Short-Time Retractable Instrumented Kalorimeter Experiment Research Articles

STRIKE Heat Flux Reconstruction by Using Neural Networks: Application to the Experimental Results

To characterize the SPIDER negative ion beam in terms of beam uniformity and divergence during short pulse operations, an instrumented calorimeter named short-time retractable instrumented kalorimeter experiment (STRIKE) has been designed and operated. STRIKE is made of 16 1D carbon fiber-carbon composite (CFC) tiles, intercepting the whole beam and observed on the rear side by two infrared (IR) cameras. To know the energy flux profile hitting the front surface and then the beam parameters, it is necessary to solve an inverse non-linear problem, mathematically ill-posed, upon knowing the non-linear characteristics of the tiles and the 2-D temperature pattern measured on the rear side of the tiles themselves. Most of the conventional methods used to solve this inverse problem are unbearably time-consuming; when fully operative STRIKE receives 1280 beamlets, each one characterized by at least five features, a ready-to-go tool to determine the beam condition is highly recommended. In this work, the inverse problem in stationary conditions is faced by using a neural network (NN) model, pursuing different training approaches. The NN is trained by associating features extracted from the 2-D temperature profile, obtained by a fitting process to the heat flux profile parameters. The proposed method is then applied to experimental STRIKE data from the beam campaigns.

Assessment of the SPIDER beam features by diagnostic calorimetry and thermography.

The full-size ITER ion source prototype SPIDER (Source for the Production of Ions of Deuterium Extracted from a Radio frequency plasma) has recently started beam operation, whose objective is to produce 100 keV, 60 A hydrogen negative ions for 1 h. The source is presently operated in the volume regime, and the beam power is consequently limited. In such a configuration, the high resolution calorimeter STRIKE (Short-Time Retractable Instrumented Kalorimeter Experiment), even though uncooled, may be used instead of the SPIDER beam dump without limiting the beam-on time. STRIKE is formed by unidirectional carbon fiber-carbon matrix (CFC) composite tiles that are exposed to the beam while their temperature is recorded by using two infra-red cameras. This setup, thanks to the moderate broadening of the temperature profile guaranteed by the anisotropy of CFC, allows for the determination of detailed features of the beam current distribution (spatial resolution is about 2 mm). Furthermore, positively biasing the CFC tiles permits a direct electrical measurement of the negative ion beam current. Besides the total beam current and beam uniformity, which can be retrieved both by calorimetry and electrical measurement, beamlet divergence and deflection can be determined by infra-red thermography. This contribution describes the characterization of the SPIDER negative ion beam as a function of the source and accelerator parameters by means of the diagnostic calorimeter STRIKE in the volume regime.

Neural network based prediction of heat flux profiles on STRIKE

The instrumented calorimeter STRIKE (Short-Time Retractable Instrumented Kalorimeter Experiment) has been designed with the main purpose of characterizing the SPIDER (Source for Production of Ion of Deuterium Extracted from Radio Frequency plasma) negative ion beam in terms of beam uniformity and divergence during short pulse operations. STRIKE is made of 16 1D Carbon Fiber Composite (CFC) tiles, intercepting the whole beam and observed on the rear side by infrared (IR) cameras. The front observation presents some drawbacks due to optically emitting layer caused by the excited gas between the beam source and the calorimeter, and the material sublimated from the calorimeter surfaces due to the heating itself. This paper proposes a Neural Network-based approach to solve the inverse non-linear problem of determining the energy flux profile impinging on the calorimeter, considering the 2D temperature pattern measured on the rear side of the tiles. Most of the conventional methods used to evaluate the inverse heat flux are unbearably time consuming; since the objective is having a tool for heat flux evaluation for STRIKE real time operation, the need to have a ready-to-go instrument to understand the beam condition becomes stringent. For this reason, in this paper, a Multi-Layer Perceptron has been used to solve the problem. Once properly trained, the neural networks provide a fast evaluation of the impinging flux. Furthermore, there is no need to optimize any parameter since this operation is already included in the self-adjustment of the network weights during the training. The achieved results show the reliability of the proposed method both with stationary and non-stationary heat fluxes.

Applicability of inverse heat flux evaluation to thermographic measurements in SPIDER

To study and optimize negative ion production, the SPIDER (Source for Production of Deuterium Extracted from RF plasma) prototype (beam energy 100keV, current 48A) is under construction in Padova, Italy. The instrumented calorimeter STRIKE (Short-Time Retractable Instrumented Kalorimeter Experiment) has been designed with the main purpose of characterizing the SPIDER negative ion beam in terms of beam uniformity and divergence during short pulse operations. STRIKE is made of 16 1D Carbon Fibre Composite (CFC) tiles, intercepting the whole beam and observed on the rear side by infrared (IR) cameras.As the front observation is not convenient, it is necessary to solve an inverse non-linear problem to determine the energy flux profile impinging on the calorimeter, starting from the 2D temperature pattern measured on the rear side of the tiles. The aim of the paper is to give an overview about the transfer function technique which may be used to retrieve the flux profile, dealing with non-linearity and non-stationariness.

Final design of the diagnostic calorimeter for the negative ion source SPIDER

The Short-Time Retractable Instrumented Kalorimeter Experiment (STRIKE) diagnostic will be used to characterize the SPIDER negative ion beam during short pulse operation (<10s) to verify the degree of attainment of ITER requirements about the maximum allowed beam non-uniformity (±10% is required). After a preliminary design developed in the last few years, the complete STRIKE diagnostic system has been recently subjected to a final review and is now ready for construction. The main components of the system are: an array of 16 tiles made of unidirectional Carbon Fibre Composite (CFC) with the fibre direction parallel to the thickness, which will be exposed to the high power density beam (20MW/m2 peak) exiting from the SPIDER Beam Source; a supporting and positioning system, based on a set of structures made of stainless steel, with controlled moving systems, to be installed within the SPIDER vacuum vessel; a set of thermal, electric and thermo-graphic sensors to thoroughly characterize the beam. The development of the engineering design of the supporting and positioning system and an overview of the complete diagnostic system are given.

