2 Basic conceptsThe physical quantities are measured with instruments. The instrument should measure always the same value if they were perfectly accurate. In reality the instruments are not perfectly accurate, so the measure differs from the real value of the physical quantity .Measurement is the activity of comparing a number with a predefined pattern, involving the existence of measurement units. These units are essentially arbitrary; i.e. create and agree to use them.The basic units are the simple measurements of time, length, mass, temperature , amount of substance, electric current and light intensity. The derived units are comprised of basic units, e.g., velocity (m/s) or density (kg/m3) .By measuring is possible to express numerically qualities (quantify ) avoiding concepts like " big / small " , " strong / weak "
3 DiagnosticsPlasma diagnostics are methods, techniques whose purpose is to deduce information about the plasma from practical observations of physical processes and their effectsIn general we do not have access to the physical quantity and we need to use models, theories, simulations to interpret the results.Main quantities of interest:Magnetic (Current, Flux loop, B-fields, magnetic configuration)Kinetic (Electron and ion temperature and density, pressure)Plasma composition (impurities, wall interaction)
4 What to measure Density of particles Temperature Potential, electric field, velocities, …Energy: joule (J) but often we use 1 eV = 1.6 J (energy gain by an electron in a potential difference of 1 volt)Temperature: kelvin (K) but often we use the equivalent in eV/k (Boltzmann constant)1 eV/k = 1.6 J / 1.38 J/K = K1 eV K
5 Diagnostic characteristics Ideal diagnostics should provide measurement of plasma quantitiesDirect and independentWith good spatial and time resolution.With no perturbations (by the plasma and to the plasma)Real plasma diagnostics areOften indirect (need interpretation models as there is no direct access the physical quantity). The understanding of the associated physics process is required to interpret the resultsOften mutually dependent (need other plasma parameters)Spatial and time resolution dependent of the measurement techniquePlasma perturbation and environment noise is an issue.
6 Complementarity of diagnostics Different techniques:Except for a few quantities each plasma parameter in general can be measured by more than one technique, often with different spatial and time resolution or with the use of different interpretation models.Compatibility of different measurements:Different diagnostics may give different values for the same parameter. Compatibility is related to the validity of the interpretation models and to the correct determination of measurement errors.Complementarity:The diagnostics operating in a plasma experimental must be seen as set of complementary techniques that operate all together to provide a reliable picture of the plasma.
7 Diagnostic characteristics Local measurements (electrical probes): can only be used in cold plasma. Remote measurements are required for hot plasmas.Some plasma parameters are difficult to measure (plasma characterization limited)There is a large variety of plasma diagnostics (hot and cold). The choice of the appropriated diagnostic toll depends on the plasma condition and budget.Required temporal and spatial resolution depends on the plasma parameters (ex. gradients)
9 Complexity of diagnostics Noisy environment poses strict requirements: electric and magnetic shielding. Careful signal grounding. Optical insulation in signal transmission sometimes necessary Accessibility: Limited accessibility to diagnostic equipment in large fusion machines Reliability: Long term survival of plasma facing components, damage by irradiation High degree of automatization of control/monitoring of diagnostic equipment and of data acquisition. Consequence: High complexity and high cost of diagnostic systems.
