☢️Nuclear Fusion Technology Unit 6 – Plasma Diagnostics in Nuclear Fusion

Key Concepts and Fundamentals

• Plasma diagnostics involves measuring and analyzing various properties of plasma to understand its behavior and characteristics in nuclear fusion experiments
• Fundamental plasma parameters include density, temperature, velocity, and magnetic fields which are essential for characterizing the plasma state
• Plasma diagnostics enables researchers to monitor and control plasma conditions to optimize fusion reactions and ensure stable operation of fusion devices
• Diagnostic techniques are based on the interaction of plasma with electromagnetic waves, particles, and radiation providing valuable insights into plasma dynamics
• Understanding the principles of plasma physics, such as particle motion, collective behavior, and instabilities, is crucial for interpreting diagnostic data
  • Plasma physics principles include magnetohydrodynamics (MHD), kinetic theory, and plasma waves
• Plasma diagnostics plays a critical role in validating theoretical models and simulations used in fusion research by providing experimental data for comparison and validation
• Advances in diagnostic technologies have greatly enhanced the spatial and temporal resolution of measurements enabling more detailed studies of plasma phenomena

Plasma Parameters and Properties

• Plasma density refers to the number of charged particles (electrons and ions) per unit volume and is a key parameter affecting fusion reactions and plasma confinement
  • Typical plasma densities in fusion experiments range from $10^{19}$ to $10^{21}$ particles per cubic meter
• Electron and ion temperatures are important indicators of plasma energy and are often measured separately due to their different roles in fusion processes
  • Electron temperatures in fusion plasmas can reach tens of millions of degrees Kelvin (keV)
  • Ion temperatures are typically lower than electron temperatures but still in the range of millions of degrees Kelvin
• Plasma velocity and flow patterns influence particle transport, heat transfer, and stability of the fusion plasma
• Magnetic field strength and topology are crucial for plasma confinement and stability in magnetic fusion devices (tokamaks, stellarators)
  • Magnetic field strengths in fusion experiments can range from a few Tesla to over 10 Tesla
• Plasma pressure, which is the product of density and temperature, determines the plasma beta ($\beta$) a key parameter for plasma stability and confinement
• Plasma current and electric fields can affect particle orbits, instabilities, and plasma heating mechanisms
• Impurity concentrations and radiation losses from the plasma are important considerations for optimizing fusion performance and minimizing damage to the reactor components

Diagnostic Techniques Overview

• Diagnostic techniques can be broadly categorized into electromagnetic, particle, and optical methods each providing unique insights into plasma properties
• Electromagnetic diagnostics measure the interaction of plasma with electromagnetic waves, such as microwaves, millimeter waves, and lasers
  • Examples include interferometry, reflectometry, and polarimetry
• Particle diagnostics detect and analyze charged particles (electrons, ions, neutrals) emitted from or injected into the plasma
  • Examples include Langmuir probes, particle analyzers, and neutral particle analyzers
• Optical and spectroscopic diagnostics measure the emission, absorption, and scattering of light from the plasma providing information on plasma composition, temperature, and velocity
  • Examples include visible and UV spectroscopy, Thomson scattering, and charge exchange recombination spectroscopy
• Combinations of diagnostic techniques are often used to obtain a comprehensive understanding of plasma behavior and to cross-validate measurements
• Diagnostic systems must be designed to withstand the harsh environment of fusion plasmas, including high temperatures, intense radiation, and strong magnetic fields
• Advances in diagnostic technologies, such as high-speed cameras, ultra-fast detectors, and high-resolution spectrometers, have greatly expanded the capabilities of plasma diagnostics
• Data acquisition and processing systems are essential components of diagnostic setups, enabling real-time monitoring and analysis of plasma parameters

Electromagnetic Measurements

• Interferometry measures the phase shift of electromagnetic waves passing through the plasma to determine the line-integrated electron density
  • Interferometers typically operate in the microwave or infrared range and use heterodyne detection to measure the phase shift
• Reflectometry probes the plasma by measuring the reflection of electromagnetic waves from density gradients providing information on the electron density profile and fluctuations
  • Reflectometers can operate in various frequency ranges (microwave, millimeter wave) depending on the expected density range
• Polarimetry measures the rotation of the polarization plane of electromagnetic waves propagating through the plasma, which is proportional to the line-integrated magnetic field
  • Faraday rotation polarimetry is commonly used to measure the poloidal magnetic field in tokamaks
• Electron cyclotron emission (ECE) diagnostics measure the intensity and spectrum of microwave radiation emitted by electrons gyrating in the magnetic field providing information on the electron temperature profile
• Collective Thomson scattering measures the scattering of electromagnetic waves from plasma density fluctuations yielding information on the ion temperature and velocity distribution
• Microwave imaging techniques, such as electron cyclotron emission imaging (ECEI) and microwave imaging reflectometry (MIR), provide 2D or 3D measurements of plasma parameters with high spatial and temporal resolution

