Fundamental investigations in the field of plasma physics on the ion production in sources of negative hydrogen ions for particle accelerators
Final Report Abstract
Sources of negative hydrogen ions are applied at various particle accelerators around the world. They utilize a low pressure low temperature hydrogen plasma for the ion generation. The performance optimization of these sources is mainly based on an empirical approach up to now: the operational parameters or certain design properties are changed and the impact on the properties of the extracted H– beam is characterized. The intermediate steps of how the plasma parameters are actually influenced and how this in turn affects the source performance is typically not investigated as the compact sources are difficult to access with diagnostic methods. This project aimed at investigating these two aspects for the H– ion source of the Linac4 accelerator at CERN as the detailed understanding of these issues is of crucial importance for further source optimization. As diagnostic method, optical emission spectroscopy (OES) has been applied as it only requires access to the ion source plasma emission via a small window. The measured emissivities have been evaluated with collisional radiative models in order to obtain plasma parameters what allows for a detailed assessment of the particular H− production and destruction processes. In order to obtain spatially resolved plasma parameters which is of importance for studying the impact of certain source design aspects like external magnetic fields, the electromagnetic particle-in-cell Monte Carlo Collision code NINJA was applied in addition to OES. This code has been developed within a PhD thesis during the last years at CERN. This project offered the opportunity to perform a detailed code benchmark which was successfully carried out at diagnostically well accessible lab experiments at the University of Augsburg. At these experiments also the fundamental investigations concerning the refinement of the applied diagnostic methods and evaluation procedures have been carried out. In general, the creation of H– ions in the source can proceed via two different pathways: either in the plasma volume (via electron impact dissociation of vibrationally excited hydrogen molecules) or via the conversion of hydrogen atoms or ions on a converter surface having a low work function. From the operational point of view, it is highly desirable to run the source in volume mode as it is basically maintenance free. However, a higher H– current is achieved in surface mode, where regular evaporation of caesium on the converter surface on the basis of weeks is required for maintaining the low work function. Within the framework of the project, systematic operational parameter variations have been performed and the corresponding plasma parameters have been evaluated successfully. In addition, the impact of design properties like the externally applied magnetic fields on the plasma were characterized. The obtained plasma parameters were fed into a rate balance model which allowed for assessing those processes which are relevant for the H– generation and destruction. For volume operation, the underlying plasma process mechanisms that determine the experimentally observed source performance trends for operational parameter variations could be identified. Concerning the surface operational mode, the process of the particle conversion to H– ions cannot be included in the process balancing model. For investigating this task in detail, specialized beam formation codes are required and the corresponding studies are still ongoing. In summary, for H– ion sources applied at particle accelerators, a characterization of the plasma parameters with respect to the operational parameters and to design properties was carried out. This represents the first time that such systematic investigations were performed for these ion sources. It has been demonstrated, that with the unique interplay between results obtained from OES measurements and numerical simulations with the NINJA code valuable insights in the plasma parameters of the Linac4 ion source and in the governing processes with respect to the H– formation and destruction could be achieved. To a large extent, the experimentally observed behaviours can now be explained on a plasma-physical foundation. In general, the gained results are not limited to the Linac4 ion source but detailed insights in the plasma parameters and the processes can be obtained by OES measurements at any other H– ion source with limited experimental effort.
Publications
- Comparison of the B field dependency of plasma parameters of a weakly magnetized inductive and Helicon hydrogen discharge, Plasma Sources Sci. Technol. 25 (2016) 035015
S. Briefi, P. Gutmann, D. Rauner and U. Fantz
(See online at https://doi.org/10.1088/0963-0252/25/3/035015) - Determination of discharge parameters via OES at the Linac4 H− ion source, Rev. Sci. Instrum. 87 (2016) 02B104
S. Briefi, D. Fink, S. Mattei, J. Lettry and U. Fantz
(See online at https://doi.org/10.1063/1.4932009) - Determination of the rotational population of H2 and D2 including high-N states in low temperature plasmas via the Fulcher-α transition, J. Quant. Spectrosc. Radiat. Transfer 187 (2017) 135–144
S. Briefi, D. Rauner and U. Fantz
(See online at https://doi.org/10.1016/j.jqsrt.2016.09.015) - Experimental benchmark of the NINJA code for application to the Linac4 H– ion source plasma, New J. Phys. 19 (2017) 105006
S. Briefi, S. Mattei, D. Rauner, J. Lettry, M. Q. Tran and U. Fantz
(See online at https://doi.org/10.1088/1367-2630/aa8679) - Influence of the cusp field on the plasma parameters of the Linac4 H− ion source, AIP Conf. Proc. 1869 (2017) 030016
S. Briefi, S. Mattei, J. Lettry and U. Fantz
(See online at https://doi.org/10.1063/1.4995736) - RF power transfer efficiency of inductively coupled low pressure H2 and D2 discharges, Plasma Sources Sci. Technol. 26 (2017) 095004
D. Rauner, S. Briefi and U. Fantz
(See online at https://doi.org/10.1088/1361-6595/aa8685)
