Vacuum Vessel Baking Research Articles

Design and manufacturing of resistive wall mode control coil of JT-60SA

Resistive wall mode control coils (RWMCs) will be installed in the vacuum vessel of JT-60SA. Since 200 °C baking of vacuum vessel on JT-60SA is performed to remove the impurity in the plasma facing components, in-vessel components must be made with heat-resistant materials such as metal and ceramic or must be cooled with water. We looked for the heat-resistant organic insulator for JT-60SA RWMC and performed the heat resistant test for several resins. However, no resin passed the test (Sukegawa et al., 2015). We again performed the heat resistant test of several organic insulator in the circumstances without air and finally found that the some Bismaleimide–Triazine (BT) resin has enough heat resistance without air. We designed the eight-turn RWMC with the BT resin. Based on this design, the specification of RWMC and power supply requirement are decided. For the design of RWMC, the finite element (FEM) analysis of electromagnetic force during disruption and thermal stress during baking and operation were performed. Manufacturing of the RWMC has already launched.

Experience of pumping the vacuum vessel of SST-1 during the baking cycle with indigenously developed liquid nitrogen cooled sorption pump

To achieve ultra-high vacuum in the vacuum vessel of SST-1, the baking of vacuum vessel is done to remove impurities like water vapour, carbon-dioxide and hydrocarbons etc. The maximum baking temperature of the plasma-facing components (PFCs) are around 250 °C and the vacuum vessel is around 150 °C. The gas load of water vapour increases significantly during the baking operation. Therefore, to pump the water vapour during baking, a customized liquid nitrogen cooled sorption pump was designed, fabricated and installed on the radial port of the machine. It is the first time that liquid nitrogen cooled Cryopump was used on the SST-1 machine. The pumping of water vapour, nitrogen and impurity gases was observed during the 11 days of operation. Commercial Cryopumps are operating at ∼10 K temperature and can pump hydrogen gas however, the direct mounting of these pumps during baking results in early regeneration of the pump due to high heat load. Therefore, the experience of pumping the vacuum vessel of SST-1 with the indigenous developed sorption pump in association with the Turbo-Molecular Pump (TMP) sets is discussed systematically in the present report. The partial pressure of the water vapour using TMP sets was 1.0E-6 mbar and after incorporating the sorption cryopump with the existing vacuum system, the partial pressure reached to 1.3E-7 mbar in 48 h at 230 °C PFCs temperature. The pressure attained in the vacuum vessel was ∼1.0E-7 mbar and the partial pressure of water vapour was ∼1.1E-8 mbar after ramp-down the PFCs to 50 °C temperature.

Recent achievements in R&D on fibre optics current sensor for ITER

ITER is expected to achieve new milestones on the way to commercial fusion energy production. This will be still a research machine where numerous diagnostic systems are deployed, both already used on existing tokamaks and developed specifically for burning plasma operation. One of the innovative diagnostics is the Fibre Optics Current Sensor (FOCS). This paper describes recent results of the ITER FOCS development, concentrating on the front-end design. The FOCS sensing fibre, likely acrylate coated, will be located directly on the external surface of the vacuum vessel. During operation, it will be exposed to temperatures of 100–110 °C and multi-MGy level total radiation doses, while during vacuum vessel baking intervals the temperature will be 200–210 °C with low levels of radiation. Such temperatures are outside the nominal range for acrylate coatings even for acrylate coatings specially designed for high temperature applications. We provide experimental results that indicate that acrylate coated fibres FOCS fibres nevertheless can survive ITER environment without critical degradation and attribute this to the system design, for which the fibre operates in an inert gas (neon) atmosphere.

