Boronization: Plasma results

In October 2001 boronization technique has been tested for the first time on FTU, as required by some experimental programs, to overcome the problem of high Zeff values at low electron density.
From the point of view of the vacuum performance, the getter action of boron on the low Z impurities is seen as a strong reduction of the overall degassing rate (up to a factor 2.5) and of the pressure limit, which is reduced by a factor 1.7 after 1-2 days of operation following a fresh boronization, and lasts for a long period (> 300 discharges).
After boronization, the restart of operations is immediate (1-2 discharges) and recovery from plasma disruptions is prompt as well. Both these effects are a strong indication of the reduction of light impurities. From the point of view of the plasma characteristics, there are two main results. For an ohmic plasma the total radiated power typically drops from 70-90% down to 35-40% and for Ip=0.5 MA and , Zeff decreases from 6.0 to 2.2 (see fig.1). This is due to the strong reduction of heavy metal concentrations (up to a factor 5 in the case of molybdenum) and of the getter action of boron on the light impurities (oxygen concentration in the plasma is reduced from 2.5% to <0.5% and the carbon flux from the walls drops from 1.0x1018 to 1.1x1017 part/s/m2).

Fig.1 Line average density, ratio of the radiated power on the ohmic power, Zeff for two ohmic discharges at Ip=0.5 MA: (blue) before boronization and (red) after boronization

An unfavourable consequence of boronization with cold walls is however the difficulty in controlling the plasma density during the first two operation days (about 60 discharges). Outside the operator control, the wall can either pump or release a large amount of H, depending on the saturation degree of the surfaces facing the plasma. These phenomena are not observed on other tokamaks which operate at high wall temperature (> 150°C) because hydrogen is pumped by the B film during one pulse and then it is released immediately after the pulse. Preliminary observations suggest that a high recycling or a high edge neutral density could hinder the ITB formation on FTU.
Another consequence of boronization is the large dilution of the plasma with hydrogen particles released from the B film. The ratio of deuterium to hydrogen + deuterium fluxes, as measured by the neutral particle analyzer, can be as low as 40% after a fresh boronization, despite the pure D2 puffing, then increases slowly to 85% after about 200 discharges. The D dilution, in turn, reduces the plasma performances in terms of neutron yield. The target plasma suitable for pellet injection, shows a much lower radiated power than before boronization (Prad/Ptot¼35% against 65%, together Zeff¼1 against Zeff ¼1.4), but the neutron rate decreases up to a factor 5. The observed reduction agrees well with simulations performed with the transport code EVITA. In addition, the same code shows that neither the electron nor the ion transport coefficients show any significant difference after boronization.
The best plasma performances have been achieved only after about 100 discharges after boronization when boron is eroded by the limiter but it was still present on the chamber walls. The metal influx is lower than before boronization, because the physical sputtering by oxygen ions and atoms is strongly reduced, and it is possible to control the edge temperature with D2 gas puffing. For Ip=0.5 MA and , low oxygen (0.4%), molybdenum (0.1%) and iron (0.09%) concentrations are present in the plasma with Zeff =3.0 and a total radiated power close to 65% of the input power.
During this phase, the reduction of Zeff has allowed to reach one of the best performances of FTU in terms of the actual current drive (CD) efficiency eCD. Full CD with eCD=0.2x1020 Am-2/W has been obtained on a plasma target with: Ip=360 kA, BT=5.3 T, and PLH=1.5 MW, with only a small increase of Zeff from 1.5 to 2.2. With the same plasma target, additional power up to PLH+EC=2.6 MW have been coupled to the plasma with Zeff =3.0 as compared to 6.0 before boronization at a lower power. Very good high density plasmas are also obtained. The density limit at Ip=1.1 MA, BT=7.2T, has reached, with gas puffing only ~3.2 1020 m-3. By having reduced the radiated power, the boronization technique has allowed to study in FTU the so called RI mode plasmas at density higher than those of TEXTOR. Neon is used as a cooling gas until a fraction of 90% of the radiated power is achieved with a subsequent peaking of the density profile and an increase of the neutron yield. In this case no significant difference is found between fresh and old boronization. The relative neutron rate production increases by a factor 3-4 in both cases, but the starting level after a fresh boronization is about 5 times lower due to H dilution .
To overcome the problem of H dilution, and hopefully exceed the neutron production of the best FTU performances, use of deuterate diborane (B2D6) as working gas is scheduled in the near future.


Experimental reports