[en] The spheromak plasma confinement concept provides the opportunity to study the evolution of a nearly force-free magnetic field configuration. The plasma currents and magnetic fields are produced self-consistently, making this type of device attractive as a possible fusion reactor. At present, spheromaks are observed to have poorer particle and magnetic confinement than expected from simple theory. The purpose of this study is to examine the role of plasma density in the decay of spheromaks produced in the Maryland Spheromak experiment. Density measurements are made with an interferometer and Langmuir probe, and results are correlated with those of other plasma diagnostics to understand the sources of plasma, the spheromak formation effects on the density, and the magnitude of particle loss during the spheromak decay. A power and particle balance computer model is constructed and applied to the spheromaks studied in order to assess the impact of high density and particle loss rate on the spheromak decay. The observations and model indicate that the decay of the spheromaks is at present dominated by impurity radiation loss. The model also predicts that high density and short particle confinement time play a critical role in the spheromak power balance when the impurity levels are reduced

[en] We are using VUV spectroscopy to study the ion source region on the SABRE applied-B extraction ion diode. The VUV diagnostic views the anode-cathode gap perpendicular to the ion acceleration direction, and images a region 0--1 mm from the anode onto the entrance slit of a I m normal-incidence spectrometer. Time resolution is obtained by gating multiple striplines of a CuI- or MgF₂ -coated micro-channel plate intensifier. We report on results with a passive proton/carbon ion source. Lines of carbon and oxygen are observed over 900--1600 angstrom. The optical depths of most of the lines are less than or of order 1. Unfolding the Doppler broadening of the ion lines in the source plasma, we calculate the contribution of the source to the accelerated C IV ion micro-divergence as 4 mrad at peak power. Collisional-radiative modeling of oxygen line intensities provides the source plasma average electron density of 7x10¹⁶ cm^-3 and temperature of 10 eV Measurements are planned with a lithium ion source and with VUV absorption spectroscopy

[en] Pulsed-power driven ion diodes generating quasi-static, ∼10 MV/cm, 1-cm scale-length electric fields are used to accelerate lithium ion beams for inertial confinement fusion applications. Atomic emission spectroscopy measurements contribute to understanding the acceleration gap physics, in particular by combining time- and space-resolved measurements of the electric field with the Poisson equation to determine the charged particle distributions. This unique high-field configuration also offers the possibility to advance basic atomic physics, for example by testing calculations of the Stark-shifted emission pattern, by measuring field ionization rates for tightly-bound low-principal-quantum-number levels, and by measuring transition-probability quenching

[en] Light-ion ICF experiments on the PBFA II accelerator provide a unique opportunity to investigate ion beam source physics at the anode of an applied-B ion diode. Spectroscopic observations of the anode plasma have been made in the visible; various atoms and ions, including CII, CIII, CIV, and LiI have been identified. The spectral line profiles show strong broadening and shifting. A line-fitting code that accounts for Zeeman splitting, Stark shifting, and instrumental, Doppler, and Stark broadening has been used to analyze these profiles. Calculations have been done to check that the lines analyzed are optically thin. An electron density of ∼2x10²³ m^-3 in the anode plasma is measured from Stark shifts of Li I 2s-2p (6708 Angstrom). This is the largest electric field ever measured with spectroscopic methods. The CIV ion temperature, as derived from deconvolved Doppler broadening, is 0.5-2.5 keV. The mechanism responsible for generating the unexpected high source temperature is under investigation. Evidence for the high temperature, and further spectroscopic results, will be presented

[en] We are improving the understanding of pulsed-power-driven ion diodes using measurements of the charged particle distributions in the diode anode-cathode (AK) gap. We measure the time - and space-resolved electric field in the AK gap using Stark-shifted Li I 2s-2p emission. The ion density in the gap is determined from the electric field profile and the ion current density. The electron density is inferred by subtracting the net charge density, obtained from the derivative of the electric field profile, from the ion density. The measured electric field and charged particle distributions are compared with results from QUICKSILVER, a 3D particle-in-cell computer code. The comparison validates the fundamental concept of electron build-up in the AK gap. However, the PBFA II diode exhibits considerably richer physics than presently contained in the simulation, suggesting improvements both to the experiments and to our understanding of ion diode physics

