ITER Vacuum System Research Articles

Dynamic model of ITER Front-End Cryopumps Distribution System: Torus cryopumping and regeneration scenarios

The ITER vacuum system will use first of a kind cryopumps to provide high vacuum conditions to the torus vacuum vessel, the cryostat, and the neutral beam injectors. In order to evacuate the high gas flows required by the plasma scenarios, the cryopumps will need sequential regeneration with unprecedented high frequencies.The Front-End Cryopumps Distribution System, made of 12 cold valve boxes and 1 warm regeneration box, will provide helium to each of the cryopumps at the required pressure, temperature and mass flowrate, with dynamic synchronization with plasma pulses, in order to control the pumps behaviour during pumping and regeneration.A model of the cryopumps and their cryogenic distribution system is being developed, with the purpose of simulating the cryogenic process in dynamic scenarios and optimize the final design and the control of the system. This paper presents the dynamic model of the torus subsystem, made of 6 torus cold valve boxes, and the connected cryopumps, during operation and 100K regeneration scenarios. The preliminary simulation results will be presented, giving useful information for a refined setting of parameters like pressures, mass flow rates, and valves control. These results are suitable to be used to optimize the distribution system final design and commissioning.

Physisorption of ammonia on AISI 304 L stainless steel at different surface temperature under high vacuum conditions

The physisorption of ammonia molecules (sticking) on the walls of a stainless steel pipe (AISI 304L) has been studied at different wall temperatures (323-473K). The total amount of ammonia that is retained on the walls, once equilibrium is reached, has been measured by differentially-pumped mass spectrometry in gas exposure laboratory experiments. The results show ammonia retentions in the range of μg/cm2 resulting in a multilayer adsorption with lower amounts of stuck ammonia at higher temperatures of the stainless steel surface. The sticking coefficient follows an exponential decay evolution with time. The activation energy of the process has been estimated by an Arrhenius fit, assuming that the characteristic time for this decay is inversely proportional to the kinetic adsorption constant. A value of 0.15eV per ammonia molecule has been obtained, being in agreement with nominal values for the physisorption of small molecules or atoms (CO, N2, Ar…) that can be found in the specialized literature. The implication of these results in the possible extrapolation to the ITER vacuum system under nitrogen seeded plasma operation is also addressed.

Progress in the design of ITER front-end vacuum instrumentation and controls

The ITER vacuum system will be one of the largest and most complex vacuum systems ever to be built. Extensive instrumentation and controls are being developed to satisfy the stringent vacuum processes necessary for the successful and safe operation of the ITER Tokamak. The complexity and deep integration of the vacuum systems within the ITER machine presents a challenge to implement all of the controls necessary for reliable operation. Several thousand valves and sensors have to be implemented within the harsh environmental conditions of the Tokamak vicinity, and require engineering of instrumentation and controls with remote electronics solutions.In this paper the status of the design of field end vacuum controls and instrumentation for the ITER vacuum systems is described. Details of the progress on selection of sensors and actuator technologies are given herein and solutions for remote device operation, including those for cryogenic devices, are described together with necessary local shielding.

Gas species, their evolution and segregation through the ITER vacuum systems

This paper takes the ITER fueling requirements and current knowledge of gas balance and exhaust from operating tokamaks to predict all likely gas inputs into the ITER Vacuum systems. Areas where gas dynamics modeling is relevant to the ITER design are highlighted. The design and operation of the ITER vacuum system gives an element of segregation of different gas flows and species. This paper analyses the time dependent gas segregation in the vacuum system resulting from different temperature dependences of cryogenic sorption and condensation processes of different gas species. As a specific example, the optimal transfer of 41Ar through the vacuum system is studied with respect to its decay and the resulting effects on the design of system components.

