RF Discharge Mirror Cleaning System Development for ITER Diagnostics

This report summarizes the status of several R&D tasks devoted to characterization of the basic behavior and definition of some features of the RF discharge mirror cleaning systems for ITER spectroscopy diagnostics. First results of mirror cleaning system engineering development and its implementation on ITER are described. Key requirements and specifications for such mirror cleaning systems for ITER conditions are presented

Electron Bernstein wave heating of over-dense H-mode plasmas in the TCV tokamak via O-X-B double mode conversion

This paper reports on the first demonstration of electron Bernsteinwave heating (EBWH) by double mode conversion from ordinary (O-) to Bernstein (B-) via the extraordinary (X-) mode in an over-dense tokamak plasma, using low field side launch, achieved in the TCV tokamak H-mode, making use of its naturally generated steep density gradient. This technique offers the possibility of overcoming the upper density limit of conventional EC microwave heating. The sensitive dependence of the O-X mode conversion on the microwave launching direction has been verified experimentally. Localized power deposition, consistent with theoretical predictions, has been observed at densities well above the conventional cut-off. Central heating has been achieved, at powers up to two megawatts. This demonstrates the potential of EBW in tokamak H-modes, the intended mode of operation for a reactor such as ITER

Plasma Turbulence studied by means of Correlation-ECE in the TEM domain in TCV

Plasma Turbulence studied by means of Correlation-ECE in the TEM domain in TCV Electron temperature fluctuations in the TEM domain have been measured in TCV using the correlation-ECE diagnostics [1]. Significant broadband electron temperature fluctuations are found radially extending between 0.3<ρ<0.7 on the equatorial LFS. Their amplitude decreases with collisionality (with increasing density in OH discharges), in qualitative agreement with predictions from local, linear gyrokinetic GS2 calculations. Thus the mixing length heat diffusivity calculated from GS2 decreases with collisionality, as does the measured heat diffusivity from power balance [2]. This diagnostics allows now the study at the microscopic, turbulence level, of the previously found heat transport triangularity scaling [3], linked to changes of the resonance of TE with the TEM [4]. The decrease of transport when going from positive to negative triangularity found in TCV L-mode can now be investigated and compared to gyrokinetic predictions of instabilities or turbulence (linear/non-linear, local/global). TEM features, like the orientation of the potential cells, predicted to change with plasma shape, up/down-asymmetries, can now be measured through the correlation lengths along a horizontal line of sight, or obliquely, using a mobile mirror arrangement (ECRH launcher in reception mode). [1] V.S. Udintsev et al., Fusion Science and Technology 52 (2007) 161. [2] V.S. Udintsev, E. Fable et al., in preparation. [3] Y. Camenen et al. Nucl., Fusion 47 (2007) 510. [4] A. Marinoni et al., Plasma Phys. Control. Fusion 51 (2009) 055016. 1 present address ITER-IO, Cadarache, St Paul-lez-Durance, F 2 present address CFSA, Dept of Physics, University of Warwick, UK This work was supported in part by the Swiss National Science Foundation

Design and integration of lower ports for ITER diagnostic systems

All around the ITER vacuum vessel, forty-four ports will provide access to the vacuum vessel for remotehandling operations, diagnostic systems, heating, and vacuum systems: 18 upper ports, 17 equatorialports, and 9 lower ports. Among the lower ports, three of them will be used for the remote handlinginstallation of the ITER divertor. Once the divertor is in place, these ports will host various diagnosticsystems mounted in the so-called diagnostic racks. The diagnostic racks must allow the support andcooling of the diagnostics, extraction of the required diagnostic signals, and providing access and main-tainability while minimizing the leakage of radiation toward the back of the port where the humans areallowed to enter. A fully integrated inner rack, carrying the near plasma diagnostic components, will bean stainless steel structure, 4.2 m long, with a maximum weight of 10 t. This structure brings water forcooling and baking at maximum temperature of 240?C and provides connection with gas, vacuum andelectric services. Additional racks (placed away from plasma and not requiring cooling) may be requiredfor the support of some particular diagnostic components. The diagnostics racks and its associated exvessel structures, which are in its conceptual design phase, are being designed to survive the lifetimeof ITER of 20 years. This paper presents the current state of development including interfaces, diagnos-tic integration, operation and maintenance, shielding requirements, remote handling, loads cases anddiscussion of the main challenges coming from the severe environment and engineering requirements

Transport and turbulence reduction with negative triangularity : Correlation ECE measurements in TCV

