Proton fusion, also known as proton-proton fusion, is a type of nuclear fusion that occurs in stars like our Sun. It involves the fusion of hydrogen nuclei (protons) to form helium nuclei. While proton fusion is the dominant process in the core of the Sun, achieving controlled proton fusion on Earth is extremely challenging due to several reasons:
Coulomb Repulsion: Protons carry a positive electric charge, and as such, they repel each other due to the electromagnetic force. To bring protons close enough for fusion to occur, the repulsive Coulomb force must be overcome. This requires high temperatures and pressures.
Low Cross-Section: The probability of two protons approaching each other with sufficient energy to overcome the Coulomb barrier and fuse is relatively low. This low probability is described by the fusion cross-section, which represents the effective area for fusion to occur. The proton-proton fusion cross-section is relatively small compared to other fusion reactions, such as those involving isotopes of hydrogen or helium.
Low Energy Density: Achieving the necessary temperature and pressure for proton fusion requires a high energy density. However, maintaining such conditions in a controlled environment is challenging. It requires confining and heating the fuel to extreme conditions to overcome the energy loss through radiation and conduction.
Thermal Equilibrium: In order for sustained fusion to occur, a plasma (a hot, ionized gas) must be created and maintained. However, achieving thermal equilibrium, where the temperatures and velocities of the ions and electrons are similar, is difficult. The high temperature necessary for fusion causes the ions and electrons to have significantly different energies, making it challenging to maintain a stable plasma.
Plasma Instabilities: Plasma confinement is prone to various instabilities, such as the magneto-hydrodynamic (MHD) instabilities, which can disrupt the fusion process. These instabilities can cause the plasma to lose its confinement, leading to energy loss and the termination of fusion reactions.
Overcoming these challenges requires advanced technology and innovative approaches. Scientists are currently exploring different methods for achieving controlled fusion, such as magnetic confinement fusion (e.g., tokamaks) and inertial confinement fusion (e.g., laser-driven implosion). Although significant progress has been made, practical and sustained proton fusion on Earth is yet to be realized.