12.3 Ignition and burn physics

Ignition and burn physics are crucial in inertial confinement fusion. They determine when fusion reactions become self-sustaining and how much energy we can get out. It's all about hitting the right conditions to kickstart a fusion inferno.

The Lawson criterion is our fusion roadmap. It tells us what density, temperature, and confinement time we need to reach ignition. Once we hit that sweet spot, alpha particles take over, heating things up and keeping the fusion party going.

Ignition and Gain

Ignition Threshold and Lawson Criterion

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• Ignition threshold defines minimum conditions for self-sustaining fusion reactions
• Lawson criterion for ICF determines necessary conditions for fusion ignition
  • Combines fuel density, confinement time, and temperature
  • Expressed as $n\tau > f(T)$, where n is fuel density, τ is confinement time, and T is temperature
• Breakeven occurs when fusion energy output equals input energy
  • Crucial milestone in fusion research
  • Represented by Q = 1, where Q is the energy gain factor
• Energy gain factor (Q) measures fusion reactor efficiency
  • Ratio of fusion power output to input power
  • Q > 1 indicates net energy production
  • Q = ∞ represents ideal ignition scenario

Fuel Gain and Energy Considerations

• Fuel gain measures energy released per unit mass of fusion fuel
  • Depends on fuel composition and reaction rates
  • Higher fuel gain improves overall reactor efficiency
• Target gain in ICF includes effects of driver efficiency and energy coupling
  • Accounts for energy losses in the fusion process
  • Crucial for determining overall system performance
• Scientific breakeven achieved when fusion energy exceeds absorbed driver energy
  • Important milestone in ICF research
  • Precursor to engineering breakeven and ignition

Fusion Burn

Burn Fraction and Propagation

• Burn fraction represents the portion of fusion fuel consumed in the reaction
  • Typically expressed as a percentage
  • Higher burn fractions indicate more efficient fuel utilization
• Burn propagation describes the spread of fusion reactions through the fuel
  • Initiated by alpha particles produced in initial fusion reactions
  • Creates a chain reaction effect, sustaining the fusion process
• Factors affecting burn propagation include:
  • Fuel density and temperature distribution
  • Confinement geometry and magnetic field configuration
  • Presence of impurities or non-fuel particles

Fusion Yield and Alpha Heating

• Fusion yield measures total energy released from fusion reactions
  • Depends on burn fraction, fuel mass, and reaction energy
  • Expressed in joules or equivalent TNT mass
• Alpha heating plays crucial role in sustaining fusion reactions
  • Alpha particles (helium nuclei) produced in D-T fusion carry 20% of reaction energy
  • Deposit energy back into the plasma, maintaining high temperatures
  • Essential for achieving ignition and high energy gain
• Self-heating regime occurs when alpha heating dominates external heating
  • Marks transition towards self-sustaining fusion reactions
  • Critical for achieving high Q values and eventual ignition

Plasma Confinement

Confinement Time and Energy Balance

• Confinement time measures how long fusion plasma remains at reaction conditions
  • Crucial parameter in achieving and maintaining fusion conditions
  • Influenced by various loss mechanisms (thermal conduction, radiation, particle diffusion)
• Energy confinement time (τE) specifically relates to thermal energy retention
  • Defined as ratio of plasma thermal energy to power loss rate
  • Key factor in achieving fusion breakeven and ignition
• Particle confinement time (τp) describes retention of fuel particles in the plasma
  • Affects fuel burnup and overall reactor efficiency
  • Can differ from energy confinement time due to different loss mechanisms
• Global energy balance in fusion plasma includes:
  • Input heating power (external heating + alpha heating)
  • Power losses (radiation, conduction, convection)
  • Change in plasma thermal energy over time