Control of re-cycling and impurities in the ATC tokamak by means of gettered surfaces

Abstract

A simple technique for reducing the re-cycling rate and impurity concentration in a tokamak plasma is described. An active metal coating, in this case titanium, evaporated onto the surface of the vacuum vessel, provides a trap for neutral hydrogen and impurity atoms which would otherwise freely penetrate the plasma. With this treatment, the plasma density decays with time after the ionization of the initial filling gas is completed, in contrast to typical standard discharges which have a rising density throughout the entire period of the discharge, indicating a large gas influx. These discharges are observed to have resistances close to that of a pure hydrogen plasma, Zeff ≃ 1.0. There is a corresponding reduction in the intensity of highly ionized spectral lines of oxygen and iron as evidence of reduced impurity concentrations. The value of the effective ion charge, Zeff, can be varied, by pulsing controlled amounts of impurity gases into the hydrogen plasma.

Development of ultrahigh and extreme high vacuum technology for physics research

“…This result was achieved by evaporating titanium onto a large fraction of the vessel wall prior to a plasma discharge to suppress the impurities, combined with programmed injection of hydrogen during the discharge [73].…”

Vacuum theory: a possible solution to the singularity problem of cosmology

“…The particle flux to the limiter coincides with the particle flux from the main plasma because the recombination rate is negligible for T e > 10 eV; F p is re-written as follows: (6) and the gross particle confinement time of the main plasma column, respectively. PL is not strongly changed by increasing machine parameters because N e proportional to the major radius and the square of the minor radius, r p is also proportional to the square of the minor radius, and T e is limited as was discussed above.…”

“…In DITE tokamak [5], high-Z impurities have been shown to decrease substantially by operating the bundle divertor, but the low-Z impurities are still dominant as is often observed in the present-day conventional tokamaks. That light impurities can essentially be eliminated by cleaning the first wall has been shown by experiment [6,7]. Thus, the question arises whether it is possible to reduce the high-Z impurities after reducing the low-Z impurities.…”

Finite-zone analogues ofN-multiplet solutions of the Korteweg-de Vries equation

“…For example, in the ST tokamak, variations in the high-Z impurities were achieved by changing the level of oxygen contamination on the wall [ 1 ]. In the ATC, titanium-gettering experiments led to dramatic variations in the oxygen level [2]. In Alcator, Z e ff = 1 plasmas were obtained without oxygen and/or titanium "blankets" [3].…”