10 Accuracy vs Precision Real value may not be known DefinitionThe degree of closeness to true value.The degree to which an instrument will repeat the same value.Measurements:SingleMultiple measurements are neededReal value may not be knownDo not mix up lack of precision with plasma fluctuationsHigh accuracyLow precisionHigh accuracyHigh precisionLow accuracyLow precision
12 Scales: spaceIn large fusion experiments the spatial scales vary by 6 orders of magnitudeDebye length (< mm)Electron Larmor radius (< mm)Ion Larmor radius (mm)Turbulence scale (cm)Scale of the magnetic perturbations (cm)Gradients (cm – dimension of the experiment, m)Length along the magnetic field line ( m)
13 Scales: timeIn large fusion experiments the temporal scales vary by 12 orders of magnitudeMagnetic activity (0 – 1 MHz)Particles exchange with the wall (< Hz)Current diffusion (kHz -Hz)Magnetic equilibria, confinement ( kHz -Hz)Turbulence (1-200 kHz)Ion cyclotron frequency (> 10 MHz)Electron cyclotron frequency (10 GHz)
15 Diagnostic classification Plasma perturbationNone: Spectroscopy, Magnetic probesWeak: Micro-waves, Lasers, particle beamsStrong: Electric probes, particle beamsNatureElectromagnetic: Electric and Magnetic probesOptics: Spectroscopy (visible, X-ray, ...), InterferometerParticles: Ion beams
20 Selected low temperature plasma diagnostics Diagnostic MeasuresLangmuir probes Plasma potential, electron temperature & densityMagnetic diagnostics Plasma current, plasma waves, ….Spectroscopic Plasma composition, ion temperature & drift velocity, …….Microwave diagnostics Plasma electron density, density profile, …..Mass / energy analyser Identify species of ions, and measures their charge state and energyLaser diagnostics Density of various species in the plasma
21 Selected ITER diagnostics Diagnostic MeasuresMagnetic diagnostics Plasma current, position, shape, waves ..Spectroscopic & neutral Ion temperature, He & impurityparticle analyser systems density,Neutron diagnostics Fusion power, ion temperature profile, ….Microwave diagnostics Plasma position, shape, electron density, profile, …..Optical/IR(infra-red) systems Electron density (Line-average & profile, electron temperature profile, ….Bolometric diagnostics Total radiated power, ….Plasma-facing components & Temperature of, and particle flux operational diagnostics to First Wall, …..Neutral beam diagnostics Various parameters
24 Electrical probes Conductor inserted into the plasma Simplest diagnosticData interpretation complicated as probes perturb the plasmaLimited to the plasma region were the probes can survive or do not perturb plasmaAllows the determination of a large variety of plasma parameters (some of them only possible with probes)Potential and particle flux depends on plasma parameters
26 Magnetic measurements Essential in magnetic confinement devicesPlasma current, position, geometry, instabilitiesSensor fluxo magnéticoSignal in the sensorSignal has to be integrated (hardware or software)
30 Particle beamsIons: Heavy elements (Xenon): Require large mass elements and low magnetic field. Larmor radius has the dimension of the device:The aim is to collect the ions after crossing the plasma. Information from the plasma parameters at the ionization locationNeutral: Light elements (Lithium): The aim is to measure the ionization radiation. Neutral elements so not limited by B.
31 Heavy ion beamLarmor radius has the dimension of the device: the aim is to collect the ions after crossing the plasma. Information from the plasma parameters at the ionization location
32 Lithium beamsLight elements (Lithium): The aim is to measure the ionization radiation. Neutral elements so not limited by B.
33 Plasma radiationPlasma radiation contains important information about the plasma properties. Plasma emits electromagnetic radiation due to different physics processesComplex spectra (continuum + spectral lines) from IR to X-ray
34 Plasma radiationBremsstrahlung: Due to electron desacelaration in the ion field, used to measured the electron temperatureCyclotronic radiation: Due to rotation in the magnetic fieldce B 1/R (50 – 500 GHz)
35 Plasma radiationSpectral lines: Discrete radiation due to electron transition between energy atomic levelsFrom visible to X-rayBroadening Ti,Doppler shift velocityIntensity = f(ni, n0, Te)Only high-Z elements emit X-rays( keV, E ~ 13.6Z2 eV).Spectra: mix of continuum and linesHot plasmas: Dominated by Bremsstrahlung (10 kev, Z~1)Low temperature plasmas: Spectral lines dominate (1 – 10 eV, Z > 1)
41 Fast visible camerasAdvantages: Large temporal and spatial resolution, plug-and-playDisadvantages: Expansive (100 k €), measurement not local (different average field lines, inversion necessary), difficult to extract plasma parametersExample: Photon ultima APX-RS3,000 fps (1024 x 1024), 250,000 fps (64 x 64)