Evidence on the Role of Main-Chamber Neutrals on Density Shoulder Broadening

Main-chamber neutral density, in addition to enhanced radial transport, plays a significant role in the broadening of the density shoulder based on evidence from TCV. Density profile broadening does not occur when baffles are used to reduce the flux of neutrals into the main-chamber, even in detached conditions where high divertor collisionality is achieved without the use of impurity seeding. Radial turbulent flux remains low in the cases when density shoulder broadening is prevented. Another source for main-chamber neutrals – recycling at the inner wall – was modulated by changing the plasma-wall gap, and was also found to affect the amplitude of the density shoulder and the turbulent transport.