### Diagnostics

```PLASMA DIAGNOSTICS
CARLOS SILVA
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 "
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)
What to measure
 Density of particles
 Temperature
 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
Diagnostic characteristics
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.
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.
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
Temporal / spatial resolution
v r ( Nt ) 
N

n0
EB (nt )
N
 n (nt )
e
n0
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
High degree of automatization of control/monitoring of diagnostic
equipment and of data acquisition.
Consequence: High complexity and high cost of diagnostic systems.
Accuracy vs Precision
Accuracy
Precision
Definition The degree of The degree to which
closeness to an instrument will
true value.
repeat the same
value.
Measure
ments:
 Real value may not be
known
 Do not mix up lack of
precision with plasma
fluctuations
High accuracy
Low precision
Single
High accuracy
High precision
Multiple measurem
ents are needed
Low accuracy
Low precision
Scales: space
In large fusion experiments the spatial scales vary by 6 orders of
magnitude
 Debye length (< mm)
 Electron 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)
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)
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
Plasma Diagnostic Systems
Selected low temperature plasma diagnostics
Diagnostic
Measures
Langmuir 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
Selected ITER diagnostics
Diagnostic
Measures
Magnetic diagnostics
Plasma current, position, shape, waves ..
Spectroscopic & neutral
particle analyser systems
Ion temperature, He & impurity
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
Plasma-facing components &
operational diagnostics
Temperature of, and particle flux
to First Wall, …..
Neutral beam diagnostics
Various parameters
DIAGNOSTICS
OVERVIEW
Lectures on:
 Electrical probes (Carlos Silva)
 Magnetic probes (Bernardo Carvalho)
 Particle beams (Artur Malaquias)
 Spectroscopy (Elena Tatarova)
 Reflectometry (M.E. Manso, MT5)
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
Langmuir probes I – V characteristic
Magnetic measurements
Essential in magnetic confinement devices
 Plasma current, position, geometry, instabilities
Sensor fluxo magnético
Signal in the sensor
Signal has to be integrated
(hardware or software)
Magnetic measurements
Magnetic probes on ISTTOK
Magnetic probes
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.
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
Lithium beams
 Light elements (Lithium): The aim is
Neutral elements so not limited by B.
plasma properties. Plasma emits electromagnetic radiation due
to different physics processes
 Complex spectra (continuum + spectral lines) from IR to X-ray
 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)
 Spectral lines: Discrete radiation due to electron transition between
energy atomic levels
 From visible to X-ray
 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)
Spectral lines
Bolometer
Infra-red cameras
Infra-red cameras
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-RS
3,000 fps (1024 x 1024), 250,000 fps (64 x 64)
Fast visible cameras
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)
ISTTOK Database
 Support for Matlab (Octave), IDL, Matematica
http://baco.ipfn.ist.utl.pt/jws/DataViewer.jnlp
http://metis.ipfn.ist.utl.pt/CODAC/SDAS/Access
http://metis.ipfn.ist.utl.pt/CODAC/SDAS/Codes
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