Jul LRB L.R. Baylor, T.C. Jernigan, N. Commaux, P.Parks, M. Fenstermacher, T. Osborne, S. K. Combs, C. R. Foust, M. Hansink, B. Williams 14 th ITPA Meeting Pedestal and Edge Physics General Atomics 1-May-2008 Initial Results of Pellet Dropper Experiments on DIII-D

Jul LRB 2 ITER ELM Challenge Filaments evolving during ELM on MAST. Kirk, et al. PRL ELMs result from edge pressure clamped at ballooning limit – transient breakdowns of edge barrier – filaments are ejected from the plasma Significant fraction of the total plasma stored energy ~5% can be expelled by an ELM (>10 MJ on ITER) Significant erosion of plasma facing materials will occur for ELM bursts greater than 1 MJ. A method to eliminate or induce rapid small ELMs is badly needed for ITER. Compatibility of method with pellet fueling needs to be demonstrated.

Jul LRB 3 Fueling Pellets Injected from all Locations on DIII-D Trigger ELMs Pellets injected from the 5 different injection locations on DIII-D are observed to trigger immediate ELMs. LFS injected pellets trigger larger longer lasting D  perturbations (larger ELMs). Natural ELM frequency on DIII-D is higher than available pellet fueling frequency so no chance to test ELM pacing with fueling pellets. DIII-D PoP 2000

Jul LRB 4 Pellet cloud releases from pellet and expands along a flux tube. Density from the cloud expands along flux tube at the sound speed c s. Temperature ‘cold wave’ travels along the flux tube at the thermal speed. Heat is absorbed in the cloud resulting in a temperature deficit far from the cloud. Pressure decays and expands along the flux tube with a lower pressure far from the cloud. Strong local cross field pressure gradients result along the flux tube that form on  s time scales. What Causes an ELM Trigger from a Pellet? L nene cscs TeTe qeqe L PePe L  

Jul LRB 5 ITER Penetration Requirement AUG Trigger Range Non-RMP HFS RMP HFS Non-RMP LFS RMP LFS Depth of Pellet when ELM is Triggered in DIII-D is Much Less Than Top of the Pedestal Data from DIII-D indicates that the pellet triggers an ELM before the pellet reaches half way up the pedestal. (% Ped is % of T e pedestal height) » Note: Deeper penetration is needed with RMP applied Pe (kPa)  Electron Pressure Pedestal DIII-D s Pre-pellet and ELM Pellet Location when ELM Triggered % Ped

Jul LRB 6 Pellet ELM Pacing Led to 2X Higher ELM Frequency on AUG Initial pellet ELM pacing shown for 1 second on AUG. (Lang et al NF 2004). Small fueling pellets from HFS doubled the natural ELM frequency. Further optimization needed to reduce strong fueling induced confinement decay and greatly increase the natural ELM frequency.

Jul LRB 7 Pellet Dropper for ELM Pacing on DIII-D The dropper was developed from existing equipment to make a simple ELM trigger tool. It uses a batch extruder with pellet cutter to supply sub 1mm D 2 pellets at up to 50 Hz for triggering ELMs on DIII-D The extruder is cooled with a G-M cryocooler and LN 2 for simplified installation and operation Gravity acceleration limits pellet speeds to ~ 10 m/s Low speed and small pellet size minimize fueling, but should make strong enough density perturbations to trigger ELMs DIII-D Platform Pellet Dropper V+3 at 0 o Microwave Cavity

Jul LRB 8 Pellet Dropper Operation Pictures of extruded solid deuterium (14K) and pellet chopped from extrusion Data from microwave cavity showing mass of each pellet.

Jul LRB 9 Pellet Dropper Now Operational on DIII-D NGS ablation model shows pellets should nearly reach T e pedestal in DIII-D H-mode. Good simulation of expected ITER 3mm pellet penetration depth. Initial operation will be to determine penetration depth and ELM triggering.

Jul LRB 10 Target Plasma for the Pellet Dropper is the ITER Scenario ELMs of this size on ITER could be 15 MJ per ELM The dropper is designed to drop 50 Hz pellets to trigger 50 Hz ELMs that have smaller energy loss Can the dropper be used to Obtain 5X increase in ELM Frequency? Density P NBI W tot Divertor D  ORNL Filterscope Data Time (s) E. Doyle 2008 DIII-D ITER Like Scenario

Apr2008 LRB Pellet Dropper Initial Results – April 2008 The 1-mm 10/ms deuterium pellets dropped during L-mode Ohmic plasmas travel in a vertical path and are observed in the divertor D  signals and edge interferometer. In H-mode NBI plasmas the dropper pellets are observed to “skip” along the SOL and are swept toroidally. No interferometer signals for these pellets – no penetration inside the separatrix. The largest pellets appear to coincide with ELMs and there may be a cause and effect, which is under investigation. Drag and ion ablation in the SOL are candidates for the strong toroidal deflection and poor penetration. Pellets with more momentum normal to the plasma are needed with this vertical trajectory. Tangential view from fast framing camera – images and movies by J. Yu Ohmic L-mode NBI H-mode

