Long-term carbon transport and fuel retention in gaps of the main toroidal limiter in TEXTOR

Abstract

The 1.1-1.5 mm wide gaps between tiles of the main toroidal belt limiter in TEXTOR were utilized to study the long-term impurity deposition and fuel retention in gaps. The tiles were exposed during a full tokamak campaign of 9365 s of plasma to various discharge conditions and wall conditioning, accumulating of up to 30 µm thick layers at the gap entrance. It was found that (i) gaps trap impurities twice as efficient as the top surface, (ii) the deposition in the toroidal gaps is twice as high as in the poloidal, (iii) carbon deposition decays with a fall-off length of about 0.7 mm towards the gap bottom, (iv) deposition on the bottom is significantly higher than on the adjacent side walls of gaps, (v) the amount of deuterium scales with the amount of carbon with D/C varying from 3% to 30% depending on the surface temperature.

Post-mortem analyses of gap facing surfaces of tungsten tiles of T-10 ring limiter

“…The striking effect is that the surfaces in gaps, including the gap facing surfaces (GFS), SS holder facing surfaces (HFS), and the step between GFS and HFS are nonuniformly dark and violet and appear to be covered by a nonuniform film. This observation is in contrast with measurements of film deposition in castellated structures in TEXTOR [1,22], where films thickness decreased towards the gap bottom with a fall-off length of about 1 mm.…”

“…Carbon deposits were observed in many large and small-sized tokamaks either working with graphite plasma-facing components (PFC) or having graphite PFC in previous campaigns . Deposits in gaps and remote areas were observed, for example, in TEXTOR [1,14,22], DIIID [2], JET [3,7,9,12,20,21], JT-60U [8], TFTR [9], KSTAR [11], Tore Supra [13,15], Alcator [17], and RFX [6]. The effect was interpreted as co-deposition of carbon with hydrogen and deposition of volatile hydrocarbons formed due to chemical sputtering of carbon [18,19].…”

Studies of protection and recovery techniques of diagnostic mirrors for ITER

“…Therefore, in order to deduce the absolute CXA flux to the given location, one needs to know the neutral recycling flux at one location in TEXTOR and to multiply it by the relation ratio provided by EIRENE. The long-term experiment in TEXTOR described in [17], lasting 9365 s provides an excellent basis for our studies. The neutral recycling flux was evaluated in [17] for the ALT tile marked with 'ALT' in figure 8.…”

“…The long-term experiment in TEXTOR described in [17], lasting 9365 s provides an excellent basis for our studies. The neutral recycling flux was evaluated in [17] for the ALT tile marked with 'ALT' in figure 8. The recycling fluence was found to be 3 × 10 21 part cm −2 .…”

“…The relation coefficient provided by EIRENE for CXA at the Mirror Station location was estimated to be 1.0 × 10 −6 . Converting the aforementioned fluence from [17] into flux and multiplying the obtained recycling flux consequently by the duration of plasma operation and by the relation coefficient we obtain the total fluence of CX atoms to the aperture of the Mirror Station of 1.3 × 10 15 CXA cm -2 . Having compared this value with the amount of carbon found on contaminated mirrors and provided on figure 2, measured to be of order of 10 16 C at.…”