1. magnetic confinement - ENEA - Fusione

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18 1. MAGNETIC CONFINEMENT 1.1 Tokamak Physics Another result of boronisation is that hydrogen particles released from the B film dilute the plasma. The ratio of deuterium to hydrogen + deuterium fluxes, as measured by the neutral particle analyser, can be as low as 40% after a fresh boronisation, despite pure D 2 puffing. The ratio then increases slowly to 85% after about 200 discharges. The D-dilution, in turn, reduces the plasma performance in terms of neutron yield. The target plasma used for pellet injection (n _ e =1.7×1020 m -3 ) shows a much lower radiated power than before boronisation (P rad /P tot ≈35% against 65% and Z eff ≈1 against Z eff ≈1.4), but the neutron rate decreases by up to a factor of 5. This decrease is in good agreement with simulations performed with the EVITA transport code [1.14]. The same code also shows that neither the electron nor the ion transport coefficients show any significant difference after boronisation. Zeff Prad/Pohm(%) ne(x10 19 m -3 ) The best plasma performance 0.0 0.5 1.0 1.5 following boronisation was Time (s) achieved only after about 100 discharges, when the boron had been eroded by the limiter but was still present on the chamber walls. The metal influx was lower than before boronisation because physical sputtering by oxygen ions and atoms was strongly reduced, and it was possible to control the edge temperature with D 2 gas puffing. For I p =0.5 MA and (n _ e =0.4×1020 m -3 ), low oxygen (0.4%), molybdenum (0.1%) and iron (0.09%) concentrations were present in the plasma with Z eff =3.0 and a total radiated power close to 65% of the input power. During this phase, the reduction in Z eff allowed one of the best performances of FTU to be reached in terms of the actual CD efficiency η CD . Full CD with η CD =0.2x10 20 Am -2 /W was obtained on a plasma target with I p =360 kA, B T =5.3 T, n _ e =0.4×1020 m –3 ) and P LH =1.5 MW, with only a small increase (from 1.5 to 2.2) in Z eff . With the same plasma target, additional power P LH+EC =2.6 MW was coupled to the plasma with Z eff =3.0, compared to 6.0 before boronisation, at lower power. Very good highdensity plasmas were also obtained. With gas puffing, the density limit at I p =1.1 MA, B T =7.2 T reached, with gas puffing only, n e =3×10 20 m -3 . By reducing the radiated power, the boronisation technique has made it possible to study the so-called radiative improved mode plasmas at higher densities than those of TEXTOR [1.15]. Neon is used as the injection gas until a fraction of 90% of the radiated power is achieved, with a subsequent peaking of the density profile and an increase in the neutron yield. In this case, no significant difference was found between the fresh and old boronisation. The relative neutron rate production increases by a factor of 3-4 in both cases, but the starting level after a fresh boronisation is about five times lower due to H dilution. To overcome the problem of H dilution and hopefully to exceed 5 4 3 2 1 100 80 60 40 20 12 10 8 6 4 2 0 Fig. 1.11 - a) Line-averaged density; b) ratio of radiated to Ohmic power; c) Z eff for two Ohmic discharges at I p =0.5 MA: (blue) before boronisation and (red) after boronisation. [1.14] V. Zanza. http://efrw01.frascati. enea.it/Software/Unix/F TUcodici/evita [1.15] B. Unterberg et al., J. Nucl. Mater. 266-269, 75 (1999)
1. MAGNETIC CONFINEMENT 19 the neutron production of the best FTU performances, deuterate diborane (B 2 D 6 ) is going to be used as the working gas in the near future. 1.1.7 Radiative improved mode in Ohmic plasmas 1.1 Tokamak Physics In many tokamaks, controlled injection of gas impurities (mainly noble gases such as Ne, Ar and Kr) into the plasma has been found to lead, in certain conditions, to an improved confinement regime, the so-called radiative improved (RI) mode. The interest in this regime for FTU lies in the fact that it can be obtained with different magnetic configurations (circular or elongated plasmas, limiter or divertor) and different heating systems (neutral beam injection, ICRH and Ohmic). Moreover, it couples good energy confinement with a large fraction of radiation losses (up to 90% of total input power), thus alleviating the problems of plasma-wall interactions. Of course the price to be paid is a larger Z eff and greater plasma dilution. [1.16] M.Z. Tokar et al., Plasma Phys. Control. Fusion 41, B317 (1999) [1.17] M. Bessenrodt- Weberpals et al., Plasma Phys. Control. Fusion 34, 443 (1992) The strategy to look for the RI mode in FTU is based on the interpretation given by TEXTOR [1.16]: impurity injection attenuates the growth rate of the ion temperature gradient (ITG) instability. This leads to a smaller particle outflow and hence to peaking of the density profile. As a consequence, ITG turbulence is further attenuated, or even quenched. In cases where ITG turbulence is the dominant heatloss mechanism, an increase in energy confinement is achieved. In addition, it has been found that in TEXTOR the energy confinement time increases with density. An experimental campaign was started at FTU at the end of 2001 to explore the possibility of a RI mode in Ohmically heated plasmas. The aim was to reproduce the improved Ohmic confinement (IOC) regime of ASDEX. [1.17]. The plasma target was chosen so as to clearly see this regime, if it really did exist in FTU. At magnetic field B T =6 T, the plasma current was programmed to be at 0.8-0.9 MA to avoid the insurgence of MARFEs. The operational density was set at 10 20 m -3 in order to be well into the saturated Ohmic confinement (SOC) regime, where energy confinement is independent of density. In FTU the critical density to access SOC is ~0.8 10 20 m -3 . Deuterium gas puffing was interrupted at 0.45 s, just at the beginning of the current flat–top, according to the experience on ASDEX. A neon puff (10-30 ms duration) was injected at 0.6 s, just at the beginning of the current flat–top. The standard FTU diagnostics was used to obtain the experimental results. Fig. 1.12 - a) Lineaveraged density; b) Z eff from bremmstrahlung; c) radiated power; d) neutron yield for a discharge without Ne (red) and with Ne (violet) puffing of 20 ms at 0.6 s. 1012 (s-1) 105 (W) Zeff 1019 (m3) 10 5 3.5 3 2.5 2 1.5 10 5 1 0.5 a) b) c) d) x x 0 0 0.5 1 Time (s) x x x x x x x x 01 02 03 04 x Figure 1.12a shows the central line-averaged density for a discharge with a Ne puff of 20 ms compared to a reference discharge without Ne. Two different phases can be seen: First, there is a slow increase in density after Ne injection, which cannot be fully accounted for by the electrons contributed by Ne ionisation. The Ne concentration can be roughly derived from the variation in Z eff (fig. 1.12b). Radiation
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