Introduction to the Diagnosis of Magnetically Confined Thermonuclear - PowerPoint PPT Presentation

Plasma radiation Bolometry X-Ray 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 arturo.alonso@ciemat.es version 0.1 (March 7, 2011) Core diagnostics II: Bolometry and Soft X-rays, A. Alonso, copyleft 2010 1 / 22

Plasma radiation Bolometry X-Ray Outline Plasma radiation 1 Continuum spectrum Bolometry: total radiated power 2 The metal-resistor bolometer Sample experimental bolometric data Soft X-ray diagnostic: core MHD phenomena 3 X-Ray Imaging Systems Sample experimental SXR data Core diagnostics II: Bolometry and Soft X-rays, A. Alonso, copyleft 2010 2 / 22

Plasma radiation Bolometry X-Ray Outline Plasma radiation 1 Continuum spectrum Bolometry: total radiated power 2 The metal-resistor bolometer Sample experimental bolometric data Soft X-ray diagnostic: core MHD phenomena 3 X-Ray Imaging Systems Sample experimental SXR data Core diagnostics II: Bolometry and Soft X-rays, A. Alonso, copyleft 2010 3 / 22

Plasma radiation Bolometry X-Ray 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 2010 4 / 22

Plasma radiation Bolometry X-Ray Plasma radiation mechanisms e − state Radiation type 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 2010 5 / 22

Plasma radiation Bolometry X-Ray Spectral regions Spectral Region Wavelength / Energy Region Near Infrared 700-1200 nm / 1-2 eV Visible 400-700 nm / 2-3 eV Ultraviolet (UV) 200-400 nm / 3-6 eV Vacuum Ultraviolet (VUV) 30-200 nm / 6-40 eV Extreme Ultraviolet (EUV) 10-30 nm / 40-120 eV Soft X-Ray 0.1-10 nm / 120-12000 eV Only relatively high- Z impurities have lines above the keV ( E 0 ≈ 13 . 6 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 2010 6 / 22

Plasma radiation Bolometry X-Ray Bremsstrahlung spectra The Bremmstrahlung radiation (W/m 3 eV) emitted by a Maxwellian electron distribution colliding with an i -species ion population is [3] e − E / T e dP Brem i = Cn e n i Z 2 i g ff √ T e . dE Summed over the ion species, the total power density is dP Brem e − E / T e Z eff = 1 � = Cn 2 n i Z 2 e Z eff g ff with √ T e , i dE n e i Core diagnostics II: Bolometry and Soft X-rays, A. Alonso, copyleft 2010 7 / 22

Plasma radiation Bolometry X-Ray Recombination radiation Recombination radiation power density can be written in compact form as dP Rec = ( γ ( T e , Z i ) − 1 ) dP Brem i i . dE 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 / T e dependence on the photon’s energy. Total contiuumm radiation (Brem. + Rec.) is then dP Continuum = γ ( T e , Z i ) dP Brem i i , dE dE and γ is consequently termed the enhancement factor. Core diagnostics II: Bolometry and Soft X-rays, A. Alonso, copyleft 2010 8 / 22

Plasma radiation Bolometry X-Ray 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 2010 9 / 22

Plasma radiation Bolometry X-Ray Relative importance of radiation processes Core diagnostics II: Bolometry and Soft X-rays, A. Alonso, copyleft 2010 10 / 22

Plasma radiation Bolometry X-Ray Outline Plasma radiation 1 Continuum spectrum Bolometry: total radiated power 2 The metal-resistor bolometer Sample experimental bolometric data Soft X-ray diagnostic: core MHD phenomena 3 X-Ray Imaging Systems Sample experimental SXR data Core diagnostics II: Bolometry and Soft X-rays, A. Alonso, copyleft 2010 11 / 22

Plasma radiation Bolometry X-Ray 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 2010 12 / 22

Plasma radiation Bolometry X-Ray Viewing geometries chord detector aperture camera Core diagnostics II: Bolometry and Soft X-rays, A. Alonso, copyleft 2010 13 / 22

Plasma radiation Bolometry X-Ray 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 + ∆ T � P rad = C . 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 2010 14 / 22

Plasma radiation Bolometry X-Ray 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 R meas − R ref R meas − R ref ∆ V = V B ≈ V B R meas + R ref 2 R 0 where R = R 0 ( 1 + α ∆ T ) . Then ∆ T = k ∆ V . Core diagnostics II: Bolometry and Soft X-rays, A. Alonso, copyleft 2010 15 / 22

Plasma radiation Bolometry X-Ray Sample Bolometry data: Density limit disruption in JET G. Arnoux et al., Nucl. Fusion 49 (2009) 085038 Core diagnostics II: Bolometry and Soft X-rays, A. Alonso, copyleft 2010 16 / 22

Plasma radiation Bolometry X-Ray Outline Plasma radiation 1 Continuum spectrum Bolometry: total radiated power 2 The metal-resistor bolometer Sample experimental bolometric data Soft X-ray diagnostic: core MHD phenomena 3 X-Ray Imaging Systems Sample experimental SXR data Core diagnostics II: Bolometry and Soft X-rays, A. Alonso, copyleft 2010 17 / 22

Plasma radiation Bolometry X-Ray 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 2010 18 / 22

Plasma radiation Bolometry X-Ray AXUV photodiodes and fitlers Strong E -dependence of foil transmitance High QE for SXR photons (built-in High-Pass filter) T ∼ e − µ ( E ) d , µ ( E ) ≈ E − 3 Insensitivity to Neutrals and ECRH + Absortion edges Core diagnostics II: Bolometry and Soft X-rays, A. Alonso, copyleft 2010 19 / 22

Plasma radiation Bolometry X-Ray 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 Filter 1 different filters (select elements and thicknesses for appropriate absortion Filter 2 edges ) Core diagnostics II: Bolometry and Soft X-rays, A. Alonso, copyleft 2010 20 / 22