Inertial Confinement Fusion

Inertial Confinement Fusion (ICF) is a method that rapidly compresses and heats fuel pellets with powerful lasers or particle beams, using the fuel’s own inertia to secure the time needed for fusion reactions. It achieves fusion through a fundamentally different approach than magnetic confinement.

Basic Principles

Comparison with Magnetic Confinement

Magnetic confinement and inertial confinement achieve the Lawson criterion through contrasting approaches:

Parameter	Magnetic Confinement	Inertial Confinement
Density $n$	$\sim 10^{20}$ m$^{-3}$	$\sim 10^{31}$ m$^{-3}$
Temperature $T$	$\sim 10$ keV	$\sim 10$ keV
Confinement time $\tau$	$\sim 1$ s	$\sim 10^{-11}$ s
Confinement method	Magnetic field	Inertia

In inertial confinement, compression to over 1000 times solid density ensures sufficient reaction events even in an extremely short time for fusion to occur.

Confinement Parameter

In inertial confinement, rather than the density-time product $n\tau$, the areal density $\rho R$ is the important parameter:

$$\rho R = \int_0^R \rho(r) dr$$

The areal density required for fusion ignition is approximately 0.3 g/cm$^2$. This is the value needed for $\alpha$ particles to slow down in the fuel and maintain self-heating.

Compression and Heating

The inertial confinement fusion process is divided into four stages:

1. Ablation: Surface material is ionized and ejected outward
2. Rocket effect: Reaction accelerates fuel inward
3. Implosion: Fuel converges toward the center
4. Ignition/Burn: Fusion reactions begin at the center and a burn wave propagates outward

Laser Compression

The mainstream approach in inertial confinement fusion is compression by high-power lasers.

Required Laser Characteristics

• Pulse energy: Megajoule class (1-2 MJ)
• Pulse width: Nanosecond order
• Power density: $10^{14}$ - $10^{15}$ W/cm$^2$
• Beam uniformity: Less than 1% non-uniformity

Types of Lasers

Main lasers used:

• Nd:glass laser (wavelength 1053 nm → converted to 351 nm third harmonic)
• KrF excimer laser (wavelength 248 nm)

Shorter wavelengths provide higher ablation efficiency and also suppress plasma instabilities.

Direct Drive and Indirect Drive

There are two methods for compressing fuel with lasers: direct drive and indirect drive.

Direct Drive

A method where laser beams directly irradiate the fuel pellet.

Advantages:

• High energy coupling efficiency (10-15%)
• Simple device configuration

Challenges:

• Strict beam uniformity requirements
• Rayleigh-Taylor instability
• Laser-plasma instabilities (stimulated Raman scattering, stimulated Brillouin scattering, etc.)

Japan’s GEKKO XII and OMEGA (USA) conduct direct drive research.

Indirect Drive

A method where lasers irradiate a metal cavity called a hohlraum, and the resulting X-rays compress the fuel.

Operating principle:

1. Laser irradiates the hohlraum inner wall (gold or uranium)
2. The wall ionizes and emits X-rays (mainly soft X-rays)
3. The hohlraum interior is filled with a nearly uniform X-ray radiation field (Planckian distribution, approximately 300 eV)
4. The fuel capsule placed at the center is compressed by X-rays

Advantages:

• High X-ray irradiation uniformity
• Greater flexibility in beam arrangement
• Laser non-uniformity is mitigated

Challenges:

• Low energy coupling efficiency (about 1%)
• Complex hohlraum design and manufacturing
• Laser-plasma interactions

NIF (USA) and LMJ (France) employ indirect drive.

Fuel Target

Target Structure

A typical fuel target (for indirect drive):

Hohlraum (gold cylinder, several mm diameter)
  └─ Fuel capsule (diameter about 2 mm)
       ├─ Ablator (plastic or beryllium)
       ├─ DT ice layer (thickness about 70 μm)
       └─ DT gas (center)

Role of DT Ice Layer

Arranging DT fuel as a solid (ice) shell:

1. Maintains symmetry during compression
2. Achieves high-density state
3. Forms central hot spot

The uniformity of the DT ice layer (surface roughness below 1 μm) directly affects implosion performance, requiring extremely high-precision manufacturing technology.

Hot Spot Ignition

In an ideal implosion, a high-temperature, low-density hot spot forms at the center, surrounded by high-density, low-temperature main fuel:

• Hot spot: $T \sim 10$ keV, $\rho \sim 100$ g/cm$^3$
• Main fuel: $T \sim 1$ keV, $\rho \sim 300-1000$ g/cm$^3$

Fusion reactions begin in the hot spot, and the energy of generated $\alpha$ particles heats the main fuel, propagating the burn wave.

NIF Achievements

The National Ignition Facility (NIF) at Lawrence Livermore National Laboratory in the USA is the world’s largest laser fusion facility.

Facility Overview

Parameter	Value
Number of laser beams	192
Laser energy	Up to 2.05 MJ (ultraviolet)
Pulse width	Several nanoseconds
Peak power	Over 500 TW
Target chamber diameter	10 m

Achieving Fusion Ignition (December 2022)

On December 5, 2022, NIF achieved fusion ignition for the first time in human history:

• Laser input energy: 2.05 MJ
• Fusion output energy: 3.15 MJ
• Energy gain: Q = 1.54

This was a scientific milestone, demonstrating that fusion can produce net energy.

Caveats

However, challenges remain for practical power plant use:

• Efficiency relative to total facility power consumption (about 300 MJ/shot)
• Laser wall-plug efficiency (electricity to laser conversion efficiency)
• Target manufacturing cost and repetition rate

The overall efficiency from laser to target to fusion output is about 1%, requiring significant improvement for power generation.

Subsequent Progress

Since 2023, ignition has been achieved multiple times, with fusion output of approximately 5 MJ reported at its highest. Research continues on reproducibility and performance improvement.

Challenges for Power Plants

Repetition Rate

For a power plant to function, targets need to be imploded at a rate of several to 10 times per second (current NIF operates at about a few shots per day).

Target Manufacturing

Technology is needed to manufacture high-precision targets in large quantities (several hundred million per year) at low cost.

Driver Efficiency

Laser wall-plug efficiency needs to improve from the current approximately 1% to over 10%. Diode-pumped solid-state lasers (DPSSL) and KrF lasers are candidates.

Chamber Design

A chamber design is needed that can withstand repeated explosions, protect structures from neutrons, and efficiently recover heat.

Other Approaches

Fast Ignition

A method where a high-intensity short-pulse laser directly heats the hot spot separately from normal implosion (isotropic compression).

Advantages:

• Separates compression from hot spot formation
• Reduces required driver energy

Research is underway at Osaka University’s FIREX-I and LFEX lasers.

Shock Ignition

A method that generates a strong shock wave at the final stage of implosion to promote ignition.

Z-Pinch

A method that compresses plasma using pulsed power. Researched at Sandia National Laboratories’ Z Machine.

Related Topics

• Confinement Methods: Overview - Overview of confinement methods
• Tokamak Confinement - Mainstream magnetic confinement
• Stellarator/Helical Confinement - Steady-state magnetic confinement
• Glossary: Fusion Reaction - Fundamentals of fusion reactions