Divertor Tungsten Tile Melting and Its Effect on Core Plasma Performance
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PSFC/JA-12-42 Divertor Tungsten Tile Melting and its Effect on Core Plasma Performance 1 2 1 1 Lipschultz, B ; Coenen, JW , Barnard, HS , Howard, NT , Reinke, 1 1 1 ML , Whyte, D.G .; Wright, GM 1 M.I.T. Plasma Science & Fusion Center, Cambridge, MA, USA. 2Institute for Energy Research - Plasma Physics, Forschungszentrum Juelich GmbH, Juelich, Germany October 2012 Plasma Science and Fusion Center Massachusetts Institute of Technology Cambridge MA 02139 USA This work was supported by the U.S. Department of Energy, Grant No. DE-FC02- 99ER54512. Reproduction, translation, publication, use and disposal, in whole or in part, by or for the United States government is permitted. Submitted for publication to the Journal of Nuclear Materials PSFC/JA-12-42 Divertor Tungsten Tile Melting and its Effect on Core Plasma Performance * B Lipschultz1, J W Coenen2, H S Barnard1, N T Howard1, M L Reinke1, D G Whyte1 and G M Wright1 1M.I.T. Plasma Science & Fusion Center, 175 Albany St, Cambridge, MA, 02445, USA. 2Institute for Energy Research - Plasma Physics, Forschungszentrum Juelich GmbH, Ass. EURATOM-FZJ, Trilateral Euregio Cluster, Juelich, Germany E-mail: [email protected] Abstract: For the 2007 and 2008 run campaigns Alcator C-Mod operated with a full toroidal row of tungsten tiles in the high heat flux region of the outer divertor; Tungsten levels in the core plasma were below measurement limits. An accidental creation of a tungsten leading edge in the 2009 campaign led to this study of a melting tungsten source: H-mode operation with strike point in the region of the melting tile was immediately impossible due to some fraction of tungsten droplets reaching the main plasma. Approximately 15 grams of tungsten was lost from the tile over ~ 100 discharges. Less than 1% of the evaporated tungsten was found re-deposited on surfaces, the rest is assumed to have become dust. The strong discharge variability of the tungsten reaching the core implies that the melt layer topology is always varying. There is no evidence of healing of the surface with repeated melting. Forces on the melted tungsten tend to lead to prominences that extend further into the plasma. A discussion of the implications of melting a divertor tungsten monoblock on the ITER plasma is presented. PACS: 52.25.Vy, 52.55.Rk, 52.40.Hf, 52.55.Fa *Work supported by US DoE Cooperative Agreement No. DE-FC02-99ER54512. 1. Introduction Tungsten has been the obvious choice for reactor plasma facing components (PFCs) due to its high melting temperature, low tritium retention, relatively low nuclear activation, and low sputtering erosion rate ([1] and references therein). This led early limiter tokamaks such as PLT [2] and Alcator A [3] to use tungsten and molybdenum (similar refractory metal) for PFCs. The high core radiation levels in PLT were enough that most tokamaks, other than Alcator C[4], FTU[5], and Alcator C-Mod [6] (the first diverted tokamak with high-Z PFCs), switched to carbon PFCs. As we move towards the operation of ITER, and consider the reactor goal beyond that as well, the development of routine operation with tungsten PFCs has become of more widespread interest amongst both limiter [7] and diverted [7-9] tokamaks. Several aspects of tungsten’s compatibility with reactor operation are a serious concern – nuclear damage leading to degradation of material properties and tritium trap site production deep with the material, and the risk posed by melting to reactor operation. The latter risk is both due to enhanced levels of tungsten in the core and to degradation of material properties (PFC and coolant failure). A number of aspects of tungsten melting have been the focus of recent tokamak studies [10-12]. Those works have clearly explored and documented the dynamics of melting with resultant launching of tungsten droplets into plasmas as well as the effects on the tungsten material properties. We report the experience with melted tungsten tiles in Alcator C-Mod which complements those works with further information on the effects on operation, droplet movement and the possibility for melt layer ‘healing’. 