Introduction to the Diagnosis of Magnetically Confined Thermonuclear Plasma

Transcription

1 Introduction to the Diagnosis of Magnetically Confined Thermonuclear Plasma Core diagnostics II: Bolometry and Soft X-rays J. Arturo Alonso Laboratorio Nacional de Fusión EURATOM-CIEMAT E6 P2.10 version 0.1 (March 7, 2011) Core diagnostics II: Bolometry and Soft X-rays, A. Alonso, copyleft / 22

2 Outline 1 Plasma radiation Continuum spectrum 2 Bolometry: total radiated power The metal-resistor bolometer Sample experimental bolometric data 3 Soft X-ray diagnostic: core MHD phenomena X-Ray Imaging Systems Sample experimental SXR data Core diagnostics II: Bolometry and Soft X-rays, A. Alonso, copyleft / 22

3 Outline 1 Plasma radiation Continuum spectrum 2 Bolometry: total radiated power The metal-resistor bolometer Sample experimental bolometric data 3 Soft X-ray diagnostic: core MHD phenomena X-Ray Imaging Systems Sample experimental SXR data Core diagnostics II: Bolometry and Soft X-rays, A. Alonso, copyleft / 22

4 A loss of power and a source of information There are at least two reasons why one could be interested in the radiation emitted by the plasma: 1 It is a power loss mechanism. Remember the power balance S α + S h = S rad + S κ. (thought to take part in the physics of the density limit [2]). 2 It carries information about the plama it was emitted from. Different processes emit electromagnetic radiation in a plasma: Cyclotron acceleration by the Lorentz force in the B-field Bremsstrahlung collisions between unlike particles Recombination collisions with e capture Atomic transitions of excited e in the atom s quantum levels Core diagnostics II: Bolometry and Soft X-rays, A. Alonso, copyleft / 22

5 Plasma radiation mechanisms Radiation type e state Physical mechanism Cyclotron free-free acceleration by the Lorentz force Bremsstrahlung free-free e ion inelastic collisions Recombination free-bound e ion collision and e capture Line bound-bound e transition between atom s quantum levels Cyclotron radiation is absorbed and reemitted (plasma is optically thick in that wavelength ECE diagnostic) Brems + Recombination produce the continuum spectrum Line radiation produces the ions characterisctic spectra Total spectrum is a rather complicated mixture of lines and continuum Core diagnostics II: Bolometry and Soft X-rays, A. Alonso, copyleft / 22

6 Spectral regions Spectral Region Near Infrared Visible Ultraviolet (UV) Vacuum Ultraviolet (VUV) Extreme Ultraviolet (EUV) Soft X-Ray Wavelength / Energy Region nm / 1-2 ev nm / 2-3 ev nm / 3-6 ev nm / 6-40 ev nm / ev nm / ev Only relatively high-z impurities have lines above the kev (E Z 2 ev). Different detectors used for different spectral regions (i.e. semiconductors for IR and visible cameras or AXUV photodiodes, dispersive elements in spectrometers) Bolometers are sensitive to photon energies from 1eV to 10 kev (used as power loss monitors) Core diagnostics II: Bolometry and Soft X-rays, A. Alonso, copyleft / 22

7 Bremsstrahlung spectra The Bremmstrahlung radiation (W/m 3 ev) emitted by a Maxwellian electron distribution colliding with an i-species ion population is [3] dp Brem i de = Cn e n i Z 2 i g ff e E/Te Te. Summed over the ion species, the total power density is dp Brem = Cn 2 e E/Te de ez eff g ff, with Z eff = 1 n i Zi 2 Te n e i Core diagnostics II: Bolometry and Soft X-rays, A. Alonso, copyleft / 22

8 Recombination radiation Recombination radiation power density can be written in compact form as dp Rec i de = (γ(t e, Z i ) 1) dpbrem i de The pre-factor is a complicated sum over the possible final electron quamtum level [1]. It includes low energy cutoffs corresponding to the different bound energies of these final states For a fixed T e the recombination radiation preserves the e E/Te dependence on the photon s energy. Total contiuumm radiation (Brem. + Rec.) is then dp Continuum i = γ(t e, Z i ) dpbrem i, de de and γ is consequently termed the enhancement factor. Core diagnostics II: Bolometry and Soft X-rays, A. Alonso, copyleft / 22.

