Nuclear Fusion – the reaction that powers the sun and stars has been an elusive dream of scientific community for decades. A fusion power plant fueled by burning plasmas, having temperatures of about 100 million K, could provide limitless amounts of clean and safe energy for humankind. All this by using just a small amount of electricity and a hand full of hydrogen isotopes. Isn’t it amazing?
But the laboratory conditions cannot recreate the gravitational force that confines such highly energetic or in other words, high temperature charged particles. In fact, even the wall of the chamber in which fusion reaction is prospected to be carried out shall not be able to bear such highly energetic particle flux on its wall. We can understand this fact by considering that Tungsten is the metal having highest melting point of 3700 K. Can a crucible made of Tungsten contain a plasma having temperature an order of magnitude greater than that of Sun? No, certainly not!
To solve this problem, Fusion researchers came up with a couple of solutions:
A way to confine high temperature plasmas in a fusion device is by using the technique of Inertial confinement (ICF). The plan is to initiate nuclear fusion reactions by heating and compressing a fuel target using high-energy laser beams. But still Inertial Confinement fusion experiments have failed to reach ignition. So we shall talk more about the challenges associated with another confinement technique called magnetically confined fusion in this article.
As we know that an applied magnetic field can deflect a particle’s trajectory and a charged particle moving with a velocity ‘v’ would generally follow a helical path around a magnetic field line ( See figure 1 ) . One way to confine a plasma in a nuclear fusion device with toroidal geometry is to use a cage of magnetic field lines that would confine the particles in such a way as to prevent their leaking to the walls of the fusion chamber. Such a device is called a tokamak.
Now, we have some efficient confinement techniques, so what is stopping us from attaining fusion at a scale where we can have limitless amounts of electricity from it?
Another challenge is to heat the fusion plasmas to such high temperatures. The technique of Neutral Beam Injection (NBI) comes to the rescue.
Neutral Beam Injection
Neutral Beam Injection is the technique in which high-energy neutral particles are injected in a magnetically confined fusion device. These neutral particles then undergo collisions with the thermal plasma ions and get ionized. So now that we have high-energy charged particles rather than neutral particles, they will further transfer their energy to the thermal ions by collisions and heat up the plasma. Also, once the nuclear reactions begin, they will generate their own energetic alpha particles as a by-product of fusion reactions. These energetic particles will help in keeping the plasmas hot and the nuclear reactions self-sustaining.
But is it that simple? The answer is no.
Due to the complex geometry of magnetic field lines, there may arise some disturbances or perturbations as we say. These disturbances may give rise to certain waves such as Alfven waves in the fusion plasma. Now, the energetic particle may undergo resonant interactions with these waves, exchange energy with them and may drive them unstable.
Instabilities and Disruptions In a Nuclear Fusion Device
Imagine that you pluck a string of a guitar and it started vibrating with a particular amplitude and you hear the sound. Now what if you keep plucking the string again and again at that point with a higher force and the amplitude of vibration keeps on increasing. After some time, the string will break.
Similar things happen when you drive an old car and it starts vibrating at a particular speed. When you increase or decrease the speed slightly, the car stops vibrating. This is because at that speed, the different components of the car started vibrating with the same frequency and the frequency matching created resonance. Hence, the amplitude of vibration adds up to vibrate the car enough so that you can feel it.
The unstable modes of oscillations may then resonate with the energetic charged particles in a fusion plasma and the charged particles might deliver some of their energy to those oscillatory modes. So the amplitude of oscillations go on increasing that causes variation in charged particle trajectory and result in high-energy fluxes of energetic ions on the fusion chamber wall. Rapid growth in these instabilities may also cause disruptions in a plasma and hence poor plasma heating efficiency.
It is very important to understand the physical phenomena underlying the wave-particle interactions in a fusion plasma. Plasma physicists are using various simulation codes and comparing them with experiments to analyze the regime of operation of the neutral beam as well as alpha particles. We already have fusion devices that contain plasmas surpassing solar temperatures. But we still need to develop the technology and scientific understanding to sustain burning plasmas for hours or steady state.