Seismic analyses of the double closure plate sub-plate for the ITER electron cyclotron upper launcher during the vacuum vessel baking scenario

The Double Closure Plate Sub-Plate (DCPSP) was introduced in the Electron Cyclotron Upper Launcher (EC UL) design in order to minimize the openings exposing the interior of the Port Plug (PP) to the port cell; avoiding thus the near environment activation in case of maintenance or intervention on the In-Vessel (IV) components. The load combination Vacuum Vessel (VV) baking + Seismic level 2 is considered as one of the most relevant accidental events in terms of structural conformity affecting the DCPSP. The modal analysis of the DCPSP shows that the natural frequencies are far from the peaks of the ITER reference spectra at the PP flange. Therefore, the feasibility of analyzing the seismic event by using a static method, as a replacement of the Response Spectrum approach, is also investigated. Next, the results due to the seismic event are combined with those produced by loads on the DCPSP that occur during the VV baking scenario. The comparison of the categorized stresses and the allowable design limits showed that the mechanical integrity of the DCPSP is preserved during this load combination.

Design, manufacturing and installation of the JT-60SA vacuum vessel gravity support

In the operation of JT-60SA tokamak device, such loads as electromagnetic (horizontal and vertical loads of 2.5 MN and 7.5 MN, respectively) and seismic are imposed on the VV, and thermal expansion takes place during the baking of Vacuum Vessel (VV). The nine Vacuum Vessel Gravity Supports (GSs) have to support the loads and total dead weight of 400 tons including in-vessel components and compensate thermal deformation. Assembling to realize precise positioning of each GS to the VV is indispensable in the limited space between toroidal field coils. To meet the design requirements described above, a GS is equipped with an assembly of flexible plates (FPs) in its lower part and a stem in its upper part with external thread that is screwed upward on the internal thread of VV stub for installation. Electric discharge machining is applied to form FPs without welding. Narrow gap TIG and electron beam welding are also adopted to suppress welding distortion. By the FEM analysis-based design and successful product manufacturing, we have established a design concept and manufacturing technology of GS that has both stiffness and flexibility. The installation mock-up test of GS has also successfully finished.

Edge multi-energy soft x-ray diagnostic in Experimental Advanced Superconducting Tokamak.

A multi-energy soft x-ray (ME-SXR) diagnostic has been built for electron temperature profile in the edge plasma region in Experimental Advanced Superconducting Tokamak (EAST) after two rounds of campaigns. Originally, five preamplifiers were mounted inside the EAST vacuum vessel chamber attached to five vertically stacked compact diode arrays. A custom mechanical structure was designed to protect the detectors and electronics under constraints of the tangential field of view for plasma edge and the allocation of space. In the next experiment, the mechanical structure was redesigned with a barrel structure to absolutely isolate it from the vacuum vessel. Multiple shielding structures were mounted at the pinhole head to protect the metal foils from lithium coating. The pre-amplifiers were moved to the outside of the vacuum chamber to avoid introducing interference. Twisted copper cooling tube was embedded into the back-shell near the diode to limit the temperature of the preamplifiers and diode arrays during vacuum vessel baking when the temperature reached 150 °C. Electron temperature profiles were reconstructed from ME-SXR measurements using neural networks.

Thermal Behavior of SST-1 Vacuum Vessel and Plasma Facing Components during Baking

For plasma physics experiment, the baking of vacuum vessel (VV) as well as plasma facing components (PFC) of Steady-state Superconducting Tokamak (SST-1) is very essential. SST-1 vacuum vessel consists of ultra-high vacuum (UHV) compatible sixteen sectors in which U-shaped baking channels are embedded on inner surfaces of each of them. Similarly, PFCs are comprised of modular graphite diverters and movable graphite based limiters and stainless steel (SS 304L) tubes are brazed on the back plate of PFC for baking. Baking of SST-1 vacuum vessel and plasma facing components are carried out using nitrogen gas heating and supply system. SST-1 main vacuum vessel is baked at 150°C by circulating hot nitrogen gas at 250°C at 4.5bar gauge (g) pressure through these U-shaped channels. The plasma facing components (PFC) are baked at 250°C or more in the similar fashion by passing hot nitrogen gas through these SS brazed tubes. Thermal analysis shows that the temperature of 150°C at the vacuum vessel is achieved within ten hours if hot nitrogen gas is passed at the ramp rate of 50°C/h while thermal shields are maintained at 85K. It is also observed that the baking of either of them at a given temperature could be possible through radiation if one of them is maintained at desired temperature. The vacuum vessel at room temperature could be baked to 150°C due to radiation from PFC after 40hours when PFC alone is baked at 150°C. The mass flow rate required to bake SST-1 vacuum vessel at 150°C is 1.074kg/s while that for raising PFCs temperature to 150°C is 0.57kg/s. The mass flow rate required to bake PFCs at 250°C is 0.80kg/s.