Apr2008 LRB ELM frequency evolution due to pellets ? – April time (s) Density (10 19 m -3 ) Divertor D  P NBI (MW) Dropper pellets ECRH Dropper pellets injected during ECRH pedestal control experiments : The ELM frequency is higher (~40 Hz) than without the pellets (~10 Hz) Shot

Apr2008 LRB DIII-D Platform Pellet Dropper V+3 at 0 o Microwave Cavity Pellet Dropper Future Plans for DIII-D The low pellet speed at a glancing angle to the plasma edge limits the effectiveness in triggering ELMs. (No obvious proofs of increase in the ELM frequency) The plan is to modify the entry tube to deflect the pellets so they hit more normal to the plasma surface. – Less SOL to travel through – Higher momentum normal to the plasma Future modifications as necessary to obtain reliable ELM trigger – larger pellets and or faster.

Jul LRB 14 ITER will have 2 pellet injectors that each provide D 2, DT, T 2 pellets ~10Hz, ~30Hz). Inside wall pellet injection for efficient fueling beyond the pedestal utilizing a curved guide tube. Maximum pellet survival speed is 300 m/s. Pellet injector must operate for up to 1 hour continuously and produce ~ 1.5 cm 3 /s (0.3 g/s) of ice. A LFS tube is being added for pellet ELM pacing. Pellet speed maximum probably ~500 m/s. ITER Pellet Fueling and ELM Pacing Challenges Pellet Path in ITER

Jul LRB 15 1/  E f pel Pellet Throughput/Plasma Volume (mm 3 /s-m 3 ) DIII-D JET AUG JT-60U ITER Pellet Normalized Throughput in Present Experiments is Heading Toward Projected ITER Pellet ELM Pacing Conditions The normalized pellet throughput from current pacing and fueling experiments is far from the projected operating point. The planned pellet ELM pacing experiments on JET, DIII-D, and AUG are all planning to be in a regime that is more ITER relevant in terms of normalized throughput and time between pellets scaled to energy confinement time. ITER range covers: Hz – (Time between pellets /  E )

Jul LRB 16 RMP Using Internal Coils Developed by Evans et al. is Successful at Eliminating ELMs with Reduced Density RMP technique developed by Evans et al. is successful in ITER like collisionality and shape at eliminating ELMs. DIII-D RMP and non-RMP Comparison Evans, NF2008

Jul LRB 17 Pellet Fueling (and gas fueling) into RMP ELM Suppressed plasmas can lead to a return to ELMs when high density is reached. (DIII-D result from Feb. 2008) In lower density cases with fewer pellets only a few ELMs are triggered by the pellets. The alternative of mitigation with high frequency ELMs from Pellet Pacing is needed – Pellet dropper on DIII-D – HFPI on JET Pellet Fueling During RMP Leads to ELMs when Density Increased HFS Pellets

Jul LRB 18  ITER will require ELM mitigation in order to complete its mission  RMP for ELM suppression needs further optimization to be compatible with high density operation and pellet fueling  ELM triggering by small LFS pellets a promising but still unproven technique for ITER » Further research to optimize and understand physics of pellet induced ELMs and ELM energy loss is required  Further pellet ELM pacing research is needed and is ongoing on DIII-D, JET, and AUG  The pellet injection system for ITER fueling and ELM pacing presents challenges for the technology in throughput and reliability, gas gun concept looks promising » Development is underway and expected to take ~ 4 yrs » Extruder and accelerator prototypes will be produced which can be available to test on existing tokamak devices Summary

Jul LRB 19 Plans for Future R&D ELM triggering with vertical pellets on DIII-D – test pellet size and speed necessary to trigger ELMs. Investigate ELM size and confinement properties with pellet pacing > 5x natural ELM frequency Participate in JET pellet pacing experiments » JET HFPI utilizes ORNL pellet mass diagnostics » LFS pellets enter through the RF antenna Scaling of results to ITER and development of a 3D model for pellet ELM triggering Future possible upgrade of D3DPI to use high frequency pellets with similar guide tube to ITER

Jul LRB Pellet Enters Plasma ELM Triggered ELM End n e L(10 14 m -2 ) dB r /dt (a.u.) Inner wall dB r /dt (a.u.) Outer shelf Divertor D  HFS Pellet Induced ELM Details from DIII-D TS profile time Pellet D  The ELM is triggered 0.05 ms after the pellet enters the plasma or 147 m/s* 0.05 ms = 7.4 mm penetration depth. DIII-D mm pellet injected from inner wall is ~10% density perturbation No MHD precursor to pellet induced ELM Pellet D  represents ablation of pellet in the plasma with assumed constant radial speed. Time (ms)

Jul LRB 21 ITER Pellet ELM Triggering May Provide Tool for ELM Amelioration NGS ablation model in PELLET code shows 300 m/s 3mm pellets should nearly reach T pedestal in ITER H-mode (4 keV pedestal T e ). Figure on right shows pellet size necessary to reach half of pedestal height as a function of pellet size. Pellet perturbation size is shown in blue curve.