Studies of impurity behaviour in TFTR

Central medium and low Z impurity concentrations and Z eff have been measured by X-ray spectrometry in Tokamak Fusion Test Reactor discharges during three periods of operation. These were (1) startup period, (2) Ohmic heating, and (3) Ohmic heating portion of the two neutral beam periods, distinguished mainly by different vacuum vessel internal hardware and increasing plasma current and toroidal field capability. Plasma parameters spanned minor radius a = 0.41−0.83 m, major radius R = 2.1−3.1 m, current I p = 0.25−2.0 MA, line averaged electron density e = (0.9–4.0) × 10 19 m −3 , and toroidal magnetic field B T = 1.8−4.0 T. The metal impurities came mostly from the limiter. At low densities titanium or nickel approached 1% of ne during operation on a TiC-coated graphite or Inconel limiter, respectively. Lower levels of Cr, Fe, and Ni (⪅0.1%) were observed with a graphite limiter at similarly low densities; these elements were removed mainly from stainless steel or Inconel hardware within the vacuum vessel during pulse discharge cleaning or plasma operation on an Inconel limiter and then deposited on the graphite limiter. Hardware closest to the plasma contributed most to the deposits. Subsequent discharges slowly eroded the deposits from the limiter and reduced the metal impurity levels in the plasma. Both total low Z and metal impurity concentrations, relative to n e , decreased approximately exponentially with ne and increased with I p . The metals showed much larger variations than did low Z elements, dropping to ≳10 −5 n e at high density. Low Z impurities, mainly carbon and oxygen, ranged from 10% of n e at low n e to 3% at high n e and usually dominated Z eff and power loss. In low density plasmas with a TiC-coated graphite or Inconel limiter, the respective metals contributed 30–50% of Z eff and ⪅10–20% of the central input power. The metal contributions to Z eff and radiated power were much lower with the graphite limiter.