One of the problems preventing the Multi-billion dollar ITER nuclear fusion project from scaling to provide commercial energy are disruptions of the plasma. Disruptions are sudden events that can halt fusion reactions and damage the tokamaks.
Princeton has developed an electromagnetic particle injector (EPI) which is a type of railgun that fires a high-velocity projectile from a pair of electrified rails into a plasma on the verge of disruption. The projectile, called a “sabot,” releases a payload of material into the center of the plasma that radiates, or spreads out, the energy stored in the plasma, reducing its impact on the interior of the tokamak.
Current systems release pressurized gas or gas-propelled shattered pellets using a gas valve into the plasma, but with velocity limited by the mass of the gas particles.
The risk of disruptions is particularly great for ITER, the large international tokamak under construction in France to demonstrate the feasibility of fusion power. ITER’s dense, high-power discharges of plasma, the state of matter that fuels fusion reactions, will make it difficult for current gas-propelled methods of mitigation to penetrate deeply enough into the highly energetic ITER plasma to take good effect.
On ITER, mitigation is desired in less than 20 milliseconds, or thousands of a second, from the warning of a disruption, with 10 milliseconds as ideal. Tests of the EPI prototype show that it can deliver a payload of correctly sized particles in fewer than 10 milliseconds, compared with 30 milliseconds for gas-propelled systems.
A novel, rapid time-response, disruption mitigation system referred to as the electromagnetic particle injector (EPI) is described. This method can accurately deliver the radiative payload to the plasma center on a less than 10 ms time scale, much faster, and deeper, than what can be achieved using conventional methods. The EPI system accelerates a sabot electromagnetically. The sabot is a metallic capsule that can be accelerated to desired velocities by an electromagnetic impeller. At the end of its acceleration, within 2 ms, the sabot will release a radiative payload, which is composed of low-z granules, or a shell pellet containing smaller pellets. The primary advantage of the EPI concept over gas propelled systems is its potential to meet short warning time scales, while accurately delivering the required particle size and materials at the velocities needed for achieving the required penetration depth in high power ITER-scale discharges for thermal and runaway current disruption mitigation. The present experimental tests from a prototype system have demonstrated the acceleration of a 3.2 g sabot to over 150 m s−1 within 1.5 ms, consistent with the calculations, giving some degree of confidence that larger ITER-scale injector can be developed.