Nitrogen Gas Heating and Supply System for SST-1 Tokamak

Steady State Tokamak (SST-1) vacuum vessel baking as well as baking of the first wall components of SST-1 are essential to plasma physics experiments. Under a refurbishment spectrum of SST-1, the nitrogen gas heating and supply system has been fully refurbished. The SST-1 vacuum vessel consists of ultra-high vacuum (UHV) compatible eight modules and eight sectors. Rectangular baking channels are embedded on each of them. Similarly, the SST-1 plasma facing components (PFC) are comprised of modular graphite diverters and movable graphite based limiters. The nitrogen gas heating and supply system would bake the plasma facing components at 350°C and the SST-1 vacuum vessel at 150°C over an extended duration so as to remove water vapour and other absorbed gases. An efficient PLC based baking facility has been developed and implemented for monitoring and control purposes. This paper presents functional and operational aspects of a SST-1 nitrogen gas heating and supply system. Some of the experimental results obtained during the baking of SST-1 vacuum modules and sectors are also presented here.

Baking of SST-1 vacuum vessel modules and sectors

SST-1 Tokamak is a steady state super-conducting tokamak for plasma discharge of 1000 sec duration. The plasma discharge of such long time duration can be obtained by reducing the impurities level, which will be possible only when SST-1 vacuum chamber is pumped to ultra high vacuum. In order to achieve UHV inside the chamber, the baking of complete vacuum chamber has to be carried out during pumping. For this purpose the C-channels are welded inside the vacuum vessel. During baking of vacuum vessel, these welded channels should be helium leak tight. Further, these U-channels will be in accessible under operational condition of SST-1. So, it will not possible to repair if any leak is developed during experiment. To avoid such circumstances, a dedicated high vacuum chamber is used for baking of the individual vacuum modules and sectors before assembly so that any fault during welding of the channels will be obtained and repaired. This paper represents the baking of vacuum vessel modules and sectors and their temperature distribution along the entire surface before assembly.

SST-1 Status and Plans

The Steady State Superconducting Tokamak (SST-1) is currently being refurbished in a mission mode at the Institute for Plasma Research with an ultimate objective of producing the first plasma in early 2012. Since January 2009, under the SST-1 Mission mandate, a broad spectrum of refurbishment activities have been initiated and pursued on several subsystems of SST-1. Developing sub-nano-ohm leak-tight joints in the magnet winding packs, developing single-phased LN2-cooled thermal shields, developing supercritical-helium-cooled 5-K thermal shields for magnet cases, ensuring thermal and electrical isolations between various subsystems of SST-1, testing of each of the SST-1 toroidal field (TF) magnets at 4.5 K with nominal currents, testing each of the modules and octants of the SST-1 machine shell in representative experimentally simulated scenarios, augmentation and reliability establishment of the SST-1 vacuum vessel baking system, time synchronizations among various heterogeneous subsystems of SST-1, large data-storage scenarios, and integrated engineering testing of the first phase of the plasma diagnostics are some of the major refurbishment activities. Presently, the SST-1 device integration is in full swing. The cold test of the assembled SST-1 TF and poloidal field magnets began in December 2011. Following the successful testing of the SST-1 superconducting magnet system and engineering validations of the machine shell, the first plasmas will be attempted in SST-1. The first plasma will be ~ 100-kA limiter assisted with the available volt-seconds and could possibly be assisted by ECCD/LHCD.

