Christian Day

Computational and experimental study of gas flows through long channels of various cross sections in the whole range of the Knudsen number

A computational and experimental study has been performed for the investigation of fully developed rarefied gas flows through channels of circular, orthogonal, triangular, and trapezoidal cross sections. The theoretical-computational approach is based on the solution of the Bhatnagar-Gross-Krook kinetic equation subject to Maxwell diffuse-specular boundary conditions by the discrete velocity method. The experimental work has been performed at the vacuum facility “TRANSFLOW,” at Forschungszentrum Karlsruhe and it is based on measuring, for assigned flow rates, the corresponding pressure differences. The computed and measured mass flow rates and conductance are in all cases in very good agreement. In addition, in order to obtain some insight in the flow characteristics, the reference Knudsen, Reynolds, and Mach numbers characterizing the flow at each experimental run have been estimated. Also, the pressure distribution along the channel for several typical cases is presented. Both computational and experime...

Performance of ITER-Relevant Cryopump Panels for Tritiated Gases

A cryosorption panel test arrangement was installed in the Cryogenic Forevacuum (CF) Subsystem of the Active Gas Handling System (AGHS) at JET. The pump panels were of ITER relevant design in terms of geometry and dimension, coating and sorbent material. The central objective of this investigation was to study, for the first time in such an in-depth and parametric way, the interaction of tritium and tritiated gas mixtures with the panel and the influence on pumping performance and regeneration characteristics. This paper describes how the pump was implemented in the system and summarizes the major experimental results obtained in a two-staged programme: First, the test set-up was used to pump process gases under the Trace Tritium Campaign at JET; secondly, a dedicated test campaign was performed with defined external supply of tritium via a U-bed. It is highlighted that the ITER cryosorption pumping concept achieves highest pumping speeds for tritium. No show-stoppers have been identified.

Outgassing rate measurements of stainless steel and polymers using the difference method

This paper presents a new outgassing test facility based on the difference method, which is a rarely used modified throughput technique. Furthermore, the experimental approach presented is validated based on new measurement data for three relevant materials. The facility allows to measure at a good resolution thermal outgassing rates at variable temperatures between room temperature and 300 °C. The measurement approach is discussed, the facility is described, and the measurement uncertainty is evaluated. Three materials were measured to demonstrate the new experimental approach and to compare the results obtained for stainless steel 316L and two polymers, Viton® and Vespel®. For stainless steel 316L, the influence of different pretreatments was analyzed. It was found that by baking in vacuum as well as in air (400 °C, 100 h), the outgassing rate can be reduced by almost a factor of ten, in line with predictions based on outgassing theory. The polymers were investigated, as they represent materials with qu...

Tritium pumps for ITER roughing system

The ITER roughing system provides for both the initial pump-down of the vessel itself and the regular pump-out of the batch-regenerating cryopumps. This system must have a large pumping speed and cope with the radioactive gas tritium at the same time. The present paper shall highlight the results of the ITER roughing train optimization, discuss the modification of a Roots pump for tritium, and present the results of a ferrofluidic seal test and the first tests of a tailor-made tritium-proof Roots pump with inactive gases.

Experimental results and numerical modeling of a high-performance large-scale cryopump. I. Test particle Monte Carlo simulation

For the torus of the nuclear fusion project ITER (originally the International Thermonuclear Experimental Reactor, but also Latin: the way), eight high-performance large-scale customized cryopumps must be designed and manufactured to accommodate the very high pumping speeds and throughputs of the fusion exhaust gas needed to maintain the plasma under stable vacuum conditions and comply with other criteria which cannot be met by standard commercial vacuum pumps. Under an earlier research and development program, a model pump of reduced scale based on active cryosorption on charcoal-coated panels at 4.5 K was manufactured and tested systematically. The present article focuses on the simulation of the true three-dimensional complex geometry of the model pump by the newly developed ProVac3D Monte Carlo code. It is shown for gas throughputs of up to 1000 sccm (∼1.69 Pa m3/s at T = 0° C) in the free molecular regime that the numerical simulation results are in good agreement with the pumping speeds measured. Me...