Turbulence and Transport Reduction with Negative Triangularity : Correlation ECE Measurements in TCV Due to turbulence, core energy transport in fusion devices such as tokamaks generally exceeds collisional transport by at least an order of magnitude. It is therefore crucial to understand the instabilities driving the turbulent state and to find ways to control them. Plasma shape is one of these fundamental tools. In low collisionality plasmas, such as in a reactor, changing the plasma shape from Dee-shape to inverse Dee-shape (from positive to negative triangularity δ) reduces the energy transport by a factor two: the heat flux necessary to sustain the same profiles and stored energy in a discharge with δ=-0.4 is only half of that at δ=+0.4. This is significant, since it opens the possibility of having Hmode-like confinement time within an L-mode edge; or at least with smaller ELMs. Recent correlation ECE measurements show that this reduction of transport at negative δ is reflected in a reduction by a factor of two of both 1) the amplitude of temperature fluctuations in the broadband frequency range 30-150 kHz, and 2) the fluctuation correlation length, measured at mid-radius (ρv~0.6). In addition, the fluctuations amplitude is reduced with increasing collisionality, consistent with theoretical estimates of the collisionality effect on Trapped Electron Modes (TEM). The correlation ECE results are compared to gyrokinetic code results: 1) global linear gyrokinetic simulations (LORB) have predicted shorter radial TEM wavelength λ⊥ for negative triangularity plasmas, consistent with the shorter radial turbulence correlation length λc observed. 2) At least close to the strongly shaped plasma boundary, local nonlinear gyrokinetic simulations with the GS2 code predict that the TEM induced transport decreases with decreasing triangularity and increasing collisionality, in fair agreement with the experimental observations. 3) Calculations are now extended to global nonlinear simulations (ORB5). This work was supported in part by the Swiss National Science Foundatio

Electron temperature dynamics of TEXTOR plasmas

To study plasma properties in the presence of large and small MHD modes, new high-resolution ECE diagnostics have been installed at TEXTOR tokamak, and some of the already existing systems have been upgraded. Two models for the plasma transport properties inside large m/n = 2/1 MHD islands have been found to give estimations for the heat diffusivities, which are much lower than the global plasma heat diffusivity, which is in agreement with previous measurements in different tokamaks. The 3D-reconstruction of large m/n = 2/1 modes in TEXTOR with the help of all available ECE diagnostics allows modelling the island as a structure with closed flux surfaces. The main plasma heat flux flows through the X-point area probably along stochastic magnetic field lines. The confinement is improved within the magnetic island, compared to the background plasma. This is confirmed by a temperature profile flattening and sometimes even a secondary peaking inside the island, compared to the X-point. Making use of the mode rotation, assumed to be a rigid rotor, it has been possible to obtain information on the topology of the m = 1 precursor mode leading to sawtooth collapses. It becomes clear that this precursor cannot be described by an m = 1 cold tearing mode island but by a hot crescent wrapped around a cold high-density bubble. In the future multi-chord ECE-imaging will allow this mode reconstruction without the assumption of the rotation to be rigid. From the measurements of the broadband temperature and density fluctuations one can conclude that the turbulent structures inside the q = 1 surface are separated from the turbulence outside the q = 1 surface. This fits nicely with the observation that q = 1 surface acts as a barrier for the thermal transport. Correlation length and time measured inside q = 1 are in agreement with the observed turbulent heat diffusivity. Qualitative studies of non-thermal electrons at different heating regimes (ECRH and Ohmic) at TEXTOR were done with the help of the combined 2nd -3rd harmonic X-mode ECE radiometer. It has been found that the lower energetic non-thermal electrons are directly responsive to small density changes, in contrast to the highly energetic runaways with energy up to 20 MeV. Those are only affected by a substantial density ramp up

Neutronics analysis for integration of ITER diagnostics port EP10

Shutdown dose rate calculations have been performed on an integrated ITER C-lite neutronics model with equatorial port 10. A ‘fully shielded’ configuration, optimised for a given set of diagnostic designs (i.e. shielding in all available space within the port plug drawers), results in a shutdown dose rate in the port interspace, from the activation of materials comprising equatorial port 10, in excess of 2000 μSv/h. Achieving dose rates of 100 μSv/h or less, as required in areas where hands-on maintenance can be performed, in the port interspace region will be challenging. A combination of methods will need to be implemented, such as reducing mass and/or the use of reduced activation steel in the port interspace, optimisation of the diagnostic designs and shielding of the port interspace floor. Further analysis is required to test these options and the ongoing design optimisation of the EP10 diagnostic systems

Microwave response of ITER vacuum windows

\u3cp\u3eDiagnostic systems are essential for the development of ITER discharges and to reach the ITER goals. Many of these diagnostics require a line of sight to relay signals from the plasma to the diagnostic, typically located outside the torus hall. Such diagnostics then require vacuum windows that isolate the torus vacuum and, crucially, ensure containment of hazardous substances. While such windows are routine in many fusion experiments, ITER poses new challenges. The vacuum windows are safety important components class 1 that must withstand all ITER loads. As a consequence, in many cases double disk windows are used with modified frequency response as compared to single disk windows. ITER is a long pulse machine with 20 MW microwave heating installed, giving rise to gradual heating of windows due to stray radiation. The particular microwave heating scheme at ITER may also – in case of an erroneous polarization setting – result in a refracted beam with much higher power density. This paper looks at microwave aspects of ITER windows. The microwave response as a function of frequency is calculated for proposed arrangements. From this response the impact on diagnostic performance may be assessed as well as the thermal load on the window itself.\u3c/p\u3