A Quiescent H-mode is an H-mode without ELM activity. The QH-mode was discovered at DIII-D,   but was subsequently seen at JT-60U, ASDEX-U  and JET. QH plasmas have a characteristic Edge Harmonic Oscillation (EHO) but remain below the peeling stability boundary associated with ELMs.
The absence of ELMs is positive for plasma operation as it reduces pulsed stresses on and damage to plasma facing components (mainly the divertor), but inconvenient from the point of view of impurity accumulation.
- K.H. Burrell, M.E. Austin, D.P. Brennan, et al., Quiescent H-mode plasmas in the DIII-D tokamak, Plasma Phys. Control. Fusion 44 (2002) A253
- K.H. Burrell, T.H. Osborne, P.B. Snyder, et al., Quiescent H-Mode Plasmas with Strong Edge Rotation in the Cocurrent Direction, Phys. Rev. Lett. 102 (2009) 155003
- W. Suttrop, G.D. Conway, L. Fattorini, et al., Study of quiescent H-mode plasmas in ASDEX Upgrade, Plasma Phys. Control. Fusion 46 (2004) A151