If we are to achieve our dreams of interstellar exploration beyond our Solar System, we need to explore propulsion mechanisms that allow us to travel faster and further than current rockets. Chemical rockets used today have a low fuel-efficiency of just 35% and would take 30,000 years to reach the nearest stars outside our Solar System.
There have been many proposed alternatives to interstellar travel; one of the most tangible of these is the direct fusion drive (DPD). The DPD is a conceptual rocket engine which could be used to propel and power spacecrafts using nuclear fusion. With the potential to reach speeds of up to 12% of the speed of light, this new propulsion mechanism would allow us to travel to Mars in 90 days rather than 8 months, with twice the efficiency of a chemical propulsion system.
Nuclear fusion involves the merging of two light nuclei into a single, heavier nucleus. When the lighter atomic nuclei are forced together, the resulting heavy nucleus is more tightly bound due to its higher binding energy. Subsequently, the mass of the fused nuclei amounts to slightly less than that of the constituent atoms; in keeping with energy-mass conservation, this mass deficit is released in the form of energy.
Proton-proton nuclear fusion, which naturally occurs in stars, is a 3-step process. First, the 2 protons (hydrogen atoms) fuse to create a deuterium atom, positron, and a neutrino. Next, the positron and deuterium atom collide to produce a gamma ray and an isotope of helium (He-3). Finally, two He-3 isotope combine to form a He-4 isotope and two more protons. Just one reaction cycle produces approximately 25,000,000eV of energy. This type of fusion constitutes 40% of reactions in the Sun, producing over 10% of its net energy.
In 1955, John D. Lawson demonstrated that fusion reactions require sufficient temperature, number density of charged particles, and energy confinement time. These conditions are collectively referred to as the ‘Lawson criterion’ and are quantified by the ‘fusion triple product’.
Proposed fusion reactors typically use the hydrogen isotopes deuterium and tritium. Whilst we have an abundance of deuterium on Earth, obtaining tritium is a more arduous process. Its radioactive half-life of ten years means that there is no sizeable natural source, and we must instead produce tritium from an isotope of lithium which is in abundance, namely Li-7. Regardless, this remains the best option for fusion reactors because the constituent isotopes are more reactive than regular H-atoms. This higher reactivity thus gives rise to a lower triple product, enabling us to satisfy the Lawson criterion under less extreme conditions.
But why do we require such extreme temperatures? What type of densities are optimal? And what exactly do we mean by energy confinement time?
When two nuclei are brought in close proximity, an electrostatic repulsion arises between them. This is known as a ‘Coulomb barrier’. For nuclei to merge and undergo fusion, the Coulomb barrier must be overcome using extremely high speeds, which is achieved by through extremely high temperatures. However, when temperatures soar to the extreme, particles will be moving so fast that the time frame in which particles are in close enough proximity with each other to fuse together will become increasingly small. The ideal temperature that will both overcome the Coulomb barrier and provide sufficient time for fusion sits at between 100-200million degrees.
A high density is important because sustaining the reaction relies on particles colliding and fusion. Despite this, the optimal conditions for density are surprisingly low; the ideal density is approximately one million times less dense than air. This is because, when density increases, collisions between nuclei and electrons generate increasing amounts of radiation, known as bremsstrahlung. At high densities, bremsstrahlung can dominate so heavily that all power in the plasma is radiated away.
To sustain such extreme conditions, we use plasma, which is superheated matter found in the cores of stars. The high temperature of plasma tears electrons away from their atoms to effectively form a gas of ionised particles. Whilst these temperatures are ideal for fusion, they also make the substance incredibly difficult to confine. Since plasma will melt materials that it comes into contact with, we use strong, toroidal magnets to prevent particles in the plasma from reaching the walls of the reactor.
First introduced in the mid-1960s by Soviet plasma physicists, toroidal magnets are the most developed method of plasma confinement to date. Some radiation waves are, however, powerful enough to escape the field. The escaping of these waves reduces the efficiency and can potentially halt the fusion reaction altogether if losses exceed energy input. The rate at which energy from the plasma is lost to the environment is referred to as ‘energy confinement time’, the third key factor in the fusion triple product.
With the success of the Soviet tokamak in the 1970s, fusion research gained a lot of traction. Today, many countries are participating in fusion research. The Tokamak Fusion Test Reactor (TFTR), which now resides at Princeton, has almost reached Lawson’s criterion. Meanwhile, in May 2021, China’s HL-2M Tokamak broke records when it achieved temperatures of 120 million degrees Celsius for 101 seconds, and 160 million degrees Celsius for 20 seconds. These achievements are critical milestones to achieving viable nuclear fusion reactors and provide plenty of reason to be optimistic that we will soon possess the technology required to transform interstellar travel.