plasma diagnostics carlos silva. basic concepts the physical quantities are measured with...
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PLASMA DIAGNOSTICS
CARLOS SILVA
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Basic concepts
The 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 "
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Diagnostics
Plasma diagnostics are methods, techniques whose purpose is to deduce information about the plasma from practical observations of physical processes and their effects
In 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)
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What to measure
Density of particlesTemperature Potential, electric field, velocities, …
Energy: joule (J) but often we use 1 eV = 1.6 10-19 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 10-19 J / 1.38 10-23 J/K = 11600 K
1 eV 11600 K
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Ideal diagnostics should provide measurement of plasma quantities Direct and independent With good spatial and time resolution. With no perturbations (by the plasma and to the plasma)
Real plasma diagnostics are Often 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 results
Often mutually dependent (need other plasma parameters) Spatial and time resolution dependent of the measurement
technique Plasma perturbation and environment noise is an issue.
Diagnostic characteristics
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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.
Complementarity of diagnostics
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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)
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Temporal / spatial resolution
N
ne
N
nBE tnntntN
00r )()()(v
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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 machinesReliability: Long term survival of plasma facing components, damage by irradiationHigh degree of automatization of control/monitoring of diagnostic equipment and of data acquisition.
Consequence: High complexity and high cost of diagnostic systems.
Complexity of diagnostics
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Accuracy vs Precision
Accuracy Precision
Definition
The degree of
closeness to true value.
The degree to which an
instrument will repeat the same
value.
Measurements:
Single Multiple measurements are
needed
High accuracyLow precision
High accuracyHigh precision
Low accuracyLow precision
Real value may not be known
Do not mix up lack of precision with plasma fluctuations
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Scales: space
In large fusion experiments the spatial scales vary by 6 orders of magnitude Debye 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 (10-100 m)
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Scales: time
In large fusion experiments the temporal scales vary by 12 orders of magnitude Magnetic 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)
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Diagnostic classification
Plasma perturbation None: Spectroscopy, Magnetic probes Weak: Micro-waves, Lasers, particle beams Strong: Electric probes, particle beams
Nature Electromagnetic: Electric and Magnetic probes Optics: Spectroscopy (visible, X-ray, ...), Interferometer Particles: Ion beams
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Plasma Diagnostic Systems
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Selected low temperature plasma diagnostics
Diagnostic MeasuresLangmuir probes Plasma potential, electron temperature
& density
Magnetic 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 energy
Laser diagnostics Density of various species in the plasma
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Selected ITER diagnosticsDiagnostic MeasuresMagnetic diagnostics Plasma current, position, shape, waves ..
Spectroscopic & neutral Ion temperature, He & impurity particle 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
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DIAGNOSTICS OVERVIEW
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Lectures on: Electrical probes (Carlos Silva) Magnetic probes (Bernardo Carvalho) Particle beams (Artur Malaquias) Spectroscopy (Elena Tatarova) Reflectometry (M.E. Manso, MT5)
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Electrical probes
Conductor inserted into the plasma
Simplest diagnostic Data interpretation complicated as probes perturb the plasma Limited to the plasma region were the probes can survive or do
not perturb plasma Allows the determination of a large variety of plasma
parameters (some of them only possible with probes) Potential and particle flux depends on plasma parameters
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Langmuir probes I – V characteristic
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Magnetic measurements
Essential in magnetic confinement devices Plasma current, position, geometry, instabilities
Signal in the sensor
Sensor fluxo magnético
Signal has to be integrated (hardware or software)
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Magnetic measurements
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Magnetic probes on ISTTOK
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Magnetic probes
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Particle beams
Ions: 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 location
Neutral: Light elements (Lithium): The aim is to measure the ionization radiation. Neutral elements so not limited by B.
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Heavy ion beam
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 location
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Lithium beams
Light elements (Lithium): The aim is to measure the ionization radiation. Neutral elements so not limited by B.
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Plasma radiation
Plasma radiation contains important information about the plasma properties. Plasma emits electromagnetic radiation due to different physics processes
Complex spectra (continuum + spectral lines) from IR to X-ray
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Plasma radiation
Bremsstrahlung: Due to electron desacelaration in the ion field, used to measured the electron temperature
Cyclotronic radiation: Due to rotation in the magnetic field
ce B 1/R (50 – 500 GHz)
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Plasma radiation
Spectral lines: Discrete radiation due to electron transition between energy atomic levels
From visible to X-ray Broadening Ti,
Doppler shift velocity Intensity = f(ni, n0, Te)
Only high-Z elements emit X-rays
( keV, E ~ 13.6Z2 eV).
Spectra: mix of continuum and lines Hot plasmas: Dominated by Bremsstrahlung (10 kev, Z~1)Low temperature plasmas: Spectral lines dominate (1 – 10 eV, Z > 1)
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Spectral lines
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Bolometer
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Infra-red cameras
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Infra-red cameras
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Fast visible cameras
Advantages: Large temporal and spatial resolution, plug-and-play
Disadvantages: Expansive (100 k €), measurement not local (different average field lines, inversion necessary), difficult to extract plasma parameters
Example: Photon ultima APX-RS3,000 fps (1024 x 1024), 250,000 fps (64 x 64)
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Fast visible cameras
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Fast visible cameras
Difficult to extract physical quantities.
Possible to determine the speed, size and origin of the plasma structure (need the mapping of the field lines)
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ISTTOK Database
http://baco.ipfn.ist.utl.pt/jws/DataViewer.jnlp
http://metis.ipfn.ist.utl.pt/CODAC/SDAS/Accesshttp://metis.ipfn.ist.utl.pt/CODAC/SDAS/Codes
Support for Matlab (Octave), IDL, Matematica