector, based on negative ions accelerated at 1 MV, will be tested and optimized in the SPIDER source and MITICA full injector prototypes, using a set of diagnostics not available on the ITER HNB. The RF source, where the H(-)∕D(-) production is enhanced by cesium evaporation, will be monitored with thermocouples, electrostatic probes, optical emission spectroscopy, cavity ring down, and laser absorption spectroscopy. The beam is analyzed by cooling water calorimetry, a short pulse instrumented calorimeter, beam emission spectroscopy, visible tomography, and neutron imaging. Design of the diagnostic systems is presented.

Overview of RFX-mod results

With the exploration of the MA plasma current regime in up to 0.5 s long discharges, RFX-mod has opened new and very promising perspectives for the reversed field pinch (RFP) magnetic configuration, and has made significant progress in understanding and improving confinement and in controlling plasma stability. A big leap with respect to previous knowledge and expectations on RFP physics and performance has been made by RFX-mod since the last 2006 IAEA Fusion Energy Conference. A new self-organized helical equilibrium has been experimentally achieved (the Single Helical Axis—SHAx—state), which is the preferred state at high current. Strong core electron transport barriers characterize this regime, with electron temperature gradients comparable to those achieved in tokamaks, and by a factor of 4 improvement in confinement time with respect to the standard RFP. RFX-mod is also providing leading edge results on real-time feedback control of MHD instabilities, of general interest for the fusion community.

Helical equilibria and magnetic structures in the reversed field pinch and analogies to the tokamak and stellarator

The reversed field pinch configuration is characterized by the presence of magnetic structures both in the core and at the edge: in the core, at high plasma current the spontaneous development of a helical structure is accompanied by the appearance of internal electron transport barriers; at the edge strong pressure gradients, identifying an edge transport barrier, are observed too, related to the position of the field reversal surface.The aim of this paper is the experimental characterization of both the internal and edge transport barriers in relation to the magnetic topology, discussing possible analogies and differences with other confinement schemes.

High current regimes in RFX-mod

Optimization of machine operation, including plasma position control, density control and especially feedback control on multiple magnetohydrodynamic modes, has led RFX-mod to operate reliably at 1.5?MA, the highest current ever achieved on a reversed field pinch (RFP). At high current and low density the magnetic topology spontaneously self-organizes in an Ohmical helical symmetry, with the new magnetic axis helically twisting around the geometrical axis of the torus. The separatrix of the island disappears leaving a wide and symmetric thermal structure with large gradients in the electron temperature profile. The new topology still displays an intermittent nature but its overall presence has reached 85% of the current flat-top period. The large gradients in the electron temperature profile appear to be marginal for the destabilization of ion temperature gradient modes on the assumption that ions and electrons have the same gradients. There are indications that higher currents could provide the conditions under which to prove the existence of a true helical equilibrium as the standard RFP configuration.

The tomographic diagnostic of ITER neutral beam injector

The two-dimensional (2D) density distribution of a particle beam can be reconstructed using a diagnostic based on tomographic techniques, which measures the Dα light. This paper presents investigations and tests concerning the reliability of such a diagnostic, applied to the beam produced in MITICA (Megavolt ITER Injector & Concept Advancement) the full-size prototype of the neutral beam injector, which will be used for additional heating on ITER. It aims at developing the technologies to guarantee the correct operation of the injectors to be installed in ITER. The main target of the tomographic diagnostic is the measurement of the beam uniformity with sufficient contrast and spatial resolution, and of its evolution throughout the pulse duration. In particular the ITER beam uniformity is required to be within 10%, thus the error of the tomographic reconstruction has to be lower than this value. The conceptual design of the diagnostic is presented, with a description of the layout and main components. The portholes around the vessel and the geometry of the lines of sight are arranged so as to cover the whole area of the beam. A tomographic algorithm based on the simultaneous algebraic reconstruction technique (SART) is developed to reconstruct the beam intensity profile. Phantoms reproducing different experimental beam configurations are simulated and reconstructed with this technique. The role of noise in the line-integrated signals is studied and its effect on the reconstructed emission is investigated. The unsatisfactory results suggested the introduction of a regularization algorithm, to reduce the effect of the noise in the beam reconstruction. In this way, the simulated phantoms are correctly reconstructed and their 2D spatial non-uniformity is correctly estimated, up to a noise level of 10%.

