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]. Furthermore, coronal cooling rates [28] would predict an average Z for boron of 3 in the range of divertor Te (7-30 eV). We can then write the ratio of the Mo I and W I brightnesses as

BMoI/BWI = (YMo/SXBMoI)*(SXBWI/YW) (2) The calculated ratio is shown in Figure 3 for various densities. It is not strongly dependent on the use of B+3 as the sputtering impurity and should be taken as a rough guide in this study given the uncertainties in the various quantities - that there will be a mixture of molybdenum and tungsten on the tile surfaces, and the fact that multiple impurities and D ions can contribute to the source rate and brightness. Note that Figure 3 shows that the ratio stays roughly constant as the temperature drops for a given density. The large rise in ratio at lower Te is due to the difference in the sputtering yields for W I and Mo I at those energies. Shown in Fig. 4 are the Mo I and W I brightness signals as well as their ratio for an outer divertor viewing chord that collects light from both molybdenum and tungsten tile rows (see Figures 1 and 5). The brightnesses of Mo I and W I track each other during most of the discharge with the large rise in brightness at ~ 0.7s corresponding to a rise of Te near the outer divertor strikepoint, as measured by probes embedded in the divertor plate, from ~ 20 eV to ~50 eV, followed by a drop back to 20 eV for the rest of the discharge. The corresponding density near the strikepoint is low (~ 2x1020/m3) and roughly constant in time throughout the flattop portion of the discharge (till 1.5s). There is a sudden increase in only the W I brightness, corresponding to a sudden drop in the ratio of Mo I/W I just before the end of the flattop portion of the discharge, indicating that melting has occurred. While the actual ratio of Mo I/W I, before the melt time is higher than predicted by Fig. 3 we reiterate that the ratio of Fig. 3 is a rough guide to what is expected. The occurrence of melting is further supported by the increase of the W I brightness for a viewing chord of the inner divertor surface, where there are no tungsten tiles, concomitant with the melt signature at the outer divertor. We note that the inner divertor chord integrates emission along a path through the inner and outer divertor plasma fans as shown in Fig. 1. Some amount of tungsten, in neutral form, must be crossing the inner and outer divertor spectrograph views. Reflections cannot be ruled out as contributing to the W I signals shown. However, since the W I brightness profiles (Fig. 1 insets) are reproducibly peaked, we feel reflections are unlikely. During a short vessel opening a camera was inserted and used to determine the exact location of the loosened and melting tile (and the tile that had dropped out of the divertor). Figure 5 displays that information in a plan view of the machine (looking from above). The outer divertor spectroscopic views shown in Fig 1 are located ~ 36 degrees toroidally away from the melted tile (~ 40 cm) in the counter-clockwise direction, the same direction as plasma flow along the magnetic field to the outer divertor surface. The inner divertor views are located another 36 degrees away toroidally (total ~ 80 cm). Since the mean free path for ionization of tungsten is millimeters in the C-Mod divertor, the neutral W I signal from evaporating W should not be visible at the toroidally distant spectroscopic views. Therefore, some macroscopic ‘cluster’ of W atoms, probably originating as a ‘droplet’ leaving the melt location, the size to be discussed later, must be traveling toroidally with continuous erosion of the surface leading to the emission measured. The time resolution of the spectrograph in use for measurement of the W I brightnesses is 58ms, so the brightness information cannot be utilized to discern individual (or multiple) tungsten objects, nor their velocity, coming past the view. As an aside we want to note the variability of melting effects from one discharge to another, keeping the strike point position and ICRF power constant. For example, the discharge prior to that shown in Fig. 4 led to a disruption. The tungsten injection(s) of Fig. 4 did not lead to a disruption. And then, for the following discharge, there was no brightness signature of melting at all. Such a history is typical of what is observed, implying changes in the surface structure and temperature during or between discharges. There are many cases where there is stronger evidence for multiple W droplets scattered over time and, in addition, directly reaching the main chamber as droplets. Shown in Fig. 6a are the peak brightnesses vs. time from each of four sets of chordal views including the inner and outer divertor views discussed above. A plasma current disruption occurs at 1.08s. 
 
