Compound cryopump for fusion reactors

Abstract

We reconsider an old idea: a three-stage compound cryopump for use in fusion reactors such as DEMO. The helium "ash" is adsorbed on a 4.5 K charcoal-coated surface, while deuterium and tritium are adsorbed at 15-22 K on a second charcoal-coated surface. The helium is released by raising the first surface to ~30 K. In a separate regeneration step, deuterium and tritium are released at ~110 K. In this way, the helium can be pre-separated from other species. In the simplest design, all three stages are in the same vessel, with a single valve to close the pump off from the tokamak during regeneration. In an alternative design, the three stages are in separate vessels, connected by valves, allowing the stages to regenerate without interfering with each other. The inclusion of the intermediate stage would not affect the overall pumping speed significantly. The downstream exhaust processing system could be scaled down, as much of the deuterium and tritium could be returned directly to the reactor. This could reduce the required tritium reserve by almost 90%. We used a well-established free Direct Simulation Monte Carlo (DSMC) code, DS2V. At very high upstream densities (~1020 molecules/m3 and above) the flow into the pump is choked. Enlarging the aperture is the only way to increase the pumping speed at high densities. Ninety percent of the deuterium and tritium is successfully trapped at 15 K (assuming that the sticking coefficient is 80-100% on the 15-22 K surface). On the other hand, the remaining 10% still exceeds the small amount of helium in the gas input.Comment: 12 pages, 10 figures. For publication in Fusion Engineering & Desig

Vacuum System Optimization for EAST Neutral Beam Injector

“…The gas molecules first collide with the radiation baffle and then are condensed by the low-temperature surface. Therefore, the temperature of the gas molecules that collide with the simplified low-temperature condensation surface was set at 80 K. The cryopump adopts a classic chevron radiation baffle, and the transmission probability for gas molecules on the surface of the cryogenic condensation surface is 0.23 [8,9]. Since the beam collides with the ion dumps, calorimeters, etc., there is a significant rise in temperature, and the temperature has a greater impact on the movement of gas molecules in the vacuum environment.…”

Consequences of the technology survey and gap analysis on the EU DEMO R&D programme in tritium, matter injection and vacuum

“…Having said that, all primary pump technologies that do only provide the requested pumping speed and capacity at very low pressures (such as sublimation pumps and ion pumps) are not appropriate. In summary, three pump technologies have been identified as at least partly suitable:  Cryogenic pumping (score of 50%): The standard pump technology used on modern medium-size tokamaks could be an option but require significant R&D to implement a hydrogen isotope separation function for the DIR [48,49] (R&D topic 13).…”

Effect of rotational and vibrational degrees of freedom in polyatomic gas heat transfer, flow and adsorption processes far from local equilibrium

“…Then, these estimates may be applied in deriving boundary conditions for the simulation of adsorption processes in similar set-ups. Following this procedure the sticking coefficients of several gases for cryopanels coated with activated carbon at very low temperatures have been estimated [160,161] and then, they have been applied in the numerical modeling of cryopumps used in the main pumping systems of magnetic confinement fusion reactors [116,117].…”

“…Next, the same configuration is modelled combining both DSMC and TPMC methods in [116] providing detailed information of the gas flow field such as pressure and temperature distributions, number of particles absorbed by each cryopanel, etc. In [117] a three-stage compound cryopump for use in fusion reactors such as DEMO (DEMOnstration Power Plant) is modelled based on the DSMC method.…”