ELM Control Issue Card

1. ELM Control Issue Card G. F. Counsell Culham Science Centre Presented by S. LisgoCulham Science Centre / University of Toronto 7th ITPA D-SOL meeting Toronto Nov 6–10, 2006

2. ELM CONTROL ISSUE CARDThere’s a problem with large ELMs… • ELMy H-mode is the ITER design reference scenario • H-mode scaling used to predict performance • PFC lifetime associated with natural Type-I ELMs (almost certainly) too short • ELM frequency decreases with reduced *, size increases • 2006 Physics Basis: W/Wped 5–10% • Recent power cycling results (electron beam) suggest acceptable ELM power threshold needs to be lowered • C / W showed high cycle thermal fatigue (crack formation) • new threshold: W/Wped 1–2% • Type-I ELM suppression or (very) small ELM scenario required for acceptabletarget lifetime

3. ELM CONTROL ISSUE CARDSolutions + uncertainties

4. ELM CONTROL ISSUE CARDImpact on ITER Risk: high (very) Cost: moderate Schedule impact: moderateLikelihood: high Technical: high Main Systems Affected: Depends on the ELM control/amelioration systems chosen but is likely to include the vacuum vessel, ports and possibly blanket Other Systems Affected: Control system, cooling and remote handling

5. ITER ISSUE CARD FORM Number: Submitter: Divertor & SOL ITPA, author: GF Counsell ITPA TG Groups to Consider: MHD, Divertor & SOL Title: ELM control/amelioration Action Implied for the Design Activity: Include one or more systems for amelioration or elimination of type I ELMs in the baseline design. Main Systems Affected: Depends on the ELM control/amelioration systems chosen but is likely to include the vacuum vessel, ports and possibly blanket. Other Systems Affected: Control system, cooling and remote handling. Benefit to ITER: Allows ITER to access the planned operational space for type-I ELMy H-mode without the risk of excessive divertor target erosion, from both divertor lifetime and core contamination standpoints, or damage to first wall components. Without a suitable ELM control/amelioration system, ITER may be restricted to regions of operational space with naturally small type-I ELMs, which would typically be at lower core confinement, or regions exhibiting ‘small’ ELMs (type II or ‘grassy’), which are likely to be very narrow and difficult to access. A suitable ELM control/amelioration system would also facilitate future operation with an all tungsten divertor by avoiding melt-layer formation during large type-I ELMs. Drawbacks for ITER: Any likely ELM control/amelioration system will be fairly sophisticated requiring significant design effort to minimize the impact on other aspects of the ITER design. The system is likely to require regular maintenance during operations and, like any ITER system, carries with it the potential for failure. Failure of some aspects could have direct operational and safety implications (e.g. leaks from cooling systems on in‑vessel coils). It is unlikely (at least it cannot be guaranteed with current knowledge) that any ELM control/amelioration system would be effective in all ITER plasma scenarios and some areas of operational space are still likely to be inaccessible. Schedule Implication: Negative implications - It is likely that some operational time must be devoted to optimizing the use of the ELM control/amelioration system for a particular plasma scenario. In addition, the ELM control/amelioration system is likely to form part of the plant safety systems (those intended to avoid damage to the machine). As a result, any failure in the ELM control/amelioration system (which is likely to be fairly sophisticated) would result in downtime. Positive implications – the associated reduction in divertor erosion (and damage avoidance) will reduce the possibility that the divertor cassettes would need replacement earlier than planned, with the consequent implications for machine downtime (i.e. an early shutdown). Cost Implication Estimate (Order of Magnitude): Depends on the system (or systems chosen). Perhaps 2 – 4 kIUA for a pellet injector based system, rising to 8 – 12 kIUA for a system based on magnetic perturbation (allowing for both the coils set and power supplies). In fact it is recommended that two, independent systems be installed (e.g. both pellet and magnetic perturbation based) to provide operational flexibility. The total cost is thus likely to be 10 ‑ 16 kIUA.

6. Description: Type-I ELMy H-mode is the reference operating scenario for ITER. The ELM frequency in this scenario is expected to be of order 1 Hz and the threshold for acceptable divertor lifetime is set at around 3000 full performance pulses. The divertor should therefore expect to encounter around 106 ELMs during its lifetime. Analysis of target erosion due to sublimation and melting from ELM energy loads indicates that the tolerable ELM energy loss from the core for the target to survive 106 ELMs is around 10  1 MJ, being at the higher end of this range for a tungsten target. Even the most optimistic estimates currently available, assuming that so-called ‘convective’ ELMs can be accessed in the ITER QDT=10 scenario, indicate that average losses are likely to be 10 MJ per ELM. Allowing for a statistical spread in ELM losses, as observed on all current devices, up to 30% of ELMs would have losses of 11 MJ or more and 1% would have losses exceeding 15MJ. Less optimistic, and probably more realistic, estimates for ‘conductive’ ELMs are much higher, with average losses of more than 20 MJ. At this level, target erosion is likely to reach 2 mm/ELM (~1 mm/shot), even when factors such as vapor shielding are taken into account. It is clear that the reference scenario in ITER, at least for QDT = 10 discharges, will not be accessible unless current estimates and extrapolations are all very pessimistic. The only way to guarantee access to this scenario will be through some kind of active control of ELM energy losses. Two technologies are currently being investigated; magnetic perturbation and rapid pellets, and a combination of these systems is proposed for ITER. Quasi-helical magnetic perturbation of the pedestal region in H-modes on DIII-D has been demonstrated to completely suppress ELMs, replacing the transient energy losses with an increase in net particle transport across the boundary. This was achieved using a set of toroidally discrete coils (‘I-coils’) either side of the equatorial plane to provide a localized, n=3 resonant magnetic perturbation near the pedestal. The mechanisms for this suppression are still unclear, but may be related to ergodisation of the region or a direct coupling to plasma turbulence. Ideally, the coils should be mounted as close to the plasma as possible in order to keep the resonant perturbation across a small region at the edge. Coils mounted far from the plasma (e.g. outside the vacuum vessel) would tend to ergodise a broad region of the core plasma, with a deleterious impact on confinement. This technology is still under development and the physics is not well understood. Similar coils are being considered for installation on several devices, including ASDEX Upgrade, MAST and JET. Rapid, shallow pellet injection is routinely used on ASDEX Upgrade for ELM control. Low velocity, small pellets are used to trigger ELMs. These ablate close to the pedestal and presumably trigger ELMs by raising the pedestal pressure gradient beyond the MHD threshold. The pellet-triggered ELMs have identical behaviour to normal type-I ELMs, whose energy losses is inversely proportional to their frequency. Thus, pellet injection at a high enough frequency reduces the size of the ELMs (typically ~60 Hz is used on ASDEX Upgrade). One difference, however, is that the higher frequency ELMs do not result in a significant loss of core confinement (as is usually the case with gas puffing). Extrapolation of these results to larger devices is not straightforward, due to the delicate balance of getting the pellets to ablate at the pedestal (relates to their size and velocity), achieving a high enough repetition rate and not over‑fuelling the core plasma. Systems are currently under consideration for other devices, including JET.