  




 
 
 
 main chamber (described in Figure 5). The average brightness for each set is not shown but has similar time dependence as the peak value. In contrast to more typical melt cases (e.g. Fig. 4), the tungsten signal enhancement (core and divertor W I) lasts much longer, with W I brightness becoming strong on main chamber spectroscopic views, consistent with droplets in the main chamber at later times. The divertor W I brightness is strongest early in time as one expects since that is where the droplets originate and the droplets have to move from there to the main chamber. Note that reflections are probably dominant just prior to the disruption, as all views tend towards the same value. Figure 6b also shows that there are a number of small, step-like, increases in core radiation, which can be interpreted as droplets entering the core plasma. The radiation increases are non- negligible and, for the most part, below ~ 1 MW. Images from a camera view looking across the chamber above the melt location do confirm that there are multiple droplets at later times (1.02- 1.05s) before the disruption. That single camera view cannot be used to determine the actual location of droplets, whether inside the main plasma or in the SOL, whether near the inner or outer divertor. In cases such as that shown in Fig. 4, there is no evidence of droplets on the camera images. This is either due to the brightness of the divertor plasma overwhelming the droplet emission or the droplets have not emerged upwards from the outer divertor fan. One may ask why there are longer lasting melt cases of W I emission (Fig. 6) as compared to the more usual melt occurrences (Fig. 4) where the emission only lasts a short period. It may be that the typical case (Fig. 4) corresponds to droplets staying for a short period in the divertor plasma, with limited erosion, before leaving the plasma completely. For some reason the cases similar to that shown in Fig. 6 appear to correspond to droplets circulating in the plasma long enough to reach the core plasma. There are no obvious reasons for why droplets would have different lifetimes in the plasma. The W I brightness data during tungsten melt periods exhibit reproducible poloidal profiles which indicate that the droplets move both toroidally and poloidally through the divertor plasma. 


  
  


 

 
 
 



 


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Towards the end of the campaign, an experiment was tried where the ICRF heating was brought up very slowly over a sequence of discharges in contrast to the beginning of the campaign when the ICRF power was of order 4 MW (the Ohmic power is ~ 1 MW). The result was even more problematic than earlier in the campaign. Injections started at very low powers – 600 - 1200 kW. Several times the injections led to disruptions, even in L-mode where the impurity confinement is low. There was clearly no healing of the melt. The evidence indicates the opposite.

