Short description: Physics of the hot, magnetically confined central plasma region in a tokamak

Tokamak core plasma physics concerns the behavior of the hot, dense, magnetically confined plasma in the central region of a tokamak. In this region, the main goals are to achieve sufficiently high. The temperature of the plasma is very high, of the order of 10,000 K. The plasma also produces ultraviolet light, so for safety should not be viewed directly, particle density, and energy confinement time for fusion reactions to occur efficiently.^[1]^[2]

The core plasma is the main site where fusion performance is determined. Its properties are governed by the balance between magnetic confinement, external and internal heating, particle fueling, current evolution, and losses caused by collisional transport, turbulence, and large-scale magnetohydrodynamic (MHD) activity.^[3]

Core plasma conditions

In a tokamak, the plasma core is the hot central volume enclosed by nested magnetic flux surfaces. The magnetic field has both toroidal and poloidal components, producing helical field lines that confine charged particles and reduce direct contact with material walls.^[4]

For deuterium–tritium fusion, the core must reach temperatures of the order of tens of millions of degrees, while maintaining adequate density and confinement time. These requirements are commonly summarized by the Lawson criterion, which expresses the need for a sufficiently large product of density, temperature, and confinement time.^[1]

A central objective of tokamak core physics is therefore to produce a plasma state in which energy losses are lower than the combined heating from external systems and fusion-born alpha particles.^[5]

Magnetic equilibrium and pressure balance

The tokamak core exists in an approximate force balance between plasma pressure and magnetic forces. In idealized form, equilibrium is described by MHD, where the plasma pressure gradient is balanced by the Lorentz force generated by plasma currents and magnetic fields.^[3]

The quality of core confinement depends strongly on equilibrium shape and current distribution. Quantities such as the plasma current, safety factor, magnetic shear, elongation, and triangularity influence both transport and stability. A properly shaped plasma can improve confinement and raise the operational limits before MHD instabilities occur.^[3]

Pressure is often characterized through the dimensionless parameter beta, $β = \frac{p}{B^{2} / 2 μ_{0}}$ , which compares plasma pressure to magnetic pressure. Higher beta is desirable for efficient fusion performance, but excessively high beta can drive global instabilities that degrade or terminate confinement.^[6]

Heating of the core plasma

Core plasma heating begins with ohmic heating, produced by the toroidal plasma current. However, as temperature rises, the plasma resistivity falls, making ohmic heating alone insufficient for reactor-relevant conditions.^[7]

Additional heating is therefore supplied by auxiliary systems such as neutral beam injection and radio-frequency heating methods, including ion cyclotron and electron cyclotron resonance heating. These systems deposit energy into the plasma, shape temperature profiles, and can also help drive current non-inductively.^[7]^[8]

In deuterium–tritium operation, an especially important regime is the burning plasma regime, in which fusion-produced alpha particles deposit a substantial fraction of their energy back into the core. In that case, self-heating becomes a dominant part of the power balance.^[5]^[9]

Transport and confinement

Even when the plasma is magnetically confined, energy and particles slowly leak outward from the core. Part of this transport is collisional and classical, but in most tokamak regimes the dominant losses arise from microturbulence driven by gradients in temperature, density, and current.^[3]^[10]

The resulting confinement quality is measured by the energy confinement time $τ_{E}$ , which estimates how long thermal energy remains in the plasma before being lost. Improving $τ_{E}$ is essential because the fusion power density depends strongly on temperature and because ignition-like conditions require sufficiently low transport losses.^[1]^[2]

In many experiments, confinement improves when transport barriers form. Although edge transport barriers are associated with H-mode operation, changes in core transport can also occur, including internal transport barriers produced by favorable magnetic shear and profile control.^[10]

Profiles and self-organization

Tokamak core plasmas are described by radial profiles of temperature, density, pressure, current density, and plasma rotation. These profiles are not fixed; they evolve through the competition between heating, fueling, transport, and instabilities.^[3]

Profile shape matters because steep gradients can improve fusion reactivity while also providing free energy for turbulence and MHD modes. In practice, advanced tokamak scenarios seek optimized profiles that maximize confinement and bootstrap current while remaining stable against disruptive behavior.^[9]

The current profile is especially important because it influences magnetic shear and thus both microinstability growth and large-scale MHD stability. Controlling the current profile is a major requirement for steady-state or long-pulse tokamak operation.^[6]

Stability of the core plasma

The tokamak core can become unstable in many ways. Large-scale MHD instabilities include kink modes, tearing modes, neoclassical tearing modes, sawtooth oscillations, and ballooning-driven activity. These phenomena can flatten temperature profiles, reduce confinement, or trigger major disruptions.^[3]^[9]

Stability boundaries depend on quantities such as current, pressure gradient, safety factor, rotation, and resistivity. Operation near these limits is attractive because fusion performance generally increases with pressure, but this also increases the likelihood of instability.^[6]

An important challenge in reactor-scale plasmas is therefore to combine high core pressure with active control of instabilities through magnetic control coils, current-profile tailoring, auxiliary heating, and fast diagnostics.^[9]^[8]

Burning plasma physics

In a burning plasma, the dominant source of core heating comes from fusion-generated alpha particles rather than external power systems. This regime is qualitatively different from present-day low-gain plasmas because the plasma becomes more self-coupled: changes in confinement, pressure, or instability levels directly alter the self-heating rate.^[5]^[9]

Burning plasma physics includes the confinement and slowing-down of energetic alpha particles, their interaction with waves and instabilities, and the feedback between self-heating and global plasma performance. Understanding this regime is one of the principal scientific goals of ITER.^[5]^[11]

Role in fusion performance

The core plasma determines much of the total fusion output because fusion reactions scale strongly with central temperature and density. If the core is well confined and stable, alpha heating can reinforce high-performance operation; if transport or instabilities become too strong, the plasma cools and fusion power drops rapidly.^[1]^[2]

For this reason, tokamak core plasma physics lies at the center of magnetic fusion research. It connects equilibrium, heating, turbulence, transport, energetic-particle effects, and MHD stability into one coupled system whose optimization is essential for practical fusion energy.^[3]^[5]

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Wavefunctions and modes
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Physics:Quantum Tokamak core plasma

Core plasma conditions

Magnetic equilibrium and pressure balance

Heating of the core plasma

Transport and confinement

Profiles and self-organization

Stability of the core plasma

Burning plasma physics

Role in fusion performance

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