M. Walsh

Spatial resolution of the JET Thomson scattering system

The instrument function of the high resolution Thomson scattering (HRTS) diagnostic in the Joint European Torus (JET) has been calculated for use in improved pedestal profile analysis. The full width at half maximum (FWHM) of the spatial instrument response is (22 ± 1) mm for the original HRTS system configuration and depends on the particular magnetic topology of the JET plasmas. An improvement to the optical design of the laser input system is presented. The spatial smearing across magnetic flux surfaces is reduced in this design. The new input system has been implemented (from JPN 78742, July 2009) and the HRTS instrument function corresponding to the new configuration has been improved to approximately FWHM = (9.8 ± 0.8) mm. The reconstructed instrument kernels are used in combination with an ad hoc forward deconvolution procedure for pedestal analysis. This procedure produces good results for both the old and new setups, but the reliability of the deconvolved profiles is greatly reduced when the pedestal width is of the same order as, or less than the FWHM of the instrument kernel.

Defining the infrared systems for ITER.

The International Thermonuclear Experimental Reactor will have wide angle viewing systems and a divertor thermography diagnostic, which shall provide infrared coverage of the divertor and large parts of the first wall surfaces with spatial and temporal resolution adequate for operational purposes and higher resolved details of the divertor and other areas for physics investigations. We propose specifications for each system such that they jointly respond to the requirements. Risk analysis driven priorities for future work concern mirror degradation, interfaces with other diagnostics, radiation damage to refractive optics, reflections, and the development of calibration and measurement methods for varying optical and thermal target properties.

Fusion neutron diagnostics on ITER tokamak

ITER is an experimental nuclear reactor, aiming to demonstrate the feasibility of nuclear fusion realization in order to use it as a new source of energy. ITER is a plasma device (tokamak type) which will be equipped with a set of plasma diagnostic tools to satisfy three key requirements: machine protection, plasma control and physics studies by measuring about 100 different parameters. ITER diagnostic equipment is integrated in several ports at upper, equatorial and divertor levels as well internally in many vacuum vessel locations. The Diagnostic Systems will be procured from ITER Members (Japan, Russia, India, United States, Japan, Korea and European Union) mainly with the supporting structures in the ports. The various diagnostics will be challenged by high nuclear radiation and electromagnetic fields as well by severe environmental conditions (ultra high vacuum, high thermal loads). Several neutron systems with different sensitivities are foreseen to measure ITER expected neutron emission from 1014 up to almost 1021 n/s. The measurement of total neutron emissivity is performed by means of Neutron Flux Monitors (NFM) installed in diagnostic ports and by Divertor Neutron Flux Monitors (DNFM) plus MicroFission Chambers (MFC) located inside the vacuum vessel. The neutron emission profile is measured with radial and vertical neutron cameras. Spectroscopy is accomplished with spectrometers looking particularly at 2.5 and 14 MeV neutron energy. Neutron Activation System (NAS), with irradiation ends inside the vacuum vessel, provide neutron yield data. A calibration strategy of the neutron diagnostics has been developed foreseeing in situ and cross calibration campaigns. An overview of ITER neutron diagnostic systems and of the associated challenging engineering and integration issues will be reported.

Physical aspects of divertor Thomson scattering implementation on ITER

This paper describes the challenges of Thomson Scattering implementation in the ITER divertor and evaluates the capability to satisfy project requirements related to the range of the measured electron temperature and density. A number of aspects of data interpretation are also discussed. Although this assessment and the proposed solutions are considered in terms of ITER compatibility, they may also be of some use in currently operating magnetic confinement devices.

