Latest Results of CDB-11 Scaling of the Low Density Limit of the H-mode Threshold

1. Latest Results of CDB-11 Scaling of the Low Density Limit of the H-mode Threshold J A Snipes ITER Organization, St. Paul-lez-Durance, France

2. nthmin Increases with Increasing Toroidal Field Ohmic H-modes are achieved down to 0.34  1020 m-3 ICRF H-modes cannot be achieved below 0.69  1020 m-3 with up to 2.5 MW of ICRF power • nthmin increases with increasing toroidal field • Ohmic H-mode threshold powers are lower than ICRF thresholds

3. C-Mod Data Suggests nthmin Increases with BT ITPA Database 2007-08 RF C-Mod 2007-08 OH C-Mod Y. Martin, 2008 • C-Mod, AUG, and JFT-2M data suggest nthmin increases nearly linearly with increasing toroidal field but JET and JT-60U show a weaker trend

4. Conclusions • The low density limit to the H-mode threshold, nthmin, at 5.4 T was around 1  1020 m-3, whereas previously it was 0.8  1020 m-3 • The relative threshold power vs density curve was independent of plasma current  unrelated to the Greenwald density limit • Edge Te decreases with increasing density except at 0.6 MA • At low q95 = 3.3, near the ITER q95, large sawtooth crashes cause short dithering H-modes even up to twice the power threshold scaling with ICRF but a long Ohmic H-mode is obtained with much lower input power • The C-Mod data indicate that Ohmic H-mode threshold powers are significantly lower than ICRF H-mode threshold powers

5. The H-mode threshold low density limit is nthmin 0.4  1020 m-3 at 2.2 T whereas previously it was 0.8 – 1.0  1020 m-3 at 5.4 T • This lower density limit approaches the low density limit of nthmin 0.25  1020 m-3found in DIII-D, JET, AUG, and JT-60U • It was, however, difficult to achieve these very low densities without getting runaway electrons • There was no evidence of locked modes • The latest C-Mod data suggest that the H-mode threshold low density limit increases nearly linearly with toroidal field but JET and JT-60U data show a much weaker trend • The H-mode threshold low density limit in ITER at 5.3 T may have nthmin> 0.8  1020 m-3 requiring ~ 2.5  Pthresh to reach H-mode at the prescribed target density of ne = 0.5  1020 m-3

6. Further Work • Further experiments are required to understand the differences in the H-mode threshold between Ohmic and ICRF heating • Measurements of fast ion losses would help to answer the question of whether or not they are responsible for reduced ICRF heating efficiency at low density • Further experiments on other devices comparing different heating schemes (NBI, ECH, Ohmic, ICRF, etc) would help to check if similar differences in the H-mode threshold low density limit are observed • Need further experiments on JET, DIII-D, and ASDEX-Upgrade to check the BT and Ip dependences of the H-mode threshold low density limit • Need a testable theory of the H-mode threshold low density limit