By means of Optical Emission Spectroscopy (OES) the line-of-sight integrated light emission out of the plasma is investigated. Although no detailed evaluation of OES measurements has been carried our during this work, a brief overview is given within this section since measurement results from this work will be compared to former OES measurements from [35].
Spectrometers, optics and a detector with an accessible wavelength range of the extended visible light (e.g. 250–900 nm) are used for a wavelength resolved measurement of the plasma emission. There are two modes of operation at the IPP prototype source:
• As a routine diagnostic, OES is used for monitoring the plasma stability or for the identification of impurities. For this purpose, no information of the absolute light emission is necessary. In addition to a spectroscopic system, also photo diodes are used with a faster temporal resolution of several 100 Hz after amplification. An interference filter in front of the photo diodes narrows the detected wavelength range at the desired transition (e.g. a filter with central wavelength of 852 nm and ∆λFWHM = 10 nm is used to monitor the Cs 852 nm emission).
• OES is also used for the evaluation of plasma parameters of selected pulses. For this purpose, the measurement setup (window, optical head, fibers and the spectrometer itself) has to be absolutely calibrated what is done at the IPP test facilities by means of a calibrated Ulbricht sphere, which produces perfect diffuse light at known wavelength-dependent intensity.
The line emissivity ik [photons · m−3s−1] of an optical transition from an excited
intensity I(λ) by ik = λ2 Z λ1 I(λ)dλ. (3.20)
The transition probability (Einstein coefficient for spontaneous emission Aik)
yields the particle density ni in the upper state i:
ik = niAik. (3.21)
For the evaluation of the density in the ground state or plasma parameters as the electron density or temperature, models have to be applied which determine the population and depopulation of the upper states in sufficient precision as a function of the plasma parameters. A state can be populated or depopulated by many different excitation or de-excitation processes, e.g. by collisions with electrons, by photon absorption or emission, dissociative excitation for molecules and many more. Also ionization and recombination processes have to be considered; beside volume recombination the recombination at the wall can play an important role. Three cases can be classified in dependence of the electron density:
• ne & 1024 m−3 :
At high electron densities, a local thermal equilibrium establishes, where each excitation process is in equilibrium with its de-excitation process – with the exception of processes in which photons are involved, since photons can have long mean free paths. The population of excited states follows the Boltzmann distribution in this case – in contrary, Planck’s law for the description of the emitted radiation is usually not fulfilled.
• ne . 1016 m−3 :
For low electron densities, a so-called corona model can be applied, if excitation processes by heavy particle collisions are neglegible: due to the low density, excited states are solely excited by electron collision from the ground state and de-excited solely by spontaneous emission. Balancing of these two processes yields the rate equation:
n0neX0i(Te) = ni
X
k<i
with n0 denoting the ground state density (which is equal to the particle density of the species in this case) and X0i(Te) the excitation rate coefficient for electron collision excitation from the ground state 0 to the excited state i. • 1016 m−3
. ne . 1024 m−3:
The electron density in the IPP prototype source is in between the validity of these two models; in this regime so called collisional radiative models need to be applied. In a collisional radiative model, the relevant excitation and de-excitation processes are included for the relevant particle species. A collisional radiative model for H and H2 based on the flexible solver Yacora [80] is applied at the IPP neutral beam test facilities for quantitative evaluation of OES measurements [81], giving access to a multitude of plasma parameters. An overview is given in [41].
The lines of sight at the prototype ion source have a diameter of 1 cm, which is defined by the collimator optics in combination with the used optical fiber. The optical fibers have an enhanced UV transmission, enabling also measurements at low wavelengths in the near UV. At the LOS in the extended boundary layer, which will be shown in figure 3.15, either a photo diode with interference filter is attached, or a survey spectrometer24 is used, which creates automatically a time trace of several emission lines for each pulse with a temporal resolution of 150 ms. In this work, only the emissivity of certain emission lines will be shown for comparison with other diagnostics in the extended boundary layer. In general, the measured emissivity ik is linked to the density of the particular particle species,
electron density and effective emission rate coefficient Xem
ik,eff(Te, ne, ...):
ik = n0neXik,effem (Te, ne, ...). (3.23)
Thus, the Cs line emission at 852 nm scales with the Cs density. In the same way, the Balmer line emission of hydrogen atoms, as Hβ (486 nm for H) or Hγ
(434 nm for H) scales with nH. The emission of the Fulcher system of hydrogen molecules (between 590 nm and 650 nm), which consists of multiple lines based on a electronic transition split by their vibrational and rotational levels, scales with
nH2. Hence, the ratio between the Hβ or Hγ emission and the Fulcher emission scales with the density ratio nH/nH2 for constant plasma parameters.
24PLASUS EmiCon system, wavelength range 200–870 nm, wavelength resolution 400 pm per
For the production of negative hydrogen ions in the IPP prototype source, the flux of dissociated hydrogen ions and atoms towards the plasma grid is an
important parameter, since the flux of surface produced H− into the plasma
directly depends on it, as long as no limitation due to space charge effects occurs (see section 2.4). As the dissociation mainly takes place in the driver, a detailed characterization of the driver plasma including the determination of the density ratio nH/nH2 is desirable. This has been carried out at an axial LOS through the driver towards the plasma grid in the past [35]. Due to the higher plasma density and temperature, the line-of-sight integrated emission of this LOS is dominated by the driver emission.