2. Background Prior to the 2007 run campaign a full toroidal row of solid tungsten tiles (Figure 1) were installed in the outer divertor of Alcator C-Mod in the region of the strike point. Each of the 120 tiles in the row was made up of 8 tungsten lamellae, each lamella 4 mm thick, held together with a TZM bolt. During the 2007 and 2008 run campaigns over 3000 tokamak discharges were made (roughly 4000 seconds of divertor operation) with no discernible signature of tungsten in the core plasma; The strongest tungsten line in the spectral range of our McPherson VUV spectrometer (W XXXII, 132 Angstroms), described elsewhere [13], is not resolvable from an iron line at roughly the same wavelength except when smaller, nearby tungsten lines are evident[14]. The W/Fe line was weak and the smaller lines were absent during the 2007-2008 period and thus not very useful as a quantitative measure of tungsten radiation at such levels. Instead, the 2007-2008 tungsten concentration was estimated using the total radiated power and contributions from all major radiators (molybdenum from other tiles, Ar used for core rotation measurements) for high- power (4MW ICRF) H-mode discharges. This gives an upper bound of the 2007-2008 W concentration of ~ 1-2x10-5. For reference, molybdenum, which is used for PFCs in the rest of the divertor targets and limiters, typically dominates core impurity radiation. Studies indicated that the dominant source of Mo reaching the core plasma was from the outer limiters and top of the outer divertor due to ICRF-enhanced sheaths and resultant sputtering [15-17] with core Mo concentrations in the range 10-5 to 10-3. During the vacuum break following the 2008 run campaign a poloidal set of molybdenum tiles at a single toroidal location were removed from the inner and outer divertors to study the material migration of W away from the one outer divertor toroidal row due to sputtering erosion [18]. When the tiles were restored to the divertor (along with W tiles) before the 2009 campaign, the tungsten tiles appear to have not been torqued properly – the probable reason for their loosening during the run period and thus the melting described herein. 3. Characterization of the melt effects and characteristics During the 2009 run campaign startup period, as the ICRF antennas were being conditioned to deliver increased power, the average disruptivity was ~9-10%, similar to the previous campaign. The disruptivity abruptly increased concurrent with the start of highest ICRF power (4-5 MW). Only two of the first 12 discharges (see Figure 2) survived until current rampdown at 1.5 seconds – what we consider a full-length discharge. It was clear from core VUV spectral measurements that tungsten was the cause of the disruptions. The working hypothesis quickly formed that a tile had broken or come loose and the following actions were taken: 1) For the 13th-16th discharges of the sequence the plasma magnetic equilibrium was switched from lower- to upper-single null (LSN to USN) such that the single-null strike point was away from the tungsten tiles. As shown in Figure 2 the disruptivity dropped during those discharges. There was a tungsten injection that may have led to the disruption in the USN sequence of discharges. From the 17th discharge through the end of the day (#24) the equilibrium was switched back to LSN with strike point on the tungsten row and the high rate of disruptions returned. Starting with discharge #25 in the sequence (the following day), the strike point was located sufficiently above the row of tungsten tiles that operation could continue and the disruptivity returned close to normal levels. The higher strike point location was generally kept for the remainder of the campaign. At any point when the strike point was lowered back down to the tungsten tile row tungsten injections re-occurred. At this point in the campaign a problem with tungsten tiles had been determined but the location and the exact cause (melting or sputtering) had not been identified, although melting was the most likely candidate process. Examination of the divertor spectroscopic data reveals a clear signature of melting. In the case of sputtering we can predict the ratio of the brightnesses of neutral Mo and W lines in the same spectrum which are each a measure of the local influx of those impurities: The line brightness of the Mo I line (same for W I) can be written as ΓDCB+3*YX = BMoI*SXBX (1) where BX is the measured brightness of either the Mo I (386.4 nm) or W I line (400.9 nm), YX is +3 the sputtering rate for impurity X due to the B ion flux to the surface, CB+3 is the concentration +3 of B in the divertor plasma, SXBX is the inverse photon efficiency [19-21] for either Mo I or W +3 I [22-24], and ΓD the D ion flux to the surface. The use of B as the sputtering impurity is meant only as an approximation for a mix of charge states for B (the most abundant impurity in C-Mod due to boronization) in the divertor plasma and also for some amount of less-dominant impurities. We know from C-Mod experience that the Mo source rate in the divertor cannot be explained by D+ sputtering alone and the Mo influxes are best fit by assuming B+3 as the primary sputtering impurity [25,15]. Such an explanation is supported by mass spectroscopy measurements in the C-Mod far SOL [26] as well as similar analyses of the sputtering of tungsten by carbon in ASDEX-Upgrade [27].