Outgassing measurements for the ITER EC H&CD Upper Launcher

In most of applications involving both vacuum and high temperatures, outgassing of structural materials is a critical issue. For instance, this is the case of fusion test devices, where the gas released from the vessel walls contaminates the plasma. Four upper ports in the ITER vacuum vessel are reserved for Electron Cyclotron Heating and Current Drive (EC H&CD) Upper Launchers (UL), which have to provide plasma MHD stabilization by localized deposition of high power microwave beams. The structural material foreseen for the UL is the 316L(N)-IG stainless steel. It has to withstand temperatures in the range 120–150 °C during normal operation and 240 °C during the baking process. One of the preferred manufacturing routes for the UL is the Hot Isostatic Pressing (HIP) which is a very sophisticated method to manufacture structural components of complex geometry with good mechanical properties. The materials for use in the ITER vacuum systems have to comply with the outgassing limits given in the ITER vacuum handbook, but no outgassing data for HIPed stainless steel are available in literature, thus they must be obtained by experimental measurements. In this paper, measurements of partial outgassing rates are shown and discussed for stainless steel prototype samples AISI 316LN (on which the 316L(N)-IG is based) and AISI 317LMN, obtained by powder and solid HIPing method. A variant of the gas throughput method in vacuum systems was used for the measurements which were carried out over periods larger than 8 h and at different temperatures.

Conductance modelling of ITER vacuum systems

This paper presents the full network model of the ITER torus vacuum system including the toroidal and radial conductances inside the divertor cassettes, the by-pass leaks between and inside the cassettes, the conductances between the divertor cassettes and the vacuum vessel and inside the pumping ducts up to the cryopumps. The model was built in ITERVAC which is a program for smooth simulation of the mass flow through ducts at isothermal conditions in a wide flow regime including the difficult to describe transitional flow regime that is most relevant to the application for ITER. Simulations were performed for the burn and dwell mode of ITER; deuterium was used as model gas within the simulations. The results show for the burn phase (with four torus cryopumps in pumping mode) a strong back streaming of gas into the plasma which can be 30% higher than the actual throughput to the torus cryopumps. For the dwell phase the effect of pumping with four and eight torus pumps was investigated, respectively. It is shown in this paper that the throughput can be increased between 30% (end of dwell) and 60% (start of dwell) by using eight pumps.

The vacuum systems of ITER

ITER is a large vacuum facility which comprises many service, diagnostic and monitoring vacuum sub-systems as well as three large cryogenic pumping systems for evacuation and maintenance of the requested pressure levels. The presence of hydrogen (including the radioactive isotope tritium) and exclusion of other gases defines the parameters of fusion vacuum systems. This paper will focus on the areas of the ITER vacuum systems which require customized developments and cannot rely on commercial solutions. The complex pumps have been tailored for the very specific applications and requirements at ITER, characterized by high magnetic and radiation fields, excellent availability and maintainability and, especially, the need to be tritium-compatible. An outline of the development path which was needed to come up with a sound design for the ITER cryopumps is given and the status of the programme in view of the imminent manufacturing phase is described in this paper.

The ITER vacuum systems

ITER is a large vacuum facility which comprises many service, diagnostic and monitoring vacuum sub-systems as well as three large cryogenic pumping systems for evacuation and maintenance of the required pressure levels. Control of the gas throughput is one of the key issues affecting the performance and achievable burn time of a fusion reactor. The main pumping systems are the torus exhaust pumping, the cryopumps for the neutral beam injection systems for plasma heating, and the cryopumps for the ITER cryostat. All customized cryosorption pumps are force-cooled with supercritical helium and share a similar modular design of cryosorption pumping panels. For regeneration of the cryopumps as well as for roughing down the system volumes prior to operation, four identical sets of forepump trains are used. This paper will focus on the areas of the ITER vacuum systems which require customized developments and cannot rely on commercial solutions. The complex pumps have been tailored for the very specific applications and requirements at ITER, especially characterised by the need to be tritium compatible. An outline of the development path which was needed to come up with a sound design for the ITER cryopumps is given. The way of development is culminating in the manufacturing of 1:1 scale prototypes, which will be extensively tested in dedicated test facilities to ensure compatibility with all design requirements.

ITER primary cryopump test facility

A cryopump as ITER primary vacuum pump is being developed at FZK under the European Fusion Technology Programme. The ITER vacuum system comprises 16 cryopumps operating in a cyclic mode which fulfills the vacuum requirements in all ITER operation modes. Prior to the construction of a prototype cryopump, the concept is tested on a reduced scale model pump. To test the model pump, the TIMO facility is being built at FZK in which the model pump operation under ITER environmental conditions, except for tritium exposure, neutron irradiation and magnetic fields, can be simulated. The TIMO facility mainly consists of a test vessel for ITER divertor duct simulation, a 600 W refrigerator system supplying helium in the 5 K stage and a 30 kW helium supply system for the 80 K stage. The Model Pump Test Programme will be performed with regard to the pumping performance and cryogenic operation of the pump. The results of the model pump testing will lead to the design of the full scale ITER cryopump.