Cylindrical energy analyzer for heavy ion beam diagnostics

Resumen

Measurements of plasma potential and density, along with its fluctuations are essential to understand the dynamics of nuclear fusion plasmas. The main objective of this thesis was to enhance the capability of the heavy ion beam diagnostics (HIBD) at ISTTOK tokamak to measure plasma potential by developing an improved high resolution 90º cylindrical energy analyzer. In addition, the thesis uses the heavy ion beam probes (HIBP) setup, already installed in the TJ-II stellarator, to obtain 2D contour maps for the potential and density profiles in different plasma scenarios. These results will contribute to the validation of plasma potential asymmetry models and their fluctuations. The results obtained during the thesis are summarized following. ISTTOK tokamak 90º CEA is conceptually developed, designed, and commissioned on HIBD at ISTTOK. The single-channel prototype tested has demonstrated measurements plasma potential and its fluctuation, being consistent with simulation estimation. The highlights of the design concept, and experimental results obtained at ISTTOK are presented below. 90º Cylindrical Energy Analyzer (CEA) 90º cylindrical analyzer has been operated in an innovative deceleration operation mode. In this mode, the central line trajectory is kept at a positive potential resulting in beam retardation. Simulations have estimated 5 times increase in the coefficient of energy dispersion and 45 times decrease in angular aberration coefficient in deceleration mode in comparison with traditionally used normal mode. Numerical simulation estimates the energy resolution of ∆E/E = (3–5) × 10−4 with a strong decrease of the angular aberration in the range of θ = ± 2º for five times deceleration (kE = 5). The 3-D layout of the CEA was upgraded by adding following design modifications. i) Additional pair of guard rings were included to compensate for the fringing field distorting the equipotential at the end of the analyzer due to the external SS cross chamber. ii) The deceleration grid is added to maintain the retarding field at the analyzer exit, thus allowing beam detection at ground potential. iii) SEE grid is added to suppress the secondary electron emission from reaching the detector and interfere with real signals. A prototype ½-size 90º CEA has been investigated both numerically and in experiments with electron beam in a test facility. The experimental results verify the expected higher sensitivity in deceleration mode over conventionally used normal mode. Electrostatic Input Module (EIM) The internal elements comprising EIM were optimized (geometry and biasing voltages) to achieve a secondary beam dimension of 8 mm × 2.5 mm without any overlapping between the four beams from different slits and without any loss of current at the CEA entrance slit. A new design of Einzel lens is presented that provides additional control over the beam shape at the input of the analyzer (using side strip electrodes). A prototype setup of EIM consisting of additional elements necessary during real installation was built and installed on HIBD at ISTTOK to verify and validate the simulations. The alignment parameters obtained in this chapter were crucial to provide reference biasing values for the combined operation of the EIM and 90º CEA. Potential measurements at ISTTOK The 90o cylindrical energy analyzer operated in two-times deceleration mode has been successfully applied in ISTTOK HIBD to measure the plasma potential and its fluctuations with the following results. An indirect method of calibration estimated absolute plasma potential to be φ ~ -340 V with an estimated calibration error Δφ = ± 70V as measured in the plasma core. Experimental relative calibration established the CEA resolution of ΔE/E ~ 2 x10-3 as obtained with external sinusoidal modulation of beam energy. The author has designed a customizable workbench in SIMION software for Electrostatic input module and cylindrical energy analyzer for ISTTOK HIBD. It can be used for the charged particle trajectory calculations in the future experiments and upgrade.   TJ-II Stellarator The thesis reports on the three experiments performed at TJ-II stellarator using the unique dual HIBP installation at TJ-II. The results are summarized below. The plasma response to edge biasing strongly depends on the heating scenario, being maximum in ECRH and NBI scenarios with positive and negative biasing, respectively. This phenomenology is consistent with the asymmetric I-V electrode characteristic. However, within experimental uncertainties and in the explored plasma scenarios, no evidence of core plasma potential flux surface asymmetries was found. The HIBP was used in an energy scanning mode to successfully obtain 2D poloidal contour plots of plasma potential and density and their fluctuations in low density plasmas sustained by ECRH in the TJ-II stellarator for one-third of the core plasma poloidal area with the following conclusions: The 2D map for the absolute plasma potential has a local maximum in the plasma core as expected in low-density high-temperature scenarios. It shows about 1 cm mismatch with vacuum magnetic calculations that could be partially explained by instrumental effects. The 2D map for the absolute plasma potential shows poloidal symmetry within the experimental error ± 25 V. The 2D map for the plasma potential RMS shows poloidal symmetry within ± 5 V experimental uncertainty. Density fluctuations appear both at the positive and negative Itot gradient regions for ECRH plasmas. Normalized density fluctuations are stronger in the negative Itot density gradient than in the positive gradient region. Frequency spectra are dominated by frequencies below 100 kHz with different spectral characteristics in the positive and negative gradient regions that are affected by the ECRH scenario (on vs. off -axis heating). Comparison of plasma potential profile for the discharges sustained only by NBI 1 (co-injection) and NBI 2 (counter-injection) infers less negative DC Plasma potential for NBI 2 as compared to NBI 1. These results are in consistency with the predicted better confinement of fast ions in counter configurations.