Prototype of a Diagnostic Calorimeter for BATMAN: Design and Preliminary Measurements

The ITER neutral beam injection system is being designed to provide 33 MW of heating from two injectors, with an upgrade to 50 MW possible with a third injector; such heating power will be provided by accelerating negative ions to high energies, 1 MeV, and neutralizing them. These neutrals are then injected into the tokamak where they impart their energy by collisions. To study and optimize negative ion production, the SPIDER prototype is under construction in Padova, Italy, whose beam has an energy of 100 keV and a current of 48 A. The instrumented calorimeter Short-Time Retractable Instrumented Kalorimeter Experiment (STRIKE) has been designed with the main purpose of characterizing the SPIDER negative ion beam in terms of beam uniformity and beam divergence during short operations (several seconds). STRIKE is made of 16 1-D carbon fiber composite (CFC) tiles, intercepting the whole beam and observed from the rear side by infrared (IR) cameras. With two identical samples of the CFC material and the IR camera under assessment, a reduced version of the entire calorimeter has been built, with the purpose of characterizing its diagnostic properties. This mini-STRIKE was used in the BATMAN experiment at Max Planck Institut fur Plasmaphysik (Garching, Germany). As the beamlet divergence in BATMAN is large, the beamlets overlap each other in the measurement position; therefore, the mini-STRIKE for BATMAN includes a copper mask, facing the beam and featuring eight apertures. Thus, eight different portions of the beam can be simultaneously sampled, with minimal overlapping of the power coming from adjacent apertures. The copper mask was actively cooled, and calorimetry was carried out. The temperature in the center of the mask and some positions along the CFC tiles was also measured. In this paper, the design of the system is presented. The calorimetry system is presented in detail as well as the procedure adopted for calorimetrical data analysis. The results of calorimetry are presented together with preliminary correlations with the BATMAN beam features.

High energy flux thermo-mechanical test of 1D-carbon-carbon fibre composite prototypes for the SPIDER diagnostic calorimeter

Operation of the thermonuclear fusion experiment ITER requires additional heating via injection of neutral beams from accelerated negative ions. In the SPIDER test facility, under construction in Padova, the production of negative ions will be studied and optimised. STRIKE (Short-Time Retractable Instrumented Kalorimeter Experiment) is a diagnostic used to characterise the SPIDER beam during short pulse operation (several seconds) to verify if the beam meets the ITER requirements about the maximum allowed beam non-uniformity (below ±10%). The major components of STRIKE are 16 1D-CFC (Carbon-Carbon Fibre Composite) tiles, observed at the rear side by a thermal camera. This contribution gives an overview of some tests under high energy particle flux, aimed at verifying the thermo-mechanical behaviour of several CFC prototype tiles. The tests were performed in the GLADIS facility at IPP (Max-Plank-Institut für Plasmaphysik), Garching. Dedicated linear and nonlinear simulations were carried out to interpret the experiments and a comparison of the experimental data with the simulation results is presented. The results of some morphological and structural studies on the material after exposure to the GLADIS beam are also given.

Characterization, test and interpretative simulations of one-dimensional Carbon Fiber Composite prototype for SPIDER experiment

For ITER operations, additional heating systems are required. One of these systems is the neutral beam injector (NBI). The SPIDER experiment, a small-scale NBI, is going to be built with the aim to optimize the beam source. For this reason it is provided with several diagnostics, among which the Short-Time Retractable Instrumented Kalorimeter Experiment (STRIKE). In this contribution, a characterization of the Carbon Fiber Composite (CFC) tiles, which are the main component of the diagnostic, is presented. Such analyses include tests with a power laser, exposure to particle beams and thermal stress tests. The results are discussed, which will drive the definition of the acceptance tests of the final supply of CFC tiles.

Design and specifications of the diagnostics for the instrumented calorimeter of Source for the Production of Ions of Deuterium Extracted from Radio frequency plasma

In the development of the neutral beam injector for ITER experiment, the test facility SPIDER is going to be built. To measure the beam parameters, several diagnostics are used. One of them is the Short-Time Retractable Instrumented Kalorimeter Experiment (STRIKE) which measures beam uniformity, beamlet divergence, and stripping losses. This contribution gives an overview of the diagnostics dedicated to these measurements: thermal cameras, thermocouples, current sensors, and electrostatic sensors. The specifications of these diagnostics have been defined according to the results of electrostatic and thermal simulations. A failure modes and effects analysis has been performed during the design. Consequently the project of the STRIKE diagnostics has been optimized to reduce the possible failure risks.

Thermal and electrostatic simulations of the diagnostic calorimeter for the Source for Production of Ion of Deuterium Extracted from RF plasma beam

To study and optimise negative ion production for the ITER neutral beam injectors, a test facility is under construction in Padova with the aim of testing beam characteristics and to verify the source proper operation. The instrumented calorimeter STRIKE (short-time retractable instrumented kalorimeter experiment) is being developed to characterise the SPIDER (Source for Production of Ion of Deuterium Extracted from RF plasma) beam during short operations. The paper presents an investigation of the response of STRIKE measurement systems. It results that biasing is necessary to cope with the influence of secondary electrons on current measurements; moreover, despite the discretisation of the recorded thermal patterns introduced by the pixels of thermal cameras, a sufficient spatial resolution is expected.