Tomography feasibility study on the optical emission spectroscopy diagnostic for the negative ion source of the ELISE test facility

A feasibility study of a spectroscopic tomographic diagnostic for the emissivity reconstruction of the plasma parameters in the large negative ion source of the test facility ELISE is described. Tomographic tools are developed to be applied to the measurements of the ELISE optical emission spectroscopy (OES) diagnostic, in order to reconstruct the emissivity distribution from hydrogen (or deuterium) plasma close to the plasma grid, where negative ions are produced and extracted to be accelerated. Various emissivity phantoms, both symmetric and asymmetric, reproducing different plasma experimental conditions have been simulated to test the tomographic algorithm. The simultaneous algebraic reconstruction technique has been applied, accounting for the OES geometrical layout together with a suitable pixel representation. Even with a limited number of 14 lines of sight (LoSs), the plasma emissivity distribution expected on the ELISE source can be successfully reconstructed. In particular, asymmetries in the emissivity pattern can be detected and reproduced with low errors. A systematic investigation of different geometrical layouts of the LoSs as well as of the pixel arrangements has been carried out, and a final configuration has been identified. Noise on the simulated experimental spectroscopic measurements has been tested, confirming the reliability of the adopted tomographic tools for the plasma emissivity reconstructions of the source plasma in ELISE with the actual OES diagnostic system.

SPIDER beam dump as diagnostic of the particle beam

The beam power produced by the negative ion source for the production of ion of deuterium extracted from RF plasma is mainly absorbed by the beam dump component which has been designed also for measuring the temperatures on the dumping panels for beam diagnostics. A finite element code has been developed to characterize, by thermo-hydraulic analysis, the sensitivity of the beam dump to the different beam parameters. The results prove the capability of diagnosing the beam divergence and the horizontal misalignment, while the entity of the halo fraction appears hardly detectable without considering the other foreseen diagnostics like tomography and beam emission spectroscopy.

Improvement of the magnetic configuration in the reversed field pinch through successive bifurcations

The reversed field pinch (RFP) is a magnetic configuration alternative to the tokamak that can be considered for a second generation of reactors. In this paper new remarkable results obtained in the RFP experiment RFX-mod are presented, showing that an internal transport barrier delimitates a large fraction of the plasma volume in a RFP when the current is raised to ∼1.5 MA. The formation of this transport barrier is related to a profound, spontaneous modification of the magnetic topology. Due to the occurrence of a saddle node bifurcation the plasma enters in the single helical axis state, which is theoretically known to be more resilient to chaos. This bifurcation is driven by the amplitude of the helical perturbation which dominates the mode spectrum.

Optical Transmission of Thermal Measurements From High Voltage Devices in High Vacuum Conditions

The paper describes a system for digitization and optical transmission of thermal measurements on high voltage devices in high vacuum environmental conditions and presents the tests conducted on a prototype. The system has been designed in particular to satisfy such technical requirements as to be mounted on the grounded grid of the SPIDER facility (a 100-keV/60-A particle accelerator) and to withstand frequent fault conditions in which the voltage of the grid transiently rises up to some tens of kV. The system is based on a circuit which samples and transmits the signals to the central acquisition system while preserving the signals and avoiding any electrical links between the high voltage device and the vacuum vessel. Moreover, the system has to be designed so as to minimize the electromagnetic noise affecting the low amplitude signals from the thermocouples (TCs). The circuit design is presented, describing the layout and the electronic components for the acquisition of the TC signals and for the data transmission via optical fiber. When SPIDER is operational, with up to 1-h pulse duration, the circuit is powered by a battery, which is in turn recharged by the energy coming from a photovoltaic cell when SPIDER is not operational and the circuit is not acquiring. Data are digitally transmitted according to RS-232 protocol for easy interfacing to the central data acquisition system. The circuit has been tested to check its proper operation, with particular care devoted to the data transmission and the recharging phase. The results are reported and discussed.

Final design of thermal diagnostic system in SPIDER ion source

The prototype radio frequency source of the ITER heating neutral beams will be first tested in SPIDER test facility to optimize H- production, cesium dynamics, and overall plasma characteristics. Several diagnostics will allow to fully characterise the beam in terms of uniformity and divergence and the source, besides supporting a safe and controlled operation. In particular, thermal measurements will be used for beam monitoring and system protection. SPIDER will be instrumented with mineral insulated cable thermocouples, both on the grids, on other components of the beam source, and on the rear side of the beam dump water cooled elements. This paper deals with the final design and the technical specification of the thermal sensor diagnostic for SPIDER. In particular the layout of the diagnostic, together with the sensors distribution in the different components, the cables routing and the conditioning and acquisition cubicles are described.