Particle Detection Methods

• Langmuir probes are inserted into the edge plasma to measure the local electron temperature, density, and plasma potential by analyzing the current-voltage characteristics
  • Langmuir probes consist of one or more electrodes biased with respect to the plasma and can provide spatially resolved measurements
• Particle analyzers, such as electrostatic analyzers and time-of-flight spectrometers, measure the energy distribution and composition of charged particles escaping from the plasma
  • Particle analyzers can be used to study plasma heating, particle transport, and plasma-wall interactions
• Neutral particle analyzers (NPA) measure the energy distribution of neutral atoms resulting from charge exchange reactions between plasma ions and background neutrals providing information on ion temperature and velocity
• Fast ion diagnostics, such as scintillator probes and Faraday cups, detect energetic ions generated by fusion reactions or external heating methods (neutral beam injection, radio frequency heating)
• Bolometers measure the total radiation power emitted by the plasma across a wide wavelength range, including X-rays, visible, and infrared light
  • Bolometer arrays provide spatially resolved measurements of radiation losses, which are important for power balance studies and impurity monitoring
• Soft X-ray diagnostics, such as diode arrays and pinhole cameras, measure the intensity and spectrum of soft X-rays emitted by the plasma providing information on electron temperature, impurity concentrations, and MHD instabilities
• Neutron diagnostics, such as activation foils, scintillators, and time-of-flight spectrometers, measure the flux and energy distribution of neutrons produced by fusion reactions yielding information on the fusion power and ion temperature

Optical and Spectroscopic Diagnostics

• Visible and UV spectroscopy measures the intensity and spectrum of light emitted by atoms and ions in the plasma providing information on plasma composition, temperature, and velocity
  • Spectroscopic techniques can be passive (observing naturally emitted light) or active (using lasers or beams to excite specific transitions)
• Thomson scattering measures the scattering of laser light from electrons in the plasma yielding local measurements of electron temperature and density
  • Incoherent Thomson scattering is used for low-density plasmas, while collective Thomson scattering is used for high-density plasmas
• Charge exchange recombination spectroscopy (CXRS) measures the emission from ions that have undergone charge exchange with neutral atoms providing information on ion temperature, velocity, and impurity concentrations
  • CXRS is often used in conjunction with neutral beam injection to enhance the signal-to-noise ratio
• Motional Stark effect (MSE) diagnostics measure the polarization of light emitted by neutral atoms in the presence of a strong electric field (induced by the plasma velocity in the magnetic field) providing information on the local magnetic field pitch angle and current density profile
• Laser-induced fluorescence (LIF) measures the fluorescence emission from atoms or ions excited by a laser providing high-resolution measurements of velocity distribution functions and plasma flow
• Coherence imaging spectroscopy (CIS) uses interferometric techniques to measure the 2D velocity distribution of ions and neutrals in the plasma with high spatial and temporal resolution
• Beam emission spectroscopy (BES) measures the emission from neutral atoms injected into the plasma (usually from a diagnostic neutral beam) providing information on plasma density fluctuations and turbulence

Data Analysis and Interpretation

• Raw diagnostic data often requires extensive processing and analysis to extract meaningful physical quantities and insights into plasma behavior
• Signal conditioning techniques, such as amplification, filtering, and digitization, are applied to improve the signal-to-noise ratio and prepare the data for further analysis
• Calibration procedures are essential to convert raw diagnostic signals into absolute physical quantities (density, temperature, velocity) taking into account the response function of the diagnostic system
• Tomographic inversion methods are used to reconstruct 2D or 3D profiles of plasma parameters from line-integrated measurements (interferometry, bolometry)
  • Examples include Abel inversion, Fourier-Bessel expansion, and maximum entropy methods
• Statistical analysis techniques, such as correlation functions, power spectra, and probability distribution functions, are used to characterize plasma fluctuations and turbulence
• Machine learning and data mining techniques are increasingly being applied to analyze large datasets from plasma diagnostics and to identify patterns and correlations that may not be apparent using traditional methods
• Uncertainty quantification and error analysis are crucial for assessing the reliability of diagnostic measurements and derived quantities
  • Sources of uncertainty include statistical noise, systematic errors, and limitations of the diagnostic technique
• Integrated data analysis approaches combine measurements from multiple diagnostics to provide a more comprehensive and self-consistent picture of the plasma state
  • Bayesian inference methods are often used to combine prior knowledge with experimental data and to propagate uncertainties

Applications in Fusion Experiments

• Plasma diagnostics are essential for the operation and optimization of fusion devices, such as tokamaks, stellarators, and inertial confinement fusion (ICF) experiments
• In magnetic confinement fusion devices, diagnostics are used to monitor and control plasma parameters to maintain stable operation and to optimize fusion performance
  • Examples include feedback control of plasma position and shape, density and temperature profiles, and impurity levels
• Diagnostics are crucial for studying plasma instabilities and transport phenomena, which can limit fusion performance and cause damage to the reactor components
  • Examples include monitoring of MHD instabilities (neoclassical tearing modes, edge localized modes), turbulence, and particle and heat transport
• In ICF experiments, diagnostics are used to characterize the implosion dynamics, compression, and heating of the fusion fuel, as well as the resulting fusion yield and energy gain
  • Examples include X-ray and neutron imaging, spectroscopy, and bang time measurements
• Diagnostic data is used to validate and improve theoretical models and numerical simulations of fusion plasmas, which are essential for predicting the performance of future fusion reactors
• Plasma diagnostics also play a key role in the development and testing of advanced fusion concepts, such as spherical tokamaks, reversed field pinches, and magnetized target fusion
• Diagnostics are essential for the safe operation of fusion reactors by monitoring the integrity of the reactor components and the levels of radiation and neutron flux
  • Examples include infrared cameras for wall temperature monitoring, neutron detectors for tritium breeding blanket optimization, and erosion monitors for plasma-facing components
• The development of advanced diagnostic techniques and their integration into fusion experiments is an ongoing research area that is critical for the progress of fusion energy science and technology