4. Post-campaign analysis of tungsten loss Based on closer examination of the tungsten tile row after the 2009 run campaign there were several reasons for enhanced heat loads and melting. First, it was found that one tile had become so loose that it had fallen out of the divertor, creating a leading edge on the next remaining tile. A leading edge is a surface perpendicular to a field line that experiences the full heat load parallel to the magnetic field as opposed to being spread out by the small field line angle of incidence to the surface. Figure 7 shows the gap where the missing tile was as well as the remaining four (out of the eight lamellae that made up the tile) that were significantly melted. The second reason for enhanced heat loads on the leading edge of the remaining tile beside the gap is that it was also loose; This created a space between the tile and its mounting surface of ~ 1-2 mm and thus the front of the tile was beyond the normal front surface of the divertor. A third reason for the tungsten tile intercepting additional heat flux is the apparent melt layer motion of tungsten. After the 2009 campaign the tungsten lamellae were removed from the machine and divertor module. The parts for the tile that had remained attached to the divertor through the run campaign were then analyzed for changes in grain size, lamellae shape and melt layer characteristics. These results of that analysis are given elsewhere [29]. Here we show in Figure 8 the profilometry measurements of two individual melted lamellae. The line corresponding to 20 mm of height in the figure indicates what had been the nominal front surface of the lamellae. An arrow gives the poloidal direction downward which is the motion of the melted tungsten. ‘Hills’, or prominent regions, are formed that extend up to 3 mm into the plasma from the nominal front surface of the divertor. Taking together the looseness of the tile (1-2 mm), the added height of the lamella (3mm) and the missing tile, we can make an estimate of the additional heat load on the melted tile and the period during a RF-heated discharge needed to reach a melting condition. At the end of the run period, when we could make the above measurements of the tiles, the overall location of the leading edge of the lamella was 4-5 mm into the plasma, combined with a field line angle with respect to the nominal surface of ~0.6 degrees, means that the remaining part of a lamella tile shadowed and received the heat load intended for of order 14 of the following tiles, each tile’s width being ~ 3 cm wide in the toroidal direction. The area on the damaged tile where the heat was deposited probably varied over the run period from the first 1-2 mm of the side of a single lamella (~ 0.3 cm2) to, of order 4 cm2 where the heat would be spread out over a number of the remaining lamellae as the melting proceeded. Taking the ratio of those melting areas to that of the 14 shadowed and one missing tile (each about 9 cm2) gives a multiplication of the nominal 5- 10 MW/m2 (for high-power RF cases, of order 4MW) normal to the surface heat flux, by a factor or 35-450. Even at 35x higher heat loads melting would be reached in of order 10 milliseconds. For lower powers, as described above for the case of melting near the end of the run campaign, the time to reach melting temperatures could be longer by factors of 5-10. The projection of the tungsten lamellae, sticking into the plasma past the nominal divertor surface, is most pronounced for the last two remaining lamella along the direction of the plasma flow, the profiles of which are shown in Figure 8. The direction of melt layer movement down the divertor plate is consistent with jxB forces due to the thermoelectric emission of electrons at high temperature, which were found to be higher than that of gravity in Textor [10]. Those forces move molten material downwards, continually replacing evaporating material. Forces due to thermoelectric currents will be higher in C-Mod and ITER than Textor due to the 5.3 Tesla magnetic field. The plasma pressure is likely pushing material toroidally towards lamellae 7 & 8 as well. jxB forces are probably dominant over the plasma pressure forces in C-Mod where there are no transients (e.g. ELMs) which would lead to thin melt layers (jxB forces act on the entire melt volume [10]). The amount of tungsten lost from both the missing and melted tungsten tiles is estimated, based on lamella weights, to be of order 15 grams. We note that there was no movement of the tungsten to the next row of tiles (molybdenum) below the tungsten row. The properties of the melted tungsten lamellae were also examined [29]. As observed in previous studies [11] the grain size in melted areas was much larger than the virgin tungsten material (~ 10 microns), reaching 100s of microns. The lamellae themselves were distorted indicating that the mechanical stresses induced by the strong gradient in temperature through the lamellae led to strong gradients in expansion with internal strains past the yield of the material. To study the migration of tungsten to molybdenum surfaces away from the melted tungsten tile, three entire outer divertor modules (out of 10 toroidally around the machine) were removed from C-Mod and brought to an ion beam facility for surface analysis. A 3 mm diameter, 2 MeV external proton beam was used to quantify the amount of tungsten deposited on the molybdenum tiles in air (through a 7.5 mm thick kapton foil) using Particle induced X-Ray emission (PIXE) spectroscopy. Zero degree beam incidence, and 45-degree detection angles were used to accommodate the geometry of the divertor modules. The tungsten areal density, was determined

