4 Search Results

Initial plasma densification by odd-parity rotating magnetic fields (RMF_o) applied to the linear magnetized Princeton field-reversed configuration (PFRC-2) device with fill gases at pressures near 1 mTorr proceeds through two phases: a slow one, characterized by a rise time $$τ_{slow}$$ ~ 100 $$μ$$s, followed by a fast one, characterized by $$τ_{fast}$$ ~ 10 $$μ$$s. The transition from slow to fast occurs at a line-integral-averaged electron density, _tn_e, near 2$$\times$$ 10¹¹ cm^–3, independent of magnetic field. Here, over most of the range of experimental parameters investigated, as the PFRC-2 axial magnetic field strength was increased, RMF_o power decreased, gas fill pressuremore » lowered, or lower atomic mass unit (AMU) fill gas used, the duration of the slow phase lengthened from 50 $$μ$$s to longer than 10 ms after the RMF_o power began. The post-fast-phase maximum n_e increases with the fill-gas AMU, exceeding 5 × 10¹³ cm^–3 for Ar. The slow phase is consistent with atomic physics processes and field-parallel sound-speed losses. The fast phase may be explained by improved axial confinement, possibly augmented by radial or axial contraction of the plasma. Another possible explanation, a large increase in electron temperature, is inconsistent with x-ray emission. The n_e behavior is discussed in relation to the E to H transition.« less

Pulse pile-up in pulse-height energy analyzers increases when the incident rate of pulses increases relative to the inverse of the dead time per pulse of the detection system. Changes in the observed energy distributions with incident rate and detector-electronics-formed pulse shape then occur. Here, we focus on weak high energy tails in X-ray spectra, important for measurements on partially ionized, warm (50–500 eV average electron energy), pure hydrogen plasma. A first-principles two-photon pulse-pile-up model is derived specific to trapezoidal-shaped pulses; quantitative agreement is found between the measurements and the model’s predictions. The model is then used to diagnose pulse-pile-up tailmore » artifacts and mitigate them in relatively low count-rate spectra.« less

A collisional-radiative (CR) model that extracts the electron temperature, T_e, of hydrogen plasmas from Balmer-line-ratio measurements is examined for the plasma electron density, n_e, and T_e ranges of 10¹⁰–10¹⁵ cm^–3 and 5–500 eV, respectively. The CR code, developed and implemented in Python, has a forward component that computes the densities of excited states up to n = 15 as functions of T_e, n_e, and the molecular-to-atomic neutral ratio r(H₂/H). The backward component provides ne and r(H₂/H) as functions of the Balmer ratios to predict the T_e. The model assumes Maxwellian electrons. Furthermore, the density profiles of the electrons and ofmore » the molecular and atomic hydrogen neutrals are shown to be of great importance, as is the accuracy of the line-ratio measurement method.« less

The first-ever successful operation of a tokamak with a large area (40% of the total plasm surface area) liquid lithium wall has been achieved in the Lithium Tokamak eXperiment (LTX). These results were obtained with a new, electron beam-based lithium evaporation system, which can deposit a lithium coating on the limiting wall of LTX in a five-minute period. Preliminary analyses of diamagnetic and other data for discharges operated with a liquid lithium wall indicate that confinement times increased by 10× compared to discharges with helium-dispersed solid lithium coatings. Ohmic energy confinement times with fresh lithium walls, solid and liquid, exceedmore » several relevant empirical scaling expressions. Spectroscopic analysis of the discharges indicates that oxygen levels in the discharges limited on liquid lithium walls were significantly reduced compared to discharges limited on solid lithium walls. Tokamak operations with a full liquid lithium wall (85% of the total plasma surface area) have recently started.« less