What is NIF?
It is a facility located in Lawrence Livermore National laboratory, California. It is one of the places on the planet trying to produce new, more efficient power planet.
NIF’s aim is to bring Star Power to Earth.
What that means is to produce energy same way the stars do (by nuclear fusion). It has the world’s largest laser used to induce nuclear fusion reaction in the deuterium-tritium fuel. It is a large laser-based Inertial Confinement Fusion (ICF) research device.
This big laser is of the size of 3 football fields side by side , which has 192 laser beams focusing on a capsule of 0.2 cm , producing enormous energy. We will look into the details later.
But first, How does that help to make a better world?
- It provides the path to clean, safe, carbon-free energy future.
- It will be able to produce a next generation of High Energy Density scientists.
- Advancing the frontier of science and leading to the understanding of Big Questions of the universe.
- And also, it enables to study basic physics of Nuclear weapons without actually testing them underground.
Let’s see this in detail.
To understand how NIF works we need basic understanding two things:
- What is laser pulse?
- How Nuclear fusion reaction happens inside the stars?
Ultrashort Laser Pulses
An ultrashort pulse laser emits light in short bursts i.e it emits laser light in pulses (and not continuously) .
It persists for extremely small time duration – from picoseconds to 10 femtoseconds (1 trillionth of a second).
And secondly, they are extremely Powerful ranging from gigawatt (1 million watts) to terawatt ( 1 million million watts).
Having high peak power , Laser pulses are used in NIF.
How is Nuclear Fusion used by stars?
How do starts burn even when there is no oxygen in space?
Answer is : They produce enormous energy by nuclear fusion.
Stars are gigantic and thus their gravitational forces pushes in, compressing star stuff . This creates conditions with extreme temperatures and pressure.
Due to this pressure, nuclei are forced into each other. Core of stars have 71% hydrogen atoms.
When hydrogen nuclei fuses with each other it produces Helium , neutron and enormous enormous energy. This reaction keeps going on producing more and more heavier atoms. This way Stars produces energy and burns for billions of years.
Credit : Wykis [Public domain], from Wikimedia Commons
This same reaction is used by NIF in order to achieve the goal of producing limitless clean energy.
But it’s not that easy to fuse two nuclei/protons together as they repel each other by huge magnetic force when brought closer.
It takes to create the same conditions that are in the core of stars ( extraordinarily high temperatures and pressure ) inside the laboratory.
Great challenge, right? NIF people are ready to take that up. Let’s see how it works.
How NIF works?
Key components include Optical Pulse Generation (OPG) system, Regenerative Amplifier, Glass Amplifiers , Optical switch , frequency converter and beam controlling machine and the Fusion Fuel.
Before firing the laser pulse, amplifiers of the system are optically pumped by 7680 flash lamps supported by capacitor bank which stores energy of 422 MJ.
Birth of Laser Pulse
The story starts here.
Weak laser pulses are produced in the master oscillator room, using ytterbium-doped optical fibre laser. ( see how LASER works?)
Optical fiber laser is a laser where gain medium is optical fiber doped with ytterbium typically.
This Optical Pulse Generation (OPG) system transports 48 laser pulses (with energy in nano joules ) into pre-amplifier modules ( PAMs).
These laser pulses are amplified several billion times before meeting the target chamber.
PAM : Pre-amplifier modules
It is 2 stage amplification here.
Regenerative Amplifier :
Inside it, Laser pulse moves back and forth 30-60 times amplifying pulse energy by 10 billion times, from nano joules to tens of millions joules.
Each pulse is then divided in to four, creating 192 pulses . They head on to Glass Amplifiers.
Glass Amplifiers – the heart of NIF
Glass Amplifiers have phosphate glass doped with neodymium atoms which gives them pinkish colour.
There are total 16 glass amplifiers per beam arranged in two units –
Power amplifier (having 5 glass pillars) and Main amplifier (having 11 glass pillars).
Flash lamps are used to excite the neodymium glass so when laser pulse pass through glass , it absorbs the enrgy and gets amplified.
Lasers glass used here are 1 meter long and half meter wide.
Each glass pillar is 4 glass slabs high and 2 glass slabs wide to accommodate 8 beams.
So there are in total 3072 glass slabs required supporting 24 beamlines in whole laser system.
Important thing to take care is, glass slabs are arranged at Brewster’s angle to each other(angle at which reflections losses are very low) .
192 beams pass through the power amplifier containing 5 glass pillars
After power amplifiers, beams moves into main amplifier- having eleven glass pillars again arranged at Brewster’s angle to each other.
Key Component : Optical Switch
A key component in laser system is a kind optical switch called pockel cells that allows the beams to pass through main amplifiers 4 times and thus amplfying it by billion times, upto 4 MJ.
Along the way are spatial filters and other optical systems associated with it, it checks the uniformity of beams and ensure that they are not mis-focused a bit.
Before taking the way towards the target , beams once again passes through power amplifier to have more boost in energy.
Till now beams are amplified by quadrallion times.
This pulse has very short duration of time , i.e femtosecond and thus providing peak power of 500 TW ( i.e 5 million million watts)
After Amplication, 24 beamlines having 8 beams each heads towards the switchyard before hitting the target chamber.
Switchyard
It re-arranges all beams in conical configuration (by using various mirrors), to impinge the target from different directions .
One of the last step is to convert infrated light to UV light in a device called frequency converter.
192 lasers enter the target chamber and focuses on holraum (a cylinder of size of a pencil eraser).
Now let’s discuss about target chamber and the target itself.
Target chamber is 10 metre wide and weighs 130,000 kg .In the centre of it is the target called hohlraum.
Hohlraum is made up of fusion fuel capsule sitting inside the gold can. Why gold is used? Because it is a very good reflector of electromagnetic radiation.
Fusion fuel is made up of deuterium and tritium (iso- topes of hydrogen) . Capsule has 3 layers – capsule wall, solid DT and gas DT.
192 beams heat the gold can , which re-emits the intense X- rays capable pushing into the capsule uniformly.
A metallic case called a hohlraum holds the fuel capsule for NIF experiments. Target handling systems precisely position the target and freeze it to cryogenic temperatures (18 kelvins, or -427 degrees Fahrenheit) so that a fusion reaction is more easily achieved.
192 Laser beams focuses on 0.2 cm fuel capsule
It highly compreses the capsule which creates extraordinarily high pressure to fuse two nuclei. From the reaction of nuclear fusion , we know helium, neutron and massive energy is produced.
Here, we are interested in the helium nucleus (alpha particle) produced. It cannot go away so far and contributes to more heating.
More heating leads to more fusion reaction in the capsule and thus again more helium/ alpha particles produced leading it to happen again and again – the Chain reaction.
Let this go on for 10 billionths of a second and BOOM !
You have your miniature star ready inside the laboratory on Earth, providing limitless clean energy.
This is what we call IGNITION – getting more energy out than we put in.
This is what NIF is working for.
We have still not reached that stage.
There are many challenges on the way. Only 10-20% of energy out of the total, contributes to ignition; because there are many types of losses in the hohlraum. It needs to have temperatures of 100 million°C ( more than the temperature of sun’s core) and pressure of 100 billion atmosphere.
“Fusion is the ultimate energy source in the universe, and one day it will power our society.” – prof. Gerald A. Navratil