Development of Advanced Exhaust Pumping Technology for a DT Fusion Power Plant

The main functions of the exhaust pumping system of a Deuterium-Tritium (DT) fusion device are to pump out the helium ash and control the divertor neutral gas density. This requires the handling of large gas throughputs at high pumping speeds (but at relatively moderate vacua). The pumped exhaust gas is then usually transported to the tritium plant for cleaning, which involves impurity removal and separation of the pure hydrogenic species for reinjection as fuel. In view of a fusion power plant, a systematic technical review of primary and roughing pump technologies is conducted in order to identify potential exhaust pumping concepts, which eliminate some of the disadvantages that eventually result from simple scale-up of the ITER solutions that are based on batchwise operating cryogenic pumps. This paper also illustrates the methodology applied to come to unbiased results and describes the final configuration, which is based on a vapor diffusion pump as primary pump together with a metal foil pump for hydrogen separation, and a liquid metal ring pump as roughing pump. All pumps are working continuously and do not require cryogenic temperatures. The new concept will reduce the tritium inventories of a power plant: 1) because of the continuous pumping characteristics of the pumps involved and 2) because the metal foil pump allows for internal recycling of the unburnt fuel species directly from the divertor to the fuelling systems, bypassing the tritium plant. A research and development program was initiated in EU to demonstrate the feasibility of this novel approach and, thus, to make it the reference solution for a fusion power plant. The current status in this effort is summarized, and the test facility to be employed is described in full detail.

Leak rate of water into vacuum through microtubes

A numerical model of water leak into vacuum through microcracks and crevices is proposed. A crevice is modeled by a long tube with a radius size of the order of 1 μm. It is assumed that the water is in its liquid phase in the inlet, while it is in its gaseous phase in the outlet. In such a situation, the evaporation of water happens inside the tube when the water pressure reaches the pressure of saturated vapor. The molecular mean free path of vapor varies in a wide range along the tube so that the flow changes from the hydrodynamic to the free-molecular regime. Under such conditions, the water leak is calculated by combining the methods of rarefied gas dynamics and continuum mechanics. The leak rate is calculated for some values of the tube radius and several temperature gradients. The pressure distribution along the tube is also reported.

Effect of the Dome on the Collisional Neutral Gas Flow in the Demo Divertor

This paper presents the analyses of the dome effect on the pumping efficiency and screening of the neutrals for the European DEMO ITER-like divertor configuration of 2015. The effect of the dome on neutral compression in the private flux region is assessed by using the DIVGAS code based on the direct simulation Monte Carlo method. The numerical analysis includes the calculation of neutral density, temperature, and pressure in the divertor plenum and the overall conductance of the subdivertor structure, which consequently affects the estimation of the effective pumping speed. It is quantified how the presence of the dome impedes the reflux of neutrals toward the plasma. In addition, due to the intermolecular interactions, the dome does not strongly influence the macroscopic quantities near the pumping duct. A strong reflux of neutrals in the domeless case, as it was previously found in the free molecular flow case, is confirmed to be present also in the case of a collisional flow.

Conceptual design of the mechanical tritium pumping system for jet DTE2

Abstract This paper describes the conceptual design of the Mechanical Tritium Pumping System (MTPS) that shall be installed and tested at JET during the next Deuterium-Tritium-Experiment (DTE2). This pump train uses a two-stage liquid ring pump in combination with a booster pump to cover a pressure regime from 10-1 Pa to 105 Pa. As working fluid for all pumps, mercury will be used for tritium compatibility reasons. Starting from the requirements to MTPS, the pumps and their arrangement will be described in this paper as well as the mercury containment strategy and safety- and control issues.

TIMO-2 - a cryogenic test bed for the ITER cryosorption pumps

The Karlsruhe Institute of Technology (KIT) has been carrying out research and development in the field of vacuum cryopumps for nuclear fusion devices over the last decade. Together with the development activities also experience in the operation of the needed cryogenic systems necessary for such type of large scale cryopumps was collected. Due to the specific requirements of a large fusion device, such as ITER, the cryogenic distribution is based on gaseous helium at the needed temperature levels rather than liquid nitrogen or liquid helium. KIT has set up a large scale research facility, called TIMO-2, fully equipped with supercritical helium supply at large flow rates to be able to perform cryogenic tests of components under ITER-relevant conditions. During first test campaigns at TIMO-2 with a large scale model cryopump the ITER cryosorption vacuum pumping concept was successfully validated. After major refurbishments and upgrades, the TIMO-2 facility is now ready for the acceptance tests of the ITER ...