11.3 Fusion reactor design considerations

Fusion reactor design is a complex balancing act. It involves optimizing plasma performance, managing heat loads, and ensuring fuel self-sufficiency. Key metrics like the fusion triple product and energy gain factor guide engineers towards viable fusion power.

Reactor components must withstand extreme conditions while maximizing efficiency. Clever designs for divertors, blankets, and shielding are crucial. Advanced diagnostics and control systems keep the plasma stable, inching us closer to practical fusion energy.

Fusion Reactor Performance Metrics

Fusion Triple Product and Lawson Criterion

• Fusion triple product measures plasma performance combines density, temperature, and confinement time
• Expressed as $n T \tau_E$ where n is plasma density, T is temperature, and τ_E is energy confinement time
• Higher values of triple product indicate better fusion conditions
• Lawson criterion establishes minimum conditions for fusion reactions to be self-sustaining
• Criterion states that $n \tau_E > 10^{20} \text{ m}^{-3} \cdot \text{s}$ for deuterium-tritium fusion
• Achieving Lawson criterion crucial for practical fusion energy production

Ignition and Power Balance

• Ignition condition occurs when fusion reactions become self-sustaining without external heating
• Requires fusion power output to exceed power losses from the plasma
• Power balance equation: $P_{fusion} + P_{heating} = P_{radiation} + P_{transport}$
• $P_{fusion}$ represents power generated by fusion reactions
• $P_{heating}$ includes external heating sources (ohmic, RF, neutral beam injection)
• $P_{radiation}$ accounts for energy lost through various radiation processes
• $P_{transport}$ represents energy lost through particle and heat transport

Fusion Energy Gain Factor

• Fusion energy gain factor (Q) measures reactor efficiency
• Defined as ratio of fusion power output to input heating power
• Q = $P_{fusion} / P_{heating}$
• Q < 1 indicates more power input than output (current experimental reactors)
• Q = 1 represents breakeven point where fusion power equals input power
• Q > 5 considered minimum for commercial viability
• Q = ∞ represents ignition condition where no external heating required

Plasma Confinement and Control

Plasma-Wall Interactions and Divertor Systems

• Plasma-wall interactions occur between hot plasma and reactor vessel walls
• Interactions lead to impurity introduction and wall erosion
• Impurities cool plasma and reduce fusion reaction rates
• Divertor systems manage plasma exhaust and impurity control
• Divertor plates positioned to intercept plasma particles leaving confinement region
• Divertor geometry designed to spread heat load over larger area
• Advanced divertor concepts include snowflake and super-X configurations for improved heat handling

Plasma Diagnostics and Control Systems

• Plasma diagnostics measure various plasma parameters (temperature, density, current)
• Techniques include magnetic probes, spectroscopy, and laser-based methods
• Thomson scattering measures electron temperature and density
• Interferometry determines plasma density profile
• Bolometry measures total radiated power from plasma
• Control systems use diagnostic data to maintain stable plasma conditions
• Feedback loops adjust magnetic fields, heating power, and fueling rates
• Real-time control crucial for maintaining fusion conditions and preventing disruptions

Reactor Components and Shielding

Blanket Design and Tritium Breeding

• Blanket surrounds plasma chamber absorbs neutrons and transfers heat
• Serves multiple functions: energy extraction, tritium breeding, radiation shielding
• Typical materials include lithium compounds and beryllium neutron multipliers
• Tritium breeding reaction: $^6\text{Li} + \text{n} \rightarrow ^4\text{He} + \text{T} + 4.8 \text{ MeV}$
• Tritium breeding ratio (TBR) measures tritium production efficiency
• TBR > 1 required for fuel self-sufficiency in D-T fusion reactors
• Advanced blanket concepts include dual-coolant and helium-cooled ceramic designs

Neutron Shielding and Radiation Protection

• Neutron shielding protects reactor components and personnel from radiation damage
• Materials with high neutron absorption cross-sections used (boron, cadmium)
• Layered shielding design combines different materials for optimal protection
• Neutron moderation slows down fast neutrons for easier absorption
• Gamma radiation shielding requires high-density materials (lead, concrete)
• Activation of reactor materials by neutron bombardment considered in design
• Remote handling systems developed for maintenance of activated components