W. Biel

Collisional excitation and emission of Hα Stark multiplet in fusion plasmas

We study the excitation of parabolic Stark states in hydrogen atoms by collisions with fast ions. It is shown that excitation cross sections are very sensitive to the angle between the electric field and the projectile velocity. The calculated collisional data are implemented in a newly developed collisional?radiative model involving parabolic quantum states of hydrogen. Our simulations are shown to explain the frequently observed non-statistical behaviour of the H? component intensities under typical conditions of a motional Stark effect (MSE). A good agreement with the MSE data from the Joint European Torus (JET) for emission of the ? and ? components (Mandl et al 1993 Plasma Phys. Control Fusion 35 1373) is obtained for the first time.

Method to obtain absolute impurity density profiles combining charge exchange and beam emission spectroscopy without absolute intensity calibration

Investigation of impurity transport properties in tokamak plasmas is essential and a diagnostic that can provide information on the impurity content is required. Combining charge exchange recombination spectroscopy (CXRS) and beam emission spectroscopy (BES), absolute radial profiles of impurity densities can be obtained from the CXRS and BES intensities, electron density and CXRS and BES emission rates, without requiring any absolute calibration of the spectra. The technique is demonstrated here with absolute impurity density radial profiles obtained in TEXTOR plasmas, using a high efficiency charge exchange spectrometer with high etendue, that measures the CXRS and BES spectra along the same lines-of-sight, offering an additional advantage for the determination of absolute impurity densities.

Non-statistical simulations for neutral beam spectroscopy in fusion plasmas

While most of the existing models for calculation of neutral beam emission and attenuation in a fusion plasma assume a statistical distribution of population for the excited states of the beam, the observed motional Stark effect data reveal significant deviations from the statistical predictions. To address this problem, we developed a new collisional-radiative model based on the parabolic states which are the true eigenstates of a hydrogen atom in an induced electric field. This model is completely resolved in the magnetic quantum numbers up to n=10 without any assumption of a statistical distribution. Calculation of collisional cross sections between the parabolic states is based on the atomic-orbital close coupling and Glauber approximations for density matrix elements. Our simulations show excellent agreement with the experimental data from the JET tokamak and also can be used to define the limits for the application of statistical approximation.

A non-statistical atomic model for beam emission and motional Stark effect diagnostics in fusion plasmas

In this work we analyze magnetic sublevel populations in a neutral beam penetrating a fusion plasma. The collisional-radiative model NOMAD was extended to include magnetic parabolic sublevels with principal quantum numbers n ≤ 10. The collisional parameters were calculated with the advanced atomic-orbital close coupling method and the Glauber approximation. The ionization by the induced electric field was also included in the model. The results of our calculations show significant deviations of the sublevel populations and, accordingly, line intensities of the σ and π components, from the statistical approximation. It is shown, for instance, that for a number of experimental conditions the total intensity of σ components is not equal to the total intensity of π components, which has a strong effect on determination of magnetic field and pitch angle in fusion devices. The results are presented for a wide range of plasma and beam parameters. The most significant deviations are observed for strong magnetic fields and high beam energies typical for the ITER plasma, where component intensity ratios may deviate by more than 20% from the statistical values.

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

The charge-exchange (CX) rate coefficients for highly ionized impurity ions play a crucial role in fusion plasma diagnostics. However, till today a substantial difference exists in data for the nl-resolved cross-sections based on the different approximations underlying the classical trajectory Monte Carlo (CTMC) calculations either based on the standard initial momentum distribution of target electron orbits (pCTMC, as in the CX data provided by Whyte et al (1998 Phys. Plasmas 5 3694) and Schultz et al (2010 J. Phys. B: At. Mol. Opt. Phys. 43 144002) or based on the alternate initial radial distribution of orbits (rCTMC, as in the calculations of Errea et al (2006 J. Phys. B: At. Mol. Opt. Phys. 39 L91). In this paper, results of new pCTMC and rCTMC calculations for CX in 16.7, 25, and 50 keV/u Ar17+ + H(1s), H(2s), and H(2p) are compared against X-ray line measurements performed at the tokamak TEXTOR. The Rydberg series (1snp–1s2) and the Kα-spectrum (1s2l–1s2) of He-like argon were measured directly in the beam-line of a 16.7–50 keV/u hydrogen injector. The intensities of the spectral lines are compared to the effective CX rate coefficients utilizing both sets of cross sections. While both data sets show good agreement with respect to the observed impact on the Kα transition, only the pCTMC data allow a consistent description of the CX resonance observed on the Rydberg lines around n ≈ 8, 9. Similar to the case of low energy ion–atom interactions reported from different tokamaks, the observed influence of CX is separable into contributions from beam particles in the ground and excited states, respectively. It is shown, that the number of beam excited states nh contributing to the CX signal, where nh is the principal quantum number, is limited to nh 10, confirming the results of recent collisional-radiative models of beam atoms in parabolic states.

