30th EPS Conference on Controlled Fusion and Plasma Physics

Supersonic expansion in the Pilot-PSI plasma jet: neutral gas and MHD models

“…The plasma thus produced expands from a divergent nozzle source into a cylindrical vacuum vessel 1 m long and 40 cm in diameter, which is continuously evacuated by booster pumps to maintain a constant vessel pressure of 4-200 Pa. Five surrounding coils can create a homogeneous coaxial magnetic field of 0.4 T inside the vessel for 2 min or even allow field strengths of 0.8, 1.2 and 1.6 T during a few seconds. The externally applied field guides the collimated plasma beam to a specific target plate situated at the opposite end of the vessel [6]. The schematic diagram in figure 1 sketches the overall experimental set-up for Pilot-PSI.…”

Comparison of effective rate coefficients for high energy charge-exchange with measurements of the Rydberg series of Ar16+at the tokamak TEXTOR

“…In both scenarios the intensities of the K α lines increase towards lower plasma densities, but in case of NBI-1 heating, this increase is much stronger due to the influence of the beam neutrals. The increase in NBI-2 heated plasmas is assigned to CX contributions from thermal background neutrals, which are still abundant in the plasma core at low plasma densities [19]. The enhancement of the He-like lines between NBI-1 and NBI-2 heated plasmas should in good approximation be proportional to the density ratio of neutral particles and electrons in the observed volume.…”

“…It is, therefore, appropriate to estimate the radial extent of the individual emission shells and the contribution to be expected from CX with the NBI-1 neutrals using simple analysis first. Figure 5(a) shows the argon ion densities n Ar q+ obtained from 2D K α spectra measured during an ohmic discharge in TEXTOR [19] along with the normalized power density profile of NBI-1/2 [26]. The measured radial distribution of the argon ions deviates significantly from the corona balance calculated for TEXTOR conditions, while the ratio n n…”

“…The resulting rate coefficients for ground state excitation are found to be 2-5 times larger than the rate coefficients for radiative and dielectronic recombination, while the relative contribution of dielectronic recombination increases towards the high principal quantum numbers. However, as the concentration of H-like argon in TEXTOR plasmas is found to be on the order of 20% relative to the density of He-like argon [19], the relative contributions from electron recombination to the observed line intensities remain weak at least for n ⩽ 12. Therefore, the spectral emissivity of the Rydberg lines is predominately due to ground state excitation.…”

“…The doubly excited upper states cannot be populated by CX due to negligible fractions of singly excited argon ions in the plasma (low-density limit approximation). One should note that this experimental fact can be utilized for transport and neutral particle studies on the basis of He-like spectra [19]. With increasing plasma density the impact of CX on the K α lines clearly reduces (figure 2(d)), since on the one hand the excitation rates become stronger and, more importantly, the penetration depth or e-folding length of the beam atoms λ becomes shorter: n ( )…”