Abstract
At the Italian National Institute for Nuclear Physics-Southern National Laboratory (INFN-LNS), and in collaboration with the ATOMKI laboratories, an innovative multi-diagnostic system with advanced analytical methods has been designed and implemented. This is based on several detectors and techniques (Optical Emission Spectroscopy, RF systems, interfero-polarimetry, X-ray detectors), and here we focus on high-resolution, spatially resolved X-ray spectroscopy, performed by means of a X-ray pin-hole camera setup operating in the 0.5–20 keV energy domain. The diagnostic system was installed at a 14 GHz Electron Cyclotron Resonance (ECR) ion source (ATOMKI, Debrecen), enabling high-precision, X-ray, spectrally resolved imaging of ECR plasmas heated by hundreds of Watts. The achieved spatial and energy resolutions were 0.5 mm and 300 eV at 8 keV, respectively. Here, we present the innovative analysis algorithm that we properly developed to obtain Single Photon-Counted (SPhC) images providing the local plasma-emitted spectrum in a High-Dynamic-Range (HDR) mode, by distinguishing fluorescence lines of the materials of the plasma chamber (Ti, Ta) from plasma (Ar). This method allows for a quantitative characterization of warm electrons population in the plasma (and its 2D distribution), which are the most important for ionization, and to estimate local plasma density and spectral temperatures. The developed post-processing analysis is also able to remove the readout noise that is often observable at very low exposure times (msec). The setup is now being updated, including fast shutters and trigger systems to allow simultaneous space and time-resolved plasma spectroscopy during transients, stable and turbulent regimes.
Highlights
At the Italian National Institute for Nuclear Physics—Southern National Laboratory (INFN-LNS) and in collaboration with the ATOMKI Laboratories, efforts have been devoted to the study, development and use of an innovative multi-diagnostic setup with advanced analytical techniques aiming to characterize the thermodynamical properties of the Electron Cyclotron Resonance (ECR)-magnetized plasmas confined in compact traps for multidisciplinary studies
This figure reports the shape of the plasma core, the so-called “plasmoid”, which is typically contained within the iso-magnetic surface fixed by the ECR condition ω RF = qB/m, where ω RF is the RF pumping frequency, B is the magnetic field and q and m are the electron charge and mass, respectively
The developed algorithm, once given the amount of time needed for information transfer between the rows and the standard acquisition time of the CCD, starts by identifying the amount of photons collected in the “wrong” position, considering the spatial distribution of the photons collected in the exposure time; once identified as wrongly collected photons, the algorithm reallocates them in the correct positions
Summary
The measurements were carried out with a room-temperature, second-generation ECR ion source installed in the ECR Laboratory of ATOMKI (Debrecen), based on a Bminimum magnetic field configuration—characterized by the superposition of an axial magnetic field (from axial coils) and a radial magnetic field (from hexapole coils)—using a basic operation pumping frequency of 13.9 GHz at a power of 200 W. To allow for SPhC-based, quantitative, space-resolved spectral analysis, the ATOMKI plasma chamber walls were covered by thin layers of materials with different X-ray fluorescence energies, whilst the plasma was made by Ar, with a fluorescence emission at around 2.96 keV In this way, even in energy-integrated images, the X-rays coming from the plasma are clearly visible, while the materials of the extraction (titanium) and injection (aluminum) endplate and of the lateral walls (tantalum) provide bright regions of X-ray emission due to bremsstrahlung and X-ray fluorescence. This figure reports the shape of the plasma core, the so-called “plasmoid”, which is typically contained within the iso-magnetic surface fixed by the ECR condition ω RF = qB/m, where ω RF is the RF pumping frequency, B is the magnetic field and q and m are the electron charge and mass, respectively. The Al mesh (with a wire diameter of 400 μm) that was placed on the injection endplate to keep the microwave resonator-like properties of the plasma chamber is clearly visible, allowing for direct inspection of the chamber interior, guaranteeing more than 60% of optical X-ray transmission
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