Quiescent H-mode plasmas in the DIII-D tokamak

TLDR

H‑mode is the preferred operating regime for future tokamaks, yet it incurs significant drawbacks from edge‑localized modes that erode divertor surfaces and limit achievable β and core transport. Quiescent H‑mode is achieved by counter‑injection of neutral beams combined with cryopumping to lower density, and by exploiting an edge harmonic oscillation that boosts particle transport without degrading energy confinement. Experiments on DIII‑D demonstrate that quiescent H‑mode is ELM‑free, provides robust density control, sustains an internal transport barrier for over 3.5 s, and reaches β_NH89 ≈ 7—more than twice the β achieved in conventional ELMing H‑mode.

Abstract

H-mode operation is the choice for next-step tokamak devices based either on conventional or advanced tokamak physics. This choice, however, comes at a significant cost for both the conventional and advanced tokamaks because of the effects of edge-localized modes (ELMs). ELMs can produce significant erosion in the divertor and can affect the β limit and reduced core transport regions needed for advanced tokamak operation. Recent experimental results from DIII-D have demonstrated a new operating regime, the quiescent H-mode regime, which solves these problems. We have achieved quiescent H-mode operation which is ELM-free and yet has good density control. In addition, we have demonstrated that an internal transport barrier can be produced and maintained inside the H-mode edge barrier for long periods of time (>3.5 s or >25 energy confinement times τE). By forming the core barrier and then stepping up the input power, we have achieved βNH89 = 7 for up to 10 times the τE of 160 ms. The βNH89 values of 7 substantially exceed the value of 4 routinely achieved in standard ELMing \mbox{H-mode.} The key factors in creating the quiescent H-mode operation are neutral beam injection in the direction opposite to the plasma current (counter injection) plus cryopumping to reduce the density. Density control in the quiescent H-mode is possible because of the presence of an edge MHD oscillation, the edge harmonic oscillation, which enhances the edge particle transport while leaving the energy transport unaffected.