Installation and Operation of New Long Pulse DNB on Alcator C-Mod

Abstract

A 50 kV, 7 amp, long-pulse (1.5-3.0 s) diagnostic neutral beam (DNB) built by the Budker Institute of Nuclear Physics in Novosibirsk has been installed on the Alcator C-Mod tokamak. The DNB is used for multiple diagnostics, including motional Stark effect (MSE), charge-exchange recombination spectroscopy (CXRS), and beam emission spectroscopy (BES). Facility required power is 1 MVA and is provided by a 5 kV electrical service. The 1.25 m3 DNB vacuum chamber is pumped using a 500 l/s turbo molecular pump, and two 50,000 l/s liquid helium cryo pumps. Mass flow controllers regulate the hydrogen gas flow to the source anode and cathode. A heated lanthanum hexaboride emitter is used for the arc cathode in order to provide an enhanced lifespan. The beam is extracted from a plasma arc source and accelerated by a set of perimeter-cooled molybdenum grids. Accelerated ions are partially neutralized in a gas cell, and the un-neutralized fraction is diverted by a bending magnet into a water cooled dump. A water cooled, instrumented movable calorimeter provides a target for test and conditioning shots. The neutralized beam is injected through a port duct into the C-Mod tokamak, and has a diameter specification of ~6 cm at the full-width half-maximum (FWHM) at the plasma. Spectroscopic measurements during initial commissioning have shown a full extracted energy fraction (at the source) of ~70% of beam current. Due to the high energy delivery capability of this diagnostic beam there is a risk of overheating the tokamak inner wall. A beam interlock system will incorporate the tokamak pulse state, plasma density, toroidal field, tokamak gas pressure, and an optical pyrometer aimed at the tokamak inner wall