This simulator models the deuterium-tritium (D+T) fusion reaction inside a tokamak, the doughnut-shaped device that uses magnetic fields to confine a hot plasma. It evaluates the triple product nTT (density x confinement time x temperature) against the Lawson criterion of about 3x10²¹ m⁻³·s·keV, the threshold a reactor must exceed to sustain burning plasma. A bird's-eye cross-section shows the vessel, field coils and glowing plasma torus.
Three sliders set plasma temperature T (1-100 keV), ion density n (10¹⁹-10²¹ m⁻³) and energy confinement time τ (0.1-10 s). The simulator computes the D-T reactivity using a fit from the NRL formulary, derives fusion power and heating power densities, and reports the energy-gain factor Q = P_fusion / P_heating. Meeting these conditions is the central engineering challenge for reactors such as ITER and future power plants.
What does this simulator actually show?
It shows a tokamak cross-section with a D-T plasma and tracks whether your chosen conditions reach fusion ignition. As you adjust the sliders it recalculates the Lawson triple product nTT, the fusion and heating power densities, and the gain factor Q, lighting an ignition badge once the plasma crosses the criterion.
What is the Lawson criterion?
The Lawson criterion is the minimum triple product of density, energy confinement time and temperature needed for a self-sustaining fusion reaction. For D-T fuel the simulator uses a threshold of roughly 3x10²¹ m⁻³·s·keV. Below it, energy leaks out faster than fusion replaces it; above it, the plasma can heat itself.
What is the Q factor?
Q is the ratio of fusion power produced to external heating power supplied, Q = P_fusion / P_heating. Q = 1 marks scientific breakeven, where the reaction releases as much energy as you put in. As Q grows very large the plasma becomes self-heating, which the simulator labels as ignition (Q to infinity).
The D+T reaction has the largest cross-section at the lowest temperatures of any fusion fuel, so it is the easiest to ignite. It fuses two hydrogen isotopes into helium-4 (carrying 3.5 MeV) and a fast neutron (14.1 MeV), releasing 17.6 MeV in total. That neutron carries most of the energy and, in a real reactor, would heat a blanket to generate electricity.
Temperature T sets how energetic the ions are in keV, controlling how often they overcome their mutual repulsion to fuse. Density n sets how many fuel ions occupy each cubic metre. Confinement time τ sets how long the plasma retains its energy before it leaks away. All three multiply together in the triple product, so raising any one helps reach the Lawson threshold.
Plasma temperatures are quoted as energies, where 1 keV corresponds to about 11.6 million kelvin. So the default of 10 keV is roughly 116 million degrees, far hotter than the centre of the Sun. Fusion devices need such extremes because the fuel ions must move fast enough to tunnel through the Coulomb barrier between their positive charges.
The fusion power density scales as roughly 0.25 x n² x reactivity x E_fus, where the reactivity is the temperature-dependent average of cross-section times velocity and E_fus is the 17.6 MeV released per reaction. The heating power density is modelled as 3nT/τ. Their ratio gives Q. The reactivity uses a parametric fit to standard D-T data from the NRL plasma formulary.
The relationships and orders of magnitude are realistic and based on standard fusion formulae, so it conveys the genuine trade-offs between temperature, density and confinement. However, the reactivity is a simplified fit and the power model ignores radiation losses, impurities, geometry and instabilities. It is an educational tool, not a reactor design code.
Hot plasma constantly leaks energy through turbulence, conduction and radiation, so keeping it confined for even one second is difficult. Tokamaks use powerful magnetic fields from toroidal and poloidal coils to trap the charged particles on nested magnetic surfaces. Plasma instabilities, disruptions and heat loads on the walls all conspire to shorten τ below the theoretical ideal.
Yes. In 2022 the National Ignition Facility, which uses laser-driven inertial confinement rather than a tokamak, achieved a fusion energy output exceeding the laser energy delivered to the fuel. Magnetic-confinement tokamaks like JET have produced large fusion power for short pulses, and ITER is being built to reach a sustained Q of about 10.
Fusion promises abundant fuel, since deuterium is plentiful in seawater and tritium can be bred from lithium, with no long-lived high-level waste and no risk of runaway meltdown. A single fusion reaction releases millions of times more energy per kilogram than burning fossil fuels. Mastering the Lawson criterion is the key step toward practical fusion electricity.