Petawatt Laser and ICF

What is a petawatt laser?

A petawatt laser is a high-intensity laser which generates peak powers of quadrillion (10^15) watts, or petawatts. They typically produce laser pulses that are extremely short, lasting only a few femtoseconds.

By concentrating energy in an incredibly small space-time volume, these lasers create extreme conditions previously accessible only in astrophysical environments, enabling breakthrough applications in inertial confinement fusion.

They use the CPA technique to perform at these high energies. They proofed to be extremely useful in lots of scientific research and enabled one of the biggest scientific breakthroughs in recent years: Inertial Confinement Fusion (ICF). Scientists hope this field of research could contribute enormously to future clean energy.

What technological advancements let to the creation of petawatt laser?

In 1960 Theodor H. Maiman invented the first laser with a peak power of kilowatts. It was the first time that a source emitted coherent light, meaning the light is synchronized in phase, which is necessary for lasers.

After a decade, peak power in pulsed operation increased from kilowatts to gigawatts and even more. Science faced a whole new opportunity for research and exploring deeper structures. However, the improvement of laser power slowed down and not much happened from the 1970s to mid-1980s.

Then, CPA was invented and changed laser physics. In 1985, Donna Strickland with Gerard Mourou invented the Chirped Pulse Amplification (CPA). (6)

Nowadays, nearly all high-intensity lasers operate under the CPA-system. For instance, the ZEUS laser (Zettawatt-Equivalent Ultrashort pulse laser System), a newly constructed multi-petawatt dual-beam facility at the University of Michigan.

How exactly do CPA laser work?

Chirped Pulse Amplification (CPA) is a concept to overcome fundamental limitations that conventional lasers face when increasing peak power.  While conventional laser systems establish high average power, attempts to increase peak power face limitations due to nonlinear optical effects and material damage. These effects become particularly critical in ultrashort-pulse lasers.

Laser systems can be divided into continuous-wave and pulsed lasers. Continuous-wave (CW) laser distribute power uniformly in time. Pulsed laser systems, however, concentrate their energy in ultrashort intervals. These pulses are usually only femtoseconds long (10^-15s). Since power is energy per time, compressing the average power of CW lasers into an ultrashort pulse drastically increases the peak power, even reaching petawatts. However, these powers are too high for a conventional laser.

To understand where the limitations arise, I will shortly explain the operation of a conventional laser.

LASER stands for Light Amplification Stimulated Emission (of) Radiation. It typically consists of a pump source, a gain medium and an optical resonator.

Under thermal equilibrium, the gain medium has more particles in a lower energy state due to Boltzmann distribution. Lasers need to establish population inversion, meaning the number of particles in an exited state is higher than the number in an unexcited state. Therefore, the pump source pumps energy into the gain medium.

Optical amplification occurs due to stimulated emission (SE in LASER), a process fist described by Albert Einstein in 1917. When a photon propagates through a gain medium with population inversion, it stimulates an excited atom to emit a coherent (identical frequency, direction…)  photon. The resonator reflects the beam of photons several times through the gain medium, creating interfering waves. This is the oscillator-based concept for conventional lasers.

But extreme powers of the pulse damage the material of the laser when reflected in the resonator.

To solve the problem, CPA lasers were developed in 1985, working like a conventional laser with a few changes. The key insight was that optical damage depends on intensity (power / area), not total energy:

1. A short pulse of low intensity is stretched in time in a grating pair before amplification to prevent damage to the optical components. This is the key difference to conventional ones and enables to work with higher energies. By stretching the pulse, the peak intensity decreases. The granting pair diffracts the pulse, meaning they typically delay shorter wavelengths more than longer ones to create the chirped pulse and prevent peak powers in the optical components.
2. The long pulse of low intensity is passed through a series of amplifiers which amplifies the stretched pulse and increase its intensity. The long pulse of low intensity enters the amplifier whose gain medium has a population inversion. The pulse stimulates the excited particles to emit identical photons. This „cloning “of the pulse is what increases the energy of the pulse. However, unlike in a conventional laser where the amplified pulse is reflected in the resonator, the amplifier in the CPA is designed for energy extraction.
3. In the end, the long pulse of high intensity is compressed back to its original duration in a grating pair by routing longer wavelengths on a longer path. A short pulse of high intensity is released, a high-peak-power-flash.

Scientists at the University of Michigan have constructed a high-power laser facility called ZEUS. In a paper („The Zeus multi-petawatt laser system “) published in October 2025, they state that the ZEUS laser uses Ti: Sapphire (Ti: Sa) as a gain medium, a solid sapphire crystal doped with titanium. This titanium doped crystal has a wider luminescent spectral bandwidth which is essential for supporting the ultra-short pulses (femtoseconds). The Titanium ions in the sapphire crystal have broad vibrionic energy levels, resulting in a big emission bandwidth.  (3)

What has been achieved?