Absolute calibration of LIDAR Thomson scattering systems by rotational Raman scattering

Absolute calibration of LIDAR Thomson scattering systems on large fusion devices may be achieved using rotational Raman scattering. The choice of calibrating gas molecule presents different options and design trade-offs and is likely to be strongly dependent on the laser wavelength selected. Raman scattering of hydrogenic molecules produces a very broad spectrum, however, with far fewer scattered photons than scattering from nitrogen or oxygen at the same gas pressure. Lower laser wavelengths have the advantage that the Raman cross section increases, sigma(Raman) proportional to 1/lambda(0)(4), but the disadvantage that the spectral width of the scattered spectrum decreases, Deltalambda(Raman) proportional to lambda(0)(2). This narrower spectrum makes measurement closer to the laser wavelength necessary. The design of the calibration technique presents a number of challenges. Some of these challenges are generic to all Thomson scattering systems. These include detecting a sufficient number of photoelectrons and designing filters that measure close to the laser wavelength while simultaneously achieving adequate blocking of the laser wavelength. An issue specific to LIDAR systems arises since the collection optics operates over a wide range of depth of field. This wide depth of field has the effect of changing the angle of light incident on the optical interference filter with plasma major radius. The angular distribution then determines the effective spectral transmission function of the interference filter and hence impacts on the accuracy of the absolute calibration. One method that can be used to increase absolute calibration accuracy is collecting both Stokes and anti-Stokes lines with optical filter transmission bands specifically designed to reduce systematic uncertainty.

Influence of stray light on visible spectroscopy for the scrape-off layer in ITER

Since ITER will be a full metallic wall machine, scattered photons from the strong emission in the divertor may distort the emission from the scrape-off layer (SOL). The influence of stray light for visible spectroscopy of H? and Be?I emissions in ITER was investigated with a ray-tracing simulation. It was found that the stray light would be always more than one order of magnitude greater than the real signal intensity for H? emission from the SOL. For Be?I emission, although the situation was better than the cases of H? measurement, the stray light could be much larger than the real signal from the SOL. In ITER, it will inevitably reduce the stray light somehow for visible spectroscopy. The effect of optical dumps embedded on first walls was investigated with the ray-tracing analysis.

ITER diagnostic challenges

ITER will explore a plasma parameter envelope currently not available in tokamaks. This will require a set of diagnostics that can follow this envelope. To implement these diagnostics in a reliable and robust way requires development of current techniques in many areas to make them applicable to ITER: they need to be operable in the ITER environment and satisfy the physics and engineering requirements. In some cases, the exploitation of new techniques will be required. While much work has been carried out in this area, significant further work remains to bring the system to implementation.

RADIATION TRANSPORT ANALYSES FOR DESIGN OPTIMISATION OF THE ITER CORE LIDAR DIAGNOSTIC

Abstract Systematic analysis of the radiation fields throughout the ITER core LIDAR diagnostic system were performed to support the design optimisation and assessment process, aiming at achieving the required performance in terms of reliability, occupational safety and interface with neighboring systems. Neutron, photon, nuclear heat and material activation responses were estimated for a variety of configurations, and improved using a combination of analytical “rules of thumb” and numerical computations with the ATTILA™ and FISPACT codes. The neutron flux at the backplate of the port plug was significantly reduced (to ∼2x107 n/cm2-s) by fine-tuning the reference geometry of the laser labyrinth, and guidelines were provided for quick estimation of the effect of future design changes. The current design has adequate lifetime of essential optical components, in particular absorption in collection windows below ∼1%, and reduced dose to workers during maintenance according to the ALARA principle.

Port-Based Plasma Diagnostic Infrastructure on ITER

Abstract The port-based plasma diagnostic infrastructure on ITER is described, including the port plugs, the interspace support structure and port cell structure. These systems are modular in nature with standardized dimensions. The design of the equatorial and upper port plugs and their modules is discussed, as well as the dominant loading mechanisms. The port infrastructure design has now matured to the point that port plugs are now being populated with multiple diagnostics supplied by a number of ITER partners - two port plug examples are given.

Conceptual design of the ITER fast-ion loss detector

A conceptual design of a reciprocating fast-ion loss detector for ITER has been developed and is presented here. Fast-ion orbit simulations in a 3D magnetic equilibrium and up-to-date first wall have been carried out to revise the measurement requirements for the lost alpha monitor in ITER. In agreement with recent observations, the simulations presented here suggest that a pitch-angle resolution of ∼5° might be necessary to identify the loss mechanisms. Synthetic measurements including realistic lost alpha-particle as well as neutron and gamma fluxes predict scintillator signal-to-noise levels measurable with standard light acquisition systems with the detector aperture at ∼11 cm outside of the diagnostic first wall. At measurement position, heat load on detector head is comparable to that in present devices.