from the intensity of the induced tungsten Lα and Lβ transition X-rays, measured with a Si(Li) detector using the techniques described in [18]. The results are shown in Fig. 9, with no subtraction of the tungsten areal density that existed prior to the 2009 campaign (discussed later) for the three divertor modules examined. Measurements of tungsten areal density were made at multiple toroidal locations for the same poloidal location. This led to the range in areal densities shown at each poloidal location. Similar measurements for a few tiles from the inner divertor and poloidal limiters gave much lower tungsten areal densities than the outer divertor. Thus, for the purposes of this study, we will assume that tungsten deposition due to the melt is negligible for surfaces other than the outer divertor. Given that the tungsten tiles had been in the tokamak for 2 run campaigns prior to the campaign when the melt occurred, we need to subtract off the tungsten due to the sputtering erosion and migration of tungsten in the 2007 and 2008 campaigns. That measurement, made prior to the 2009 campaign, is shown in Fig. 9 as a horizontal solid bar [18]. That study showed that the deposition was consistent with sputtered tungsten being mostly ionized in the outer divertor plasma and then swept back to the outer and inner divertor plates in the private flux zone. (tiles 7-8 of Figure 9 for those discharges). The deposition in the common flux region, tiles 1-5, was negligible, and is not shown. Assuming that none of the tungsten that was sputtered and migrated prior to the 2009 campaign was re-sputtered during the 2009 campaign, the two sets of measurements shown indicate that the areal density of deposition due to the melting was, at most, of order 1021/m2 over the entire divertor module. Taking this maximum value and conservatively extending it over all divertor modules one can account for <0.1 grams of tungsten re-deposited on the outer divertor. These results indicate that less than 1% of the eroded tungsten is re-deposited and adhering to PFC surfaces. Given the dearth of tungsten on molybdenum surfaces it is likely that the remainder of the tungsten ended up as dust around the chamber. Unfortunately, in the rush to replace the tungsten tiles and make the measurements described above, that dust was not properly collected and analyzed. It is striking to compare the amount of tungsten lost during the C-Mod melt experience to erosion due to sputtering. The total erosion of tungsten from the entire toroidal row of 120 tungsten tiles over ~ 3000 discharges in the 2007-2008 campaigns was ~ 0.1g [18]. In contrast, for of order 100 discharges of melting from one tile the tungsten loss was over 100x larger (~ 15 grams). There is some evidence for adherence of droplets to the divertor modules. During the measurement of W areal densities described above several localized (to parts of a single tile) enhancements (x10-100) in tungsten areal density were found. Visual inspection of the dome area immediately across from the melted tile showed what appeared to be droplets as well. There is also some evidence, at least in C-Mod, that droplets, once distributed on the floor around the chamber due to a melt occurrence, are not a strong threat to further operation. In a survey of LSN operation during the remainder of the campaign following the raising of the strike point off of the damaged tile, we did not find any clear signatures of tungsten injections, implying that the dust created was not being continuously injected. This is likely due to the fact that dust does not collect on the vertical divertor plate, but rather on more horizontal surfaces [30]. Note also that if we assume that the average tungsten dust diameter was 100 microns in diameter (discussed later) there would need to be of order 106 such tungsten dusts to account for 15 grams, indicating a low probability of their mobilization.