The ZEUS laser has achieved a power of 2PW with a pulse duration of 15 femtoseconds. However, its unique geometry allows the petawatt laser pulse to collide with a GeV-energy electron beam which creates an interaction equivalent to a Zetawatt power laser. (3)

But the ZEUS laser doesn’t hold the current record which is a laser output of above 10 petawatts as of 2024-2025 from the Extreme Light Infrastructure for Nuclear Physics (ELI-NP) in Romania and development is ongoing at facilities such as the Shanghai Superintense Ultrafast Laser Facility (SULF) in China.

What are prospects for the future?

A 2024 review from the Colorado State University states: „The next milestone for high-power lasers is reaching 100 PW, with several systems currently under design and development. “(1) 100PW is the goal for 2030 for a Chinese team and 50PW for the University of Rochester in the USA.

Additionally, a post-compression technique with incorporated CPA promises further increase laser power to Exawatt (10^18) and even Zetawatt (10^21). (1)

Another prospect for the future is the application of petawatt lasers in LPA (Laser-plasma acceleration). LPA generates compact, high-energy beams of electrons, protons and ions. The laser produces strong electric fields within the plasma, stronger than the ones in particle accelerators, to accelerate over shorter distance and with more energy.

Among the most compelling applications of petawatt lasers is Inertial Confinement Fusion (ICF). The availability of ultralight-peak-power lasers has revolutionized this field, giving hope for new and clean energy sources, different form what was possible before. This topic will be explored in detail in the following section.

What is ICF and what role do petawatt lasers play?

Inertial confinement fusion (ICF) is a method to achieve nuclear fusion by compressing and heating fuel with high-intensity lasers.

Nuclear fusion is the opposite of nuclear fission where heavy nuclei split into lighter ones. Nuclear fission provides about 9-10% of global power from about 440 reactors, but faces challenges like cost, safety and the radioactive waste.

Nuclear fusion is a way to set free energy by binding certain atoms or molecules. Stars like the sun fuse elements like hydrogen as their energy source. Nuclear fusion requires extreme conditions as the atoms only fuse under extreme pressure and temperature like the one in the sun that brings them close enough together to overcome a certain repulsive barrier. This barrier is called Coloumb barrier caused by electrostatic repulsion between positively charged nuclei. After overcoming this barrier, the strong nuclear force outweighs the repulsive electrostatic force and fuses the atoms.

The challenge of generating energy through nuclear fusion has been achieving extreme conditions to overcome the Coloumb barrier.

How exactly does ICF work?

A high-intensity laser pulse is fired on a millimetre-sized capsule designed like a hollow spherical shell. The actual fuel lies inside the capsule and consist of Deuterium and Tritium, two isotopes (more neutrons) of hydrogen.

When the high-intensity laser pulse hits the capsule (shell around the actual fuel), it explodes. This process is called ablation. The electric field of the laser pulse is strong enough to ionize the atoms of the capsule, turning it into a high-temperature plasma. The shell explodes outward due to the extreme heat.

This outward explosion creates an equal an opposite reaction according to Newtons Third Law. It pushes the fuel inward at a speed of hundreds of kilometres per second. The deuterium and tritium fuel is compressed more than 1000 times, creating sun-like conditions and a temperature of approximately 100.000.000 degree Celsius.

Those conditions allow the atoms to overcome the Coloumb barrier and fuse, releasing energy. This energy is usually in form of kinetic energy of the fusion products such as high-speed neutrons.

A barrier, for instance a lithium-containing blanket, surrounding the reactor chamber absorbs high-speed neutrons, converting their kinetic energy into thermal energy. Then, certain materials extract the thermal energy to drive turbines or generate electricity.

What has been achieved?

The concept of ICF was proposed in 1972 by several scientists. But the major breakthrough for nuclear fusion occurred on December 5, 2022, when ICF achieved scientific breakeven, meaning that the fusion reaction released more energy than the input laser energy. Specifically, 2.1 MJ of laser energy resulted in 3.15 MJ of fusion energy.

The review „Fututre for internal-fusion energy in Europe: a roadmap “from 2023 stated „a recent experiment in August 2023 provided an even larger yield: about 170% of the input laser energy “(5)

What are prospects for the future?

The review „Future of internal-fusion energy in Europe: a roadmap “published in 2023 by Cambridge University Press proposes the development of a fusion power plant based on the ICF concept. „This project aims to create a scientific basis and a technological readiness that will enable future commercialisation of laser fusion energy. The goal is to demonstrate direct drive ignition of fusion reactions with lasers and high repetition rate (HRR) high-gain laser operation using frontier laser technology and sustainable materials. This goal will be achieved on a time scale of 20–30 years. It will be facilitated by creating a European Laser Fusion Research Centre – a joint venture of several major stakeholders – including research laboratories, universities, governmental organisations and private companies. “(5)