A reactor relevant Thermonuclear Fusion Experiment requires to operate in UHV conditions and requires sophisticated wall conditioning techniques to guarantee the minimum outgassing of impurities during plasma operations to minimize the radiation losses. Careful wall conditioning is more demanding with respect to usual UHV systems due to the relatively high thermal flux to the wall surface in the order of few MW/m2 in normal working conditions, whereas in the case of abnormal operations, like for example plasma disruptions, the thermal flux can exceed 10 MW/m2in unpredictable areas of the plasma-facing wall. Moreover there are regions of plasma-facing walls, like in the divertor, where a high thermal flux is always present in all operating conditions due to the magnetically induced exhaust of plasma process species (D,T, He) and impurities that have to be quickly removed from the plasma and from the divertor region. In this region in order to mitigate the concentrated power flux to the wall, spreading the thermal power over a wider wall area by enhancing particle radiation is envisaged and induced by means of relatively "high radiating" species injected during the plasma operation. This is an example of contradictory requirement for a usual UHV system. A major contradictory requirement for a UHV system occurs in the Neutral Beam Injector necessary to give access to the DT plasma fusion ignition through a relevant flux of high energy particles created and accelerated in an auxiliary UHV chamber connected to the toroidal main vacuum vessel. The generation of a high current density of negatively charged deuterium particles requires a relatively high density plasma source represented by a weakly ionized cold plasma. This plasma source requires a not negligible stationary flow of Deuterium gas. Moreover the neutralisation of the accelerated negative particles is achieved by interaction of the negatively charged beam with a high density cloud of Deuterium neutral gas; this again requires a significant gas throughput. On the contrary the particle acceleration region and the region in which the beam is launched into the plasma requires as low a pressure as possible to limit the deterioration of the beam properties, the generation of secondary particles and the deposition of the beam energy onto the walls surrounding the beam trajectory. Such contradictory requirements are very demanding in terms of modelling the vacuum pressure profiles in all regions and are very challenging in identifying the most suitable vacuum system. In the paper the vacuum pressure and neutral particle requirements in the different regions of a reactor relevant fusion experiment like ITER will be presented. An innovative model used to evaluate the pressure profiles in different regions of the tokamak and in particular of the Neutral Beam Injectors will be introduced and various validating benchmarks will be given to confirm the robustness of the code; finally the pressure profiles in different regions of the ITER Neutral Beam Injector will be presented and discussed. At the end, from the results obtained the requirements for the UHV pumping systems and the solution adopted in ITER will be discussed.
UHV pumping systems for a reactor relevant Thermonuclear Fusion Experiment: the example of ITER Neutral Beam System
2013
Abstract
A reactor relevant Thermonuclear Fusion Experiment requires to operate in UHV conditions and requires sophisticated wall conditioning techniques to guarantee the minimum outgassing of impurities during plasma operations to minimize the radiation losses. Careful wall conditioning is more demanding with respect to usual UHV systems due to the relatively high thermal flux to the wall surface in the order of few MW/m2 in normal working conditions, whereas in the case of abnormal operations, like for example plasma disruptions, the thermal flux can exceed 10 MW/m2in unpredictable areas of the plasma-facing wall. Moreover there are regions of plasma-facing walls, like in the divertor, where a high thermal flux is always present in all operating conditions due to the magnetically induced exhaust of plasma process species (D,T, He) and impurities that have to be quickly removed from the plasma and from the divertor region. In this region in order to mitigate the concentrated power flux to the wall, spreading the thermal power over a wider wall area by enhancing particle radiation is envisaged and induced by means of relatively "high radiating" species injected during the plasma operation. This is an example of contradictory requirement for a usual UHV system. A major contradictory requirement for a UHV system occurs in the Neutral Beam Injector necessary to give access to the DT plasma fusion ignition through a relevant flux of high energy particles created and accelerated in an auxiliary UHV chamber connected to the toroidal main vacuum vessel. The generation of a high current density of negatively charged deuterium particles requires a relatively high density plasma source represented by a weakly ionized cold plasma. This plasma source requires a not negligible stationary flow of Deuterium gas. Moreover the neutralisation of the accelerated negative particles is achieved by interaction of the negatively charged beam with a high density cloud of Deuterium neutral gas; this again requires a significant gas throughput. On the contrary the particle acceleration region and the region in which the beam is launched into the plasma requires as low a pressure as possible to limit the deterioration of the beam properties, the generation of secondary particles and the deposition of the beam energy onto the walls surrounding the beam trajectory. Such contradictory requirements are very demanding in terms of modelling the vacuum pressure profiles in all regions and are very challenging in identifying the most suitable vacuum system. In the paper the vacuum pressure and neutral particle requirements in the different regions of a reactor relevant fusion experiment like ITER will be presented. An innovative model used to evaluate the pressure profiles in different regions of the tokamak and in particular of the Neutral Beam Injectors will be introduced and various validating benchmarks will be given to confirm the robustness of the code; finally the pressure profiles in different regions of the ITER Neutral Beam Injector will be presented and discussed. At the end, from the results obtained the requirements for the UHV pumping systems and the solution adopted in ITER will be discussed.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