5. Discussion The C-Mod results support the idea that the melt cannot be ‘healed’ such that normal operation can be restored. By healing we mean that any tungsten prominence recedes enough such that there is no more melting. Ideally the surface temperature of the remaining tungsten would drop back to a range not threatening for the underlying coolant channels for a water-cooled device like ITER as well. We do observe that the melting effects vary in that for some discharges there are no droplets evident on the divertor or core spectroscopy data, and in the following discharge (or before) there are. Therefore the surface is constantly changing, both due to droplets leaving the surface and tungsten melt layer movement. This would imply that there is a possibility of healing. But there is evidence for the opposite as well; the forces on the melted material lead to melt layer movement in one direction (down). The melt did not move to the next tile poloidally. The melted material thus built up faster than it was being eroded by the plasma – making more of a leading edge as time went on. Based on the above reasoning there is no reason to believe that the surface will ‘heal’. The implications of the C-Mod experience are mixed with respect to movement of the melted tungsten. If the tungsten material had moved to the next tile it would have created a leading edge there and could lead to a spread of melting. On the other hand if the flow collects in one place on a single tile then the leading edge sticks further into the plasma, intercepting even more power. Thus while the melting does not spread to other regions is good, the lack of spreading exacerbates the melting at the initial melt location. As mentioned above the melt layer motion did not take the melted tungsten to the next tile leading to buildup of tungsten along the lower edge of the melted tile. This could have been related to the fact that the next tile below (molybdenum) was cooler. But another possibility is the poloidal angle of the divertor plate; In C-Mod, as in ITER, the surface of the divertor plate is at increasing major radius as one moves down the plate (see Figure 1). Gravity would tend to pull the melted material away from the surface. This is in contrast to ASDEX-Upgrade where that poloidal angle is reversed and the melt motion led to a drip to travel along the tiles below the one that melted[12]. The size of droplets is important for the extrapolation to ITER and to compare to models of melt layer dynamics and melt layer dynamics and losses in future devices. Based on radiation cooling rates [28], if all the atoms of a ~160 micron diameter ball of W were in the core plasma at one time, they would radiate all the C-Mod input power (~ 5MW). The reality is that the real radiated power will be less if one properly takes into account the droplet lifetime being longer than an impurity confinement time as shown for one case in ASDEX-Upgrade [12]. Such a calculation is beyond this study. In addition, a droplet might pass through the core plasma without completely evaporating, thus reducing the amount it deposits inside the separatrix. The results presented earlier (e.g. the discussion of Fig. 6b) implied that each droplet leads to ≤ 1MW or radiation in the core plasma. Those stepwise increases in radiation are consistent with effective droplet diameters (that which is deposited in the plasma) ≤ 100 microns. Separately, we know from past studies of dust from Alcator C-Mod the largest size molybdenum spherical balls were 30 microns diameter [30]. The modelling of ASDEX-Upgrade droplet ablation indicated between 50 and 100 microns diameter [12] based on gravity overcoming the friction force at larger diameters (eq. 1 of [12]). Scaling the prediction of the friction force to C-Mod where the densities are higher would predict larger diameters by factors of 2-4. The results from QSPA studies of melting indicate droplets in the range of 20-60 microns [31]. Finally, in SEM photos of the melted C-Mod tungsten lamellae there is evidence for 60-160 micron balls on them. However, those large tungsten balls probably did not travel through the SOL, losing material and diameter. Based on the general experience described above (e.g. everything from dust size measurements to calculations of the droplet size) as well as the radiation increase associated with the injections of Fig. 6b, we will assume, for the purpose of asking what the C-Mod results might mean for ITER, that the tungsten droplets in the C-Mod plasma are of order 100 microns in diameter. It is worthwhile to ask what the implications of the C-Mod results are for ITER where the divertor conditions (density, temperature) and parallel power flows (~500 MW/m2) are similar to C-Mod [32,33]. The core impurity confinement time, typically correlated with energy confinement, will be much larger than on C-Mod and the droplet burnup time, indicating that once W atoms are deposited in the core they stay in for a long time. From a similar calculation as above for C-Mod, but with lower radiative cooling rates given ITER’s higher temperatures, we find that all the W atoms contained in droplets of order 0.9 mm in diameter are required radiate 150 MW in ITER. That is good. Since the melting process does not depend on machine size there is little reason to believe that ITER W droplets will be significantly different than in today’s toroidal devices and linear machines. Thus if a tungsten divertor mono-block were to start melting (failure of the next tile, failure of part of the tile…) we can assume a droplet size of ~100 microns) and that multiple droplets (~ 750) would be required to be completely deposited into the core plasma within an impurity confinement time to lead to a thermal collapse and disruption. Single 100 micron diameter droplets would lead to very small effects – a W concentration of ~10-6 and < 1MW radiation. What we cannot do is comment much on whether it is more likely or not for droplets originating from the ITER divertor to reach the core plasma than in C-Mod. Certainly the distances for a similar size droplet to travel and survive erosion are much larger (~ 10x assuming path length is proportional to R) in ITER than in C-Mod. But the physics behind the movement upwards is not well understood. Note that while ITER parallel power flows are similar to C-Mod’s there are other aspects of heat deposition on the divertor surface that are different. First, the C-Mod discharges were attached while the plan for ITER is for detached plasmas at all times thus lowering the parallel heat flux. Second, the field line angle to the surface is near 3 degrees [34], much larger than C-Mod. The latter will mean that if a melt starts, the increase in power intercepted by the ITER monoblock for each mm of growth into the plasma will be less than in C-Mod. Assuming that the movement of the droplets is clockwise and upwards then the C-Mod results are somewhat at odds with predictions by Krasheninnikov [35] and previous modeling of droplet trajectories from ASDEX-Upgrade [12]. The ASDEX-Upgrade camera measurements directly demonstrated movement toroidally in the direction of the plasma flow and upwards as we have inferred for C-Mod. The upward movement was ascribed to the component of the centrifugal force reaction by the divertor plate. The poloidal angle of the ASDEX-Upgrade outer divertor plate surface with respect to vertical is opposite to that for C-Mod. The Krasheninnikov model would then predict downward movement, which does not appear to occur. While the discussion above is somewhat optimistic that the effect of droplets on ITER core radiation will be less than in present devices, we are less optimistic with respect to what melting implies for ITER general operation. As emphasized in this paper we have found that the melt in C-Mod did not heal. This result is further emphasized by the continued efforts to place the strike point on the damage row of tiles throughout the run period with negative results. On that basis we are not optimistic that ITER could recover from a situation where all, or part of one W monoblock is lost and the next monoblock toroidally takes double the power, concentrated on the leading edge corner. This could lead to a cascade effect of each monoblock receiving more heat flux than it was designed for, leading to its loss and melting of the next monoblock toroidally. As each monoblock reached melting temperature it is likely that the cooling tube that the monoblock was attached to would fail and leak water into the chamber. While we cannot estimate the probability of that occurrence we feel that the threat to the cooling system would shut ITER down until the monoblock could be replaced.

