Patent Number: 054065941
Section: summary

The present invention relates to an injection system of cryogenic pellets formed of solid slate hydrogen isotopes in a temperature range between about 4 and 19K to be supplied to machines for the magnetic confinement of plasma (for example F.T.U.). As it is known, plasma of the above machines should be supplied during the discharge for maintaining a predetermined density. The inlet, of hydrogen isotopes into the gas phase was proved to be ineffective due to the fast ionization of the gas preventing the plasma from being penetrated. A solution of this problem is the injection of the above machines with a solid state reactant at such a rate as to assure the deposition of the fusionable material at the deepest, layers of the plasma column. Such method further allows the material removed from the pellets to be deposited more or less deeply and then the density profile of plasma to be shaped as the dimensions and the rate of the pellets change. A general improvement of the confinement parameters has been noticed in plasma having a marked density at the centre so that injection rates higher than 2 km/s are being requested for such machines today, and even higher rates (5-10 km/s) shall be reached in the future, an impossible performance for the injection systems used today. Such systems can be divided into two classes: centrifugal injection systems; PA1 pneumatic single-stage injection systems. PA1 1) the pressure at the base of the pellet falls off very quickly by dynamic effect so that a high acceleration cannot be maintained for the requested time; PA1 2) the maximum acceleration is limited by the mechanical characteristics of the pellet (about 10.sup.7 m/s.sup.2); PA1 3) difficulty of providing a sufficient propelling gas stock at high temperature (500.degree.-600.degree. C.). PA1 1) the dynamic pressure drop at the base of the pellet is partially compensated by the leading edge of the pressure pulse generated at the input of the barrel; PA1 2) the pulsing pressure allows much more higher peak values (about 1200 bars) to be reached without damaging the pellet; PA1 3) the adiabatic compression allows the gas temperature to be increased up to several thousands of degrees for a time of about 100 microseconds, which also permits the sound propagation rate in the propelling gas and then the maximum rate of the pellet to be increased. PA1 a pneumatic system operating with hydrogen and/or helium and formed of one or more two-stage or multi-stage propulsion systems, the relative inlet circuits, and one or more decompression chambers; PA1 a cryogenic device formed of a Dewar flask containing liquid helium, a circuit for transferring and recovering the cooling fluid, and one or more conventional (in situ) or alternative cryostats like that described afterwards provided each with one or more launching barrels, in which the cryogenic pellets are solidified; PA1 a vacuum system including electrovalves, electropneumatic valves, rotating and turbomolecular pumps; and PA1 a set of equipment for the automatic remote control of the whole system and for collecting and supplying diagnostic data to the central processing unit. Centrifugal injection systems are characterized by a high injection frequency (number of pellets per second double as high as the rotation frequency of the system), however, by a low rate (.ltoreq.800 m/s) and a poor shot. These features are cause of drawbacks as the low rate does not allow the plasma column to be deeply penetrated, and the poor shot causes a large proportion of pellets not to pass through the entrance window of the plasma. Pneumatic injection systems, to which the system of the present invention belongs, are on the contrary characterized by a better shot and a higher rate (1600 m/s for single pellets, 1200 m/s for multiple injections up to a maximum of 6 pellets per second in two seconds). With the injection systems of the status of art based upon single stage propulsion systems the above rates are the highest limit which can be reached due to the inherent ineffectiveness of the propulsion systems essentially depending upon the following reasons: In case of conventional systems the sonic or infrasonic launching rate causes very cumbersome decompression chambers to be used (about 800/1000 l). The present invention seeks to avoid the drawbacks and the limits of the present systems and to provide a system for producing, accelerating and letting into magnetic confinement machines one or more cryogenic pellets at higher rates than those reached nowaday with the pneumatic single-stage propulsion systems. This has been provided according to the invention by using one or more two-stage or multi-stage propulsion systems provided with a pair of special valves, better described afterwards, one of which is a control valve, the other a cutoff valve. The use of pneumatic two-stage or multi-stage propulsion systems allows the injection rate to be increased up to about, 3 km/s (with non-protected pellets) because: It should be pointed out, that the above mentioned upper limit rate is determined in the present invention by the fragility of the pellet which cannot stand accelerations beyond the limit; which is imposed by the mechanical properties of the solid and not by the limits originating from the propulsion system itself having on the contrary a higher capacity than that requested for such application. The above mentioned quick control valve of the propulsion system is operatively different from those used nowadays. Actually the quick control valves used in similar systems are essentially electropneumatic valves in which the pressure-containing member is opened by a coil and closed by the differential pressure so as to reduce the flow of the propelling gas following the pellet. In the present case, both the opening and the closure of the pressure-containing member are based upon the principle of the differential pressure and then are executed very rapidly so as to improve the efficiency of the propulsion system and to limit the flow of the propelling gas. The above mentioned cutoff valve seeks to separate the propulsion system from the launching barrel during the condensation of the pellet, and to contribute to the limitations of the flow of the propelling gas following the pellet. Its characteristic is of assuring a good vacuum sealing with a considerably reduced dead volume and with an orifice having a diameter corresponding to the inner diameter of the launching barrel. In order to couple the injection system to an user a system is provided for removing and evacuating the propelling gases, said system being formed of one or more decompression chambers which may have considerably reduced dimensions (about 100/200 l) with respect to those used up to now because of the supersonic rate of the pellets. According to another advantageous feature the cryogenic device of said system has been miniaturized in order to reduce to the minimum the consumption of liquid helium, and has been provided with a particular pipe for transferring the cooling fluid (cold vapours of helium), thus allowing a better operation and preventing thermal oscillations during the cooling. According to a preferred embodiment the injection system according to the invention includes: