Modeling of Low-Acceleration Two-Stage Guns for Tokamak Refueling

A system to damp the free piston oscillations in a two-stage light-gas gun used for hypervelocity impact experiments

“…[8][9][10][11][12][13][14][15] Since the efficiency of the penetration of the pellet in the confining magnetic field increases with the velocity and the mass of the injected projectile, 11 specific technologies have been introduced to achieve quasisteady state operations: 11,12 fast valves replace rupture disks, highpressure gas is used instead of powder in the first stage, valve systems return the piston to its initial position and refill the first and second stage volumes to the preset pressure, and dedicated mechanisms reload the breech with a new projectile after each shot. 12 Although these technologies are a fundamental starting point for the development of a high-shotfrequency gun, their implementation to a gun for impact testing represents a technological challenge.…”

“…The model was developed starting from well-known models already developed. 8,17,18 The model of Milora et al 17 is user-friendly, but does not take into account the heat exchange of the gas with the wall. Riva/Reggiori's 8,18 model, on the other hand, explicitly takes into account this heat exchange and gas blow-by between the gun wall and the piston's external surface, but its formulation is a little bit more complicated to implement in a form which is useful for a full system optimization.…”

“…The model implemented in this work is based on the equation written by Milora et al 17 for the Quickgun algorithm, and the heat exchange term and the blow-by terms suggested by Riva/Reggiori. 8,18 The code was then implemented in a symbolic language 19,20 building up macroblocks simulating all the main subsystems. The model also takes into account the possibility of using two gases together.…”

“…8,17,18 Several studies have been conducted in other laboratories 2 to evaluate the effect of the nonideal properties of gases on propellant behavior, highlighting that a real gas at a high temperature could be a better propellant than a perfect gas at the same temperature and pressure. Nonetheless, to simulate the behavior of the gas in the second stage, the hypothesis of an ideal gas is normally accepted.…”

“…Nonetheless, to simulate the behavior of the gas in the second stage, the hypothesis of an ideal gas is normally accepted. 8,17,18 Although, no specific tests have been done in our laboratory to verify this assumption, the overall calibration procedure is validated by the evidence that experimental results are correctly predicted in all the working conditions of the gun. The numerical model was calibrated and tuned by comparing the model outputs with experimental results 21,22 and proved to be a reliable tool in predicting the gun performance for a wide operational range 21,22 from a few hundred bars to a few thousand bars of maximum pressure produced in the pump tube.…”