Atomic data for beam-stimulated plasma spectroscopy in fusion plasmas

Injection of high energy atoms into a confined plasma volume is an established diagnostic technique in fusion research. This method strongly depends on the quality of atomic data for charge-exchange recombination spectroscopy (CXRS), motional Stark effect (MSE) and beam-emission spectroscopy (BES). We present some examples of atomic data for CXRS and review the current status of collisional data for parabolic states of hydrogen atoms that are used for accurate MSE modeling. It is shown that the collisional data require knowledge of the excitation density matrix including the off-diagonal matrix elements. The new datasets for transitions between parabolic states are used in an extended collisional-radiative model. The ratios between the σ- and π-components and the beam-emission rate coefficients are calculated in a quasi-steady state approximation. Good agreement with the experimental data from JET is found which points out to strong deviations from the statistical distribution for magnetic sublevels.

Collisional excitation and emission ofHαStark multiplet in fusion plasmas

We study the excitation of parabolic Stark states in hydrogen atoms by collisions with fast ions. It is shown that excitation cross sections are very sensitive to the angle between the electric field and the projectile velocity. The calculated collisional data are implemented in a newly developed collisional?radiative model involving parabolic quantum states of hydrogen. Our simulations are shown to explain the frequently observed non-statistical behaviour of the H? component intensities under typical conditions of a motional Stark effect (MSE). A good agreement with the MSE data from the Joint European Torus (JET) for emission of the ? and ? components (Mandl et al 1993 Plasma Phys. Control Fusion 35 1373) is obtained for the first time.

Collisional excitation and emission for H_alpha Stark multiplet emission in fusion plasmas

We study the excitation of parabolic Stark states in hydrogen atoms by collisions with fast ions. It is shown that excitation cross sections are very sensitive to the angle between the electric field and the projectile velocity. The calculated collisional data are implemented in a newly developed collisional?radiative model involving parabolic quantum states of hydrogen. Our simulations are shown to explain the frequently observed non-statistical behaviour of the H? component intensities under typical conditions of a motional Stark effect (MSE). A good agreement with the MSE data from the Joint European Torus (JET) for emission of the ? and ? components (Mandl et al 1993 Plasma Phys. Control Fusion 35 1373) is obtained for the first time.

Kinetics of highly excited states in Ar17+ charge exchange recombination fusion plasma spectroscopy

Charge exchange (CX) between impurities and injected neutral particles, which results in line emission from highly excited states of impurity ions, is the basis of an important technique for spectroscopic diagnostics of fusion plasmas. In this paper, we study the temporal kinetics of highly excited nl states (n ≤ 20) of H-like argon under typical conditions of a CX recombination experiment in tokamaks. A two-zone model is implemented to describe the tokamak plasma with and without a neutral beam. A detailed time-dependent collisional- radiative model describing ions of Ar15+ through Ar18+ and the atomic processes that affect the excited levels is used to determine the level densities. The relative contributions of atomic processes to population influx and outflux with and without CX are discussed in detail. The simulations show that after injection of a neutral beam, the populations of excited states oscillate between two quasi steady-state conditions, and the relaxation of ionization equilibrium is reached on times much longer than a typical plasma rotation time in a tokamak.