9 Relative importance of radiation processes Total spectrum is complicated by emission lines on top of a continuum. Impurty types and concentrations vary from machine to machine. Not much can be said in general. Nonetheless, Line radiation can be strong in the relativelly cold (T e 1 10 ev), neutral and impurity rich plasma in the Edge/SOL and divertor region. For T e of the order of the quamtum potentials of the impurites, recombination dominates the continuum (γ 2 100) In fusion reactor conditions (T e 10 kev, Z eff 1) total radiated power is mainly due to Bremsstrahlung radiation. Core diagnostics II: Bolometry and Soft X-rays, A. Alonso, copyleft / 22

10 Relative importance of radiation processes Core diagnostics II: Bolometry and Soft X-rays, A. Alonso, copyleft / 22

11 Outline 1 Plasma radiation Continuum spectrum 2 Bolometry: total radiated power The metal-resistor bolometer Sample experimental bolometric data 3 Soft X-ray diagnostic: core MHD phenomena X-Ray Imaging Systems Sample experimental SXR data Core diagnostics II: Bolometry and Soft X-rays, A. Alonso, copyleft / 22

12 To monitor radiated power Plasma emits most of its energy in a spectral range from 1 ev to 10 kev Bolometric detectors need to have high efficiency across this range The general scheme is an absorber and a thermometer absorber heats up due to the absorbed radiation thermometer measures changes in the absorber s temperature form which the instantaneous radiated power can be derived thermometer absorber weak thermal link Heat sink Core diagnostics II: Bolometry and Soft X-rays, A. Alonso, copyleft / 22

13 Viewing geometries chord detector aperture camera Core diagnostics II: Bolometry and Soft X-rays, A. Alonso, copyleft / 22

14 The metal-resistor bolometer Thin layers of metals have the required efficiency, and their resistance depends linearly on their temperature The radiated power can be computed directly form changes in the absorber temperature ( d T P rad = C + T ). dt τ C is the absorber heat capacity and τ a time constant characteristic of the thermal coupling to a heat sink. Core diagnostics II: Bolometry and Soft X-rays, A. Alonso, copyleft / 22

15 The metal-resistor bolometer Bolometers are mounted in sets of 4 in a Wheatstone bridge configuration, 2 exposed (measuring), 2 shielded (reference). The voltage drop across the bridge is V = V B R meas R ref R meas + R ref V B R meas R ref 2R 0 where R = R 0 (1 + α T). Then T = k V. Core diagnostics II: Bolometry and Soft X-rays, A. Alonso, copyleft / 22

16 Sample Bolometry data: Density limit disruption in JET G. Arnoux et al., Nucl. Fusion 49 (2009) Core diagnostics II: Bolometry and Soft X-rays, A. Alonso, copyleft / 22

17 Outline 1 Plasma radiation Continuum spectrum 2 Bolometry: total radiated power The metal-resistor bolometer Sample experimental bolometric data 3 Soft X-ray diagnostic: core MHD phenomena X-Ray Imaging Systems Sample experimental SXR data Core diagnostics II: Bolometry and Soft X-rays, A. Alonso, copyleft / 22

18 SXR diagnostics For kev plasmas continuum radiation is particularly sensitive to T e variations in the SXR part or the spectrum. SXR diagnotics: X-ray Crystal spectroscopy - High spectral resolution - information from emission lines of medium-z impurities Pulse Height Analyser - Medim spectral resolution - Photon counting detector to estimate T e AXUV cameras - Coarse spectral resolution (with filters) - arrays of diodes for tomographic inversion Core diagnostics II: Bolometry and Soft X-rays, A. Alonso, copyleft / 22

19 AXUV photodiodes and fitlers High QE for SXR photons (built-in High-Pass filter) Insensitivity to Neutrals and ECRH Strong E-dependence of foil transmitance T e µ(e)d, µ(e) E 3 + Absortion edges Core diagnostics II: Bolometry and Soft X-rays, A. Alonso, copyleft / 22

20 Filter arrangements for imputiry or T e monitoring T e : High-Pass filter from mass abosortion exponential transmission profile log T(E) E 3 Impurity lines: Band pass filter from the signal difference of two detectors with different filters (select elements and thicknesses for appropriate absortion edges) Filter 1 Filter 2 Core diagnostics II: Bolometry and Soft X-rays, A. Alonso, copyleft / 22

21 Sample SXR data: Shafranov shift in W7-AS When SXR signal is filtered for T e sensitivity, the emissivity is expected to be constant in flux surfaces. Anton, Plasma Phys. Contol. Fus. 38 (1996) 1849 Core diagnostics II: Bolometry and Soft X-rays, A. Alonso, copyleft / 22

22 Sample SXR data: tomographic reconstruction of an Alfven mode in W7-AS A. Weller, et al., Rev. Sci. Instrum. 70, 484 (1999) SVD and Tomography will be treated in another lecture. Core diagnostics II: Bolometry and Soft X-rays, A. Alonso, copyleft / 22

23 S. Von Goeler, W. Stodiek, H. Eubank, H. Fishman, S. Grebenshchikov, and E. Hinnov. Thermal x-ray spectra and impurities in the st tokamak. Nuclear Fusion, 15(2):301, Martin Greenwald. Density limits in toroidal plasmas. Plasma Physics and Controlled Fusion, 44(8):R27, BC Stratton, M Bitter, KW Hill, DL Hillis, and JT Hogan. Passive Spectroscopic Diagnostics for Magnetically-confined Fusion Plasmas. Technical report, Princeton Plasma Physics Laboratory (PPPL), Princeton, NJ (United States), Core diagnostics II: Bolometry and Soft X-rays, A. Alonso, copyleft / 22