6. Summary It is clear that the effect of tungsten melting on core operation in Alcator C-Mod was major. The magnetic equilibria had to be limited to strike point positions above the row of tungsten tiles for the remainder of the campaign. The droplets launched from the melted surface circulated toroidally around the machine and up out of the divertor. It is likely that multiple droplets are required to disrupt the C-Mod plasma. There is no evidence for processes that limit and reduce the melting of the tungsten – so called healing of the surface. This is worrisome as it presents danger for ITER operation with tungsten in the high heat flux region. If a leading edge is created, possibly through a piece of a monoblock breaking off, then the proper diagnostics are needed to monitor the coolant channel and determine whether operation is safe. On the other hand, based on the C-Mod experience, we think that the tungsten reaching the core plasma from a similar size melt will have less of an effect on the ITER core plasma than in C-Mod. The evidence from C-Mod, Textor, ASDEX- Upgrade and QSPA indicates droplet sizes of order 100 microns, small enough to have small effects on the ITER plasma. It will be important to better understand the movement of tungsten droplets through the plasma, as that will determine the efficiency of the droplets reaching the core plasma.

7. Acknowledgements The authors wish to thank B. Bazylev, S. Krasheninnikov, K. Krieger and B. LaBombard for helpful discussions. This work is supported by US D.o.E. Coop. Agreement DE-FC02- 99ER54512. Figures 2.0

normal discharge length 1.5

ID11 1.0 ID9

a) USN ID7 0.5 discharge length [s] strike point above W ID5 Inner divertor 0.0 1090811-12: exclude fizzles views 02010 30 40 50 OD15 W shot sequence OD14 5 7 9 11 Figure 2: Disruptivity of a set of OD13 Outer divertor discharges over two days after the

