Development of a fuseless small-bore railgun for injection of high-speed hydrogen pellets into magnetically confined plasmas

The most effective known way of refueling a tokamak fusion reactor is to inject high-speed pellets composed of fusion fuel (i.e., isotopes of hydrogen) at a controlled rate and velocity. To effect such a fueling scheme, in particular for contemporary and future large tokamaks, pellet speeds as high as 10 km/s and injection rates as high a 10 Hz may very well be required. Also, to prevent the onset of plasma instabilities pellet sizes need to be maintained below 3 to 4 mm in. diameter. These requirements, plus the fact that the yield strength of frozen hydrogen is extremely low (∽2 atmospheres) make the task of developing an ideal pellet injection scheme a challenge. In an attempt to meet this challenge, a fuseless small-bore railgun has been under development at the University of Illinois during the past several years. Some of the unique features of this railgun system are: 1) it is a two-stage accelerator with the first stage consisting of a combination of a hydrogen pellet generator and a gas gun, and the second stage a railgun, 2) it is a fuseless railgun in that the plasma armature is formed by electrically breaking down the propellant gas immediately behind the pellet, 3) it is a small-bore railgun with the bore size in the range of a few millimeters in diameter. Perforated sidewalls and transaugmentation schemes have also been employed in an effort to improve the railgun performance. The principal diagnostics are the magnetic probes, laser interferometry, optical emission spectroscopy, and a streak camera which are designed to determine the plasma arc length, the electron density distribution along the length of the arc, the line-averaged plasma density and temperature, and the plasma velocity change during acceleration, respectively. Using a prototype system hydrogen pellet velocities exceeding 2.8 km/s have been achieved on a 2-m-long railgun for a cylindrical pellet of 3.2-mm diameter and 4-mm length. This report presents a brief review of some of the existing hydrogen pellet acceleration techniques, an overview of the University of Illinois railgun program, the results to date, and the future plan.