Preliminary observation of medium-Zimpurities by means of the soft x-ray pulse height analysis diagnostic on EAST

Abstract

A soft x-ray pulse height analysis (PHA) system, based on a silicon drift detector (SDD), has been successfully implemented on EAST to measure spectra of soft x-ray emission. Utilizing the soft x-ray PHA diagnostic, the behavior of intrinsic and injected medium-Z impurities is first investigated on EAST and the preliminary observations are presented. There are hardly any K α emission peaks of metallic impurities including Ti (Z = 22), Cr (Z = 24), Fe (Z = 26), Ni (Z = 28) and Cu (Z = 29) in the soft x-ray spectra for an ohmic heating plasma due to lower electron temperatures, while remarkable peaks can be seen in a lower hybrid wave (LHW) heated plasma. The profiles of K α intensities of these metallic elements have been obtained with about 7 cm spatial resolution. It is observed that the K α emissions come mainly from the core plasma column of ρ 0.3 and found that their intensities are roughly proportional to the power of the LHW launched into the plasma. The Ar K α peaks can also be monitored when argon gas is puffed into the plasma. It is qualitatively confirmed that the K α lines of argon emitted from the lower ionic states are predominantly located in the outer plasma region, whereas the K α lines with higher energies are mainly from high ionization stages in the central region.

Formation of plasmoids during sawtooth crashes

“…The latter persists on a long time scale, characteristic of the current re-arrangement within the q = 1 surface, or the sawtooth period. It is characterized by a persistent 2/2 component of the perturbation, which is also experimentally observed in many cases [19,20]. In particular, this would also offer an explanation for the triggering of m/n = 3/2 neoclassical tearing modes by sawteeth, which is difficult to explain by the action of a short perturbation only during the sawtooth crash phase.…”

The q-profile effect on high-order harmonic q = 1 tearing mode generation during sawtooth crashes

“…Very recently, it was also observed again by the ECE image reconstruction technique that during the sawtooth oscillations, the high-order harmonic m=n ¼ 2=2 and m=n ¼ 3=3 modes were as strong as the m=n ¼ 1=1 mode and responsible for the final sawtooth crash. 17 And the modes were able to still exist during the subsequent final strong reconnection. In addition, it should be pointed out that not only can the high-order harmonic modes cause the partial reconnection but also contribute to supply the seed island for the neoclassical tearing modes (NTMs) 18 and lead to the final disruption.…”

“…On the other hand, it was further revealed in experiments that the high-order harmonics, such as m=n ¼ 2=2 and/or m=n ¼ 3=3 modes, were significantly generated in the low b plasmas during the sawtooth oscillations in several tokamaks. [11][12][13][14][15][16][17] In low b Ohmic discharges on TFTR, the high resolution electron cyclotron emission (ECE) image reconstruction technique was used to analyze the partial sawtooth crash. 11 It was observed that during the partial sawtooth crash, the m=n ¼ 2=2 or the m=n ¼ 3=3 islands due to the localized reconnection became dominant and merged into the m=n ¼ 1=1 mode before the final crash.…”

Comparison of anomalous Doppler resonance effects with molybdenum and graphite limiters on HT-7

“…The radius of the runaway electrons with graphite limiter is about 1/8 of the minor radius of HT-7 tokamak 8 at the onset of ADR effects. In tokamak plasma, high-Z impurities lead to a stronger central accumulation, 17,18 then the concentration of the high-Z impurities is higher near the center. The behavior of high-Z impurities leads to a higher concentration of electrons near the center due to the plasma electrical neutrality.…”