OD11 Brightness [au] views tungsten problem appeared. Discharge OD9 b) length of 1.5 s is defined to be disruption free. Discharges 13-16 were upper single 9 11 13 14 15 null (USN) and were disruption free View #
except for 15 where there was a tungsten Figure 1: The C-Mod outer divertor showing injection.
the tungsten row in red (marked with a ‘W’). The spectrometer viewing chords for the outer 0.10 (OD9, 11, 13-15) and inner divertor (ID5, 7, MoI (out. div.) (a) 9, 11) are shown. Inset are the brightness WIx100 (out. div.) 0.08 profiles of W I for a case where melting has WIx100 (in. div.) been identified, for the inner (a) and outer (b) 0.06 sterad)

divertor views shown. 2
0.04

mW / (cm 0.02 B /BWI = (YMo/SXBMoI)*(SXB /YW) 100 MoI WI 0.00 (b) 100 80 80 sputtering 60 ne=1x1020/m3 60

WI melting /B

ratio MoI/WI 40 ne=2x1020/m3 MoI 40 B 20

20 3 1090805032 ne=5x10 /m 0 20 0.0 0.5 1.0 1.5 2.0 time(s) n =10x1020/m3
e Figure 4: a) The brightnesses for Mo I and W 0 I for outer divertor chord along with the W I 0 1020304050 brightness for a chord viewing the inner Te [eV]
Figure 3: The predicted brightness ratio of divertor. The peak in W I and Mo I brightness the Mo I (386.2 nm) to W I (400.9 nm) lines at ~ 0.7s is due to a large rise in divertor Te. for a range of densities and temperatures in b) The ratio of Mo I/W I for the outer divertor the divertor. chord. Both the rise in the inner divertor W I brightness and the ratio of Mo I/W I for the outer divertor viewing chord indicate melting has occurred. 180o o 144 104 -Outer div. a) -Inner div.

103 108o e 102

c d WI brightness [au] 101 72o f -CCW main chamb.

a -CW main chamb. 1090811007, prad_2pidiodex4.4 b 100 H-mode H PIN 36o o 0
4 Figure 5: Plan view (from above)

showing the direction of the plasma [MW] flux along the field and the 10 divertor 2 sections: (a) the direction of the P incoming heat flux to the location of b) RAD the melted tile (b), the outer (c) and 0 0.80 0.85 0.90 0.95 1.00 1.05 1.10 inner (d) divertor spectroscopic views, Time[s]
and the spectroscopic views across the Figure 6: a) Time dependencies of the main chamber (e, f). peak spectrometer brightnesses corresponding to views of Figures 1 & 5: outer divertor (5 chords); inner divertor (4 chords); main chamber (4 chords) looking at a region of the wall counter-clockwise B (CCW) from the melted tile; and main chamber (2 chords) looking at a region of the wall clockwise (CW) from the melted tile. b) Input and radiated powers for the same discharge with arrows pointing to what appear to be droplet injection
signatures. The plasma goes in and out of Figure 7: Second melted tile that H-mode before staying in L-mode for the remained in place, having lost 4 out of remainder of the discharge. 8 lamellae. The direction of the incoming plasma flux along a field
line is shown.
100 profile of lamella 0-36o downstream - squares 72-108o downstream- circles o

] 144-180 downstream - stars 24 lamella 7 2 /m

lamella 8 20

Pre-2009 22 10 ]

m 1 2 m

[ Nominal lamellae 3

t 20 4

h height Tungsten density [10 5 g i 6 W e 7 h Tungsten tile row 8 18 1 0246810 poloidal position Direction of JxB & Figure 9: Tungsten areal density vs 16 gravity forces position along the outer divertor plate for 0 5 10 15 20 25 30 three of 10 toroidal outer divertor poloidal distance [mm]
sections as measured following the 2009 Figure 8: Profile of the plasma-facing run campaign. The location of the three surface of 2 (of 8) lamella farthest from modules with respect to the melt location the leading edge. The machined surface (0 degrees) is given in Fig. 5. The location is shown as a dotted line. multiple areal density data points at any one poloidal position is due to toroidal
variations along a particular plate. For comparison, data from a previous study of the amount of tungsten migration due to sputtering, is also shown as a horizontal bar labeled ‘Pre-2009’..
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