Investigation of impact of neutron irradiation on properties of InSb-based hall plates

Hall sensors based on InSb thin film layers are presently being evaluated as one of the options for ITER ex-vessel steady state magnetic sensors. The compatibility with vacuum vessel baking temperature of 220°C and with the maximum life time neutron fluence of 1018cm−2 has to be demonstrated. Recently, the InSb Hall sensors with operational temperature up to 350°C were developed. We investigated stability of these sensors type when exposed to neutron irradiation on experimental fission reactor LVR-15, and during short term post-irradiation temperature cycling. Parameters of the samples were measured before and after irradiation up to the total neutron fluence of 1017cm−2. Stability of parameters, within a few percents, of the InSb sensors highly doped by Sn was observed. Novel approach to further stabilization of sensors properties based on shielding thermal neutrons and curing the damage caused by fast neutrons by proper annealing was proposed and tested.

Thermal structural analysis of SST-1 vacuum vessel and cryostat assembly using ANSYS

Steady state super-conducting tokamak-1 (SST-1) is a medium sized tokamak, which has been designed to produce a ‘D’ shaped double null divertor plasma and operate in quasi steady state (1000 s). SST-1 vacuum system comprises of plasma chamber (vacuum vessel, interconnecting rings, baking and cooling channels), and cryostat all made of SS 304L material designed to meet ultra high vacuum requirements for plasma generation and confinement. Prior to plasma shot and operation the vessel assembly is baked to 250/150 °C from room temperature and discharge cleaned to remove impurities/trapped gases from wall surfaces. Due to baking the non-uniform temperature pattern on the vessel assembly coupled with atmospheric pressure loading and self-weight give rise to high thermal-structural stresses, which needs to be analyzed in detail. In addition the vessel assembly being a thin shell vessel structure needs to be checked for critical buckling load caused by atmospheric and baking thermal loads. Considering symmetry of SST-1, 1/16th of the geometry is modeled for finite element (FE) analysis using ANSYS for different loading scenarios, e.g. self-weight, pressure loading considering normal operating conditions, and off-normal loads coupled with baking of vacuum vessel from room temperature 250 °C to 150 °C, buckling and modal analysis for future dynamic analysis. The paper will discuss details about SST-1 vacuum system/cryostat, solid and FE model of SST-1, different loading scenarios, material details and the stress codes used. We will also present the thermal structural results of FE analysis using ANSYS for various load cases being investigated and our observations under different loading conditions.

Hydrogen retention and carbon deposition in plasma facing components and the shadowed area of JT-60U

In JT-60U, erosion/deposition analyses of the plasma facing wall have shown that local carbon transport in the inboard direction was appreciable in addition to long-range transport. The total deposition and erosion rates in the divertor region were ∼1 × 1021 C atoms s−1 and ∼−6 × 1020 C atoms s−1, respectively. About 40% of the deposition in the divertor region likely originates from the main chamber wall. At the plasma facing surfaces of the divertor region, the highest hydrogen concentration in the (H + D)/C ratio and the retention amount were found to be ∼0.13 and ∼1 × 1023 atoms m−2, respectively. In the plasma-shadowed area underneath the divertor region with a vacuum vessel baking temperature of 420 K, redeposited layers of ∼2 µm thickness were found with a high hydrogen concentration of ∼0.75 in (H + D)/C, which was nearly the same level as that observed in JET. Large deuterium retention was also observed at the main chamber wall covered with boron layers. Their H + D retention and (H + D)/C were ∼1 × 1023 atoms m−2 and ∼0.16, respectively, for the vacuum vessel temperature of 570 K. Such a high deuterium retention is most likely caused by D retained in the boron layers. Nevertheless, the integration of this retention over the whole main chamber wall results in significant inventory and needs further investigation.