Historically, humans have relied largely on fossil fuels such as coal and oil to generate the energy needed for daily life, and the demand for energy is predicted to increase substantially in the coming years. In fact, global energy demand is expected to reach 17.7 billion tons of oil by 2030, which is an increase of 55 percent since 2005 – and about 80% of current fuel consumption can be attributed to fossil fuels. However, due to both supply issues and the harmful effects of fossil fuels on air quality, pollution, and the global climate, it is becoming increasingly clear that fossil fuels are not a sustainable long-term source of energy. For these reasons, many experts are adamant that a cleaner, more sustainable form of energy must be adopted soon, and nuclear energy offers a promising possibility.
Nuclear reactors make use of a phenomenon called nuclear fission. Nuclear fission of heavy elements was first discovered in 1938, with the first nuclear reactor being built in 1942. In fission reactions, energy is released when an atom’s nucleus is split into two or more smaller nuclei. Typically, the splitting of nuclei occurs when atoms are bombarded by neutrons, which results in an unstable isotope that can undergo fission. In turn, the neutrons that are released when the nuclei split can be continuously used to trigger more fission, resulting in a self-sustaining chain reaction. The heat produced from this reaction can be used to heat water, creating steam that is then used to power turbines and generate electricity.
While all current nuclear reactors use the process of fission to produce power, there have been many attempts to harness the power of nuclear fusion instead. In doing so, scientists hope to develop a safe, waste-free method of creating large amounts of energy. However, nuclear fusion is a complicated process with its own set of challenges that must be overcome before application can become widespread, and several facilities around the world are striving to harness its power.
Nuclear fusion involves the combining of atomic nuclei. It is this same phenomenon that powers stars like the sun. Like nuclear fission, fusion reactions can produce massive amounts of energy that can then be used to generate electricity; however, optimizing the process and ensuring its safety pose large challenges. For instance, on the sun, the force of gravity is strong enough to allow fusion to take place, but the environment on earth is very different. The atoms must be heated to extremely high temperatures – over 50 million degrees Celsius – and properly contained so that fusion reactions can occur.
In order to contain these sweltering hot fusion reactions, experimental chambers called tokamaks are being explored. Inside a tokamak’s donut-shaped vacuum chamber, gaseous hydrogen fuel receives the heat and pressure necessary to turn it into plasma – an ionized gas that is essentially a ‘soup’ of electrons, protons, and neutrons. Under these conditions, the hydrogen atoms are able to fuse together and generate energy. There is no long-term nuclear waste produced, the machine can be turned off with the press of a button, and there is no risk of nuclear accidents based on the conditions and quantity of fuel within the vessel. The tokamak was first developed in the 1950s, but its safe and stable operation is something scientists are still trying to optimize.
One challenge associated with nuclear fusion reactors is obtaining diagnostic information about the plasma within the vacuum chamber. Plasma rotates as it heats up, and this causes it to emit x-rays; analyzing the energy and width of the x-ray emission lines yields information about the temperature and speed of the plasma. This quantitative information assists in controlling the magnetic fields necessary to keep the tokamak operating smoothly. In order to accomplish this analysis, a high-resolution x-ray spectrometer with an accuracy of around 1 eV is required. The spectrometer contains a bent single crystal that acts as an analyzer by diffracting a small range of x-ray energies. Each x-ray energy that is diffracted will hit a different position on the face of the area detector. The pixel position on the detector dictates the energy of the photon. However, although the spectrometer’s detector can pick up the diffracted signal and intensity, the actual energy of the photons is unknown. To convert the photon into an accurate energy value, an external x-ray source such as an x-ray tube is required to calibrate the spectrometer.
Working with Luis F. Delgado-Aparicio at the Princeton Plasma Physics Laboratory, Proto was able to create a custom cadmium x-ray tube to enable calibration of these spectrometers. The wavelengths of the x-ray lines emitted from these particular plasmas were around 3 keV, making cadmium an ideal x-ray source for calibrating the spectrometer, as its characteristic L-lines are within that region. Due to its low melting point (approximately 321°C), cadmium tends to be difficult to implement in a working x-ray tube. Thanks to Proto’s in-house x-ray tube facility, the ideal x-ray tube could be designed and manufactured, allowing users to obtain meaningful information about the plasma’s speed and temperature. Eventually, this type of calibration tool could provide a better understanding of plasma behaviour, leading to better plasma production and the creation of a viable fusion reactor.