Design of a tomographic hard X-ray spectrometer for suprathermal electron studies with ECRH

1. Camera #1 Tomography and poloidal mode number (m) TCV main parameters[4]: R =0.88m; a=0.23m; B0tor=1.45T; Ip<1MA; k≤3; -0.7≤δ ≤0.7 m=0 axial symmetry (One camera) m=1 axial asymmetry (Two cameras) m=2 doublet structure (Three cameras) Local RF Power density achievable in TCV > 20 MW/m^3 Detector Line of sight X2 heating: 6 steerable launchers Power: 0.5 MW each Frequency: 82.7 GHz Density limits: 4X1019 m-3 Pulse length: 2s X3 heating: 1 upper steerable launcher Power:1.5 MW (3 gyrotrons) Frequency:118 GHz Density limits:1.1X1020 m-3 Pulse length: 2s Upper spread border line Lower spread border line Collimator metal foil RF field-particle resonance interaction (ECRH; ECCD) C1 C2 C3 Suprathermal electron population is generated C4 Bremsstrahlung emission (hard X-rays) due to electron-ion collisions. ECE emission due to the Larmor motion predominantly from suprathermals on HFS C5 X2 generates suprathermal electrons and contributes to enhance the X3 power absorption [5] C8 C6 Simulated Emissivity M=0 +m =2 + Gauss peak Cameras 1,4,5 Cameras 1,2,34,,5 Detector (Ad) dd,a C9 C7 dΩ θd,p Collimator Slit aperture (Aap) θa,p Δsx Δstoroidal L(ρ,φ) z y Detector Integrated Measurements Inversion Methods dL Local emissivity R Li Plasma 4) Chord Signal integration 3) Pixel Grid and T matrix generation Simulated Emissivity m =1 Simulated Emissivity m = 0 Cameras 2,5 Cameras 1,2,3,4,,5 Cameras 2,5 Cameras 1,2,3,4,5 Cameras 1,4,5 Cameras 1,4,5 Simulated Emissivity m =2 Cameras 1,4,5 Cameras 1,2,3,4,5 Simulated Emissivity (C) Cameras 1,3,5 Cameras 1,2,4,5 Chord brightness [a.u.] International Workshop on Burning Plasma Diagnostics Varenna, Italy, September 24-28 2007 Design of a tomographic hard X-ray spectrometer for suprathermal electron studies with ECRH Introduction and motivations The ECCD and ECRH system in TCV Electron cyclotron resonance heating (ECRH) and current drive (ECCD)[1], disruptive instability events and sawtooth activity have been demonstrated to produce suprathermal electrons in fusion devices[2]. The importance of these phenomena for fusion reactors renders suprathermal electron generation and dynamics a key topic in the physics of burning plasmas[3]. Here some significant results from the TCV tokamak[4] are briefly reviewed and a preliminary design of a novel tomographic hard X-ray spectrometer proposed for TCV is discussed. The design is aided by simulations of tomographic reconstruction. Overview of experimental results Proposed hard X-ray spectrometric system Spectrometer: main parameters Detectors[10]:CdTe (or CZT) Dimensions: 1mm X 1mm area X 2mm thickness Detector per camera: ~ 35-40 Number of cameras: from 2 to 9 (trade-off between benefits for tomographic reconstruction and costs) Spatial resolution: 2 cm Time resolution: down to 1 ms Energy resolution: ~ 5keV at 60keV of photon energy Energy range: 20-200 keV Compact and flexible camera design, wide coverage for plasmas at different vertical locations Suprathermal density propagation in space after short ECCD pulses measured by HFS ECE [7] Collimator design: Divergent Soller collimator[11] with radial detector array Tungsten metal foil enables good photon stopping power with compact size. The design of the collimator aperture can be adjusted for each single detector in order to control the étendue and its relative line spread in the plasma region. The spatial resolution of the system is related to the typical chord separation in the plasma region that is ~2cm. The extreme compactness and flexibility of this design enable it to be adapted to other fusion devices. Fast electron broadening from transport observed in many ECRH discharges (resulting in ECCD profile broadening) [6] The HXR camera on loan from TORE SUPRA [8] Now definitively reclaimed Clearly evidenced the LFS-HFS asymmetry of the poloidal bremsstrahlung distribution (possibly related to trapped particles) • Other diagnostics used: • high-field-side electron cyclotron emission (ECE) radiometer [9] • oblique ECE • diamagnetic loop coil • New diagnostics being installed: • Tangential HXR camera • Vertical ECE Tomographic reconstruction and diagnostic validation 5) Tomographic inversion and reconstruction 2)Thin chord approximation • 3D Geometric integration The tomographic algorithm that generates the reconstructed plasma emissivity uses the pixel grid method and the minimum Fisher information in order to condition the solution [12] The chord brightness: Tomographic reconstruction enables to visualize and determine quantitatively the detailed evolution of the 2D shape of the emitting structure in the plasma as a result of diffusion phenomena, instabilities or other perturbations as well as of ECRH and ECCD deposition. The power emitted by the plasma along the chord L(ρ,φ) is: is independent of the particulars of detector area, aperture size, etc. and is therefore a convenient quantity to express chord-integrated data in. Several tomographic reconstructions have been performed in order to validate the camera performance in recovering the shape and the intensity of simulated 2D plasma emission patterns. The simulations indicate as expected an increasing in the quality of the tomographic reconstruction when cameras viewing the plasma from different directions are added. Conclusions • Suprathermal electron dynamics is a crucial element of ECRH physics and MHD phenomena • important for fusion • A high-resolution, spectroscopic, tomographic hard X-ray camera system is being designedfor the TCV tokamak • Simulations of tomographic reconstruction from varying postulated emissivity patterns underway to determine the best detector distribution and to optimize the reconstruction algorithm References: [1].N. J. Fisch, Rev. Mod. Phys. 59, 175 (1987). [2].P. V. Savrukin et al, Plasma Phys. Control. Fusion 48, B201 (2006). [3].R. J. La Haye et al, Nucl. Fusion 46, 451 (2006). [4].F. Hofmann et al, Plasma Phys. Conrol. Fusion 36, B277 (1994). [5].S. Coda et al, 28th EPS conference on Control. Fusion and Plasma Phys. Funchal, 18-22 June 2001. ECA Vol.25A, 301 (2001). [6].S. Coda et al, Nucl. Fusion 43, 1361 (2003). [7].S. Coda et al, Plasma Phys. Conrol. Fusion 48, B359 (2006). [8].Y. Peysson, S. Coda, F. Imbeaux, Nucl. Inst. Meth. A 458, 269 (2001). [9]. I. Klimanov et al, Plasma Phys. Control. Fusion 49, L1 (2007). [10].T. Takahashi and S. Watanabe, IEEE Trans. Nucl. Sci. 48, 950 (2001). [11].W. Soller, Phys. Rev. 24, 158 (1924). [12].M. Anton et al, Plasma Phys. Control. Fusion. 38, 1849 (1996).