[en] We are using time- and space-resolved visible spectroscopy to measure applied-B ion diode dynamics on the 20 TW Particle Beam Fusion Accelerator II. Doppler broadening of fast Li atoms, as viewed parallel to the anode, is used in a charge-exchange model to obtain the Li⁺ ion divergence within 100 μm of the anode surface. The characteristic Stark/Zeeman shifts in spectra of alkali neutrals or singly-ionized alkaline-earths are used to measure the strong electric (10⁹ V/m) and magnetic (∼6 T) fields in the diode gap. Large Stark shifts within 0.5 mm of the anode indicate the LiF emits with a finite field threshold rather than with Child- Langmuir-type emission, and the small slope in the electric field indicates an unexpected build-up of electrons near the anode. In the diode gap, we aim to unfold fields to quantify the time-dependent ion and electron space-charge distributions that determine the ion beam properties. Observed electric field non-uniformities give local beam deflections that can be comparable to the total beam microdivergence. We are implementing active laser absorption and laser-induced fluorescence spectroscopy on low-density Na atoms injected into the diode gap prior to the power pulse. The small Doppler broadening in the Na spectra should allow simultaneous electric and magnetic field mapping with improved spatial resolution

[en] This work describes a pulsed Na atomic beam source developed for spectroscopic diagnosis of a high-power ion diode on the Particle Beam Fusion Accelerator II. The goal is to produce a ∼ 10¹²-cm^-3-density Na atomic beam that can be injected into the diode acceleration gap to measure electric and magnetic fields from the Stark and Zeeman effects through laser-induced-fluorescence or absorption spectroscopy. A ∼ 10 ns fwhm, 1.06 microm, 0.6 J/cm² laser incident through a glass slide heats a Na-bearing thin film, creating a plasma that generates a sodium vapor plume. A ∼ 1 microsec fwhm dye laser beam tuned to 5,890 angstrom is used for absorption measurement of the Na I resonant doublet by viewing parallel to the film surface. The dye laser light is coupled through a fiber to a spectrograph with a time-integrated CCD camera. A two-dimensional mapping of the Na vapor density is obtained through absorption measurements at different spatial locations. Time-of-flight and Doppler broadening of the absorption with ∼ 0.1 angstrom spectral resolution indicate that the Na neutral vapor temperature is about 0.5 to 2 eV. Laser-induced-fluorescence from ∼ 1 x 10¹²-cm^-3 Na I 3s-3p lines observed with a streaked spectrograph provides a signal level sufficient for ∼ 0.06 angstrom wavelength shift measurements in a mock-up of an ion diode experiment

[en] Surface-plasma ion sources are critical to generating high-brightness, high purity ion beams on applied-B ion diodes. The source plasma must meet requirements for species, thickness, purity, degree of ionization, conductivity, formation timescale, and engineering feasibility. Laser ionization schemes have been used on high-power ion diodes with encouraging but inconsistent results in diode impedance and beam purity. The authors are characterizing two laser-driven Li⁺ schemes for 100--200 cm² sources: a single 10 ns Nd:YAG pulse at 0.3--1 J/cm², or the 2-laser LEVIS scheme. Recognizing anode surface contamination as a key issue in 10^-5--10^-6 Torr pulsed power vacuum, they subject well-characterized 0.5 microm LiAg films on stainless substrates to extended 150--400 C heating and plasma discharge sputter cleaning after pumpdown in a test chamber, prior to pulsing the source lasers. A detailed diagnostic set includes absolutely-calibrated, streaked, CCD-imaged spectrographs for plasma properties, 2-color laser deflection for plasma/neutral expansion, ion time-of-flight spectrograph with ionizer for species content, and a tunable narrow-band dye laser for near-resonant absorption. The drive laser uniformity is measured with a CCD. Initial results on uncleaned LiAg show a single laser at >0.3 J/cm² can generate thin dense plasma, with acceptable expansion speeds at or below 2 cm/micros

[en] We report on the continuing development of the LEVIS (Laser Evaporation Ion Source) lithium active ion source for the 15-cm radial focussing ion diode on PBFA-11. We found previously that DC-heating of the anode surface to 150 degrees C maximum for 5 hours resulted in a pure lithium beam. This paper discusses the characterization of LEVIS source uniformity by Faraday cup arrays and multiple lines of sight for visible light spectroscopy. These diagnostics give some evidence of nonuniformity in both A-K gap electric fields and ion current density. Despite this, however, the measured focal spot size appears smaller than with a passive LiF source operated in the same magnetic field topology. Experiments using a curved anode for vertical beam focussing show reduced ion beam turn-on delay by 5 ns by altering the magnetic field topology as well as anode curvature. Another 3--5 ns reduction was achieved by switching from a passive LiF to the active LEVIS source

[en] The authors are using time- and space-resolved visible spectroscopy to measure applied-B ion diode dynamics on the 20 TW Particle Beam Fusion Accelerator II. Doppler broadening of fast Li atoms, as viewed parallel to the anode, is used in a charge-exchange model to obtain the Li⁺ ion divergence within 100 μm of the anode surface. The characteristic Stark/Zeeman shifts in spectra of alkali neutrals or singly-ionized alkaline-earths are used to measure the strong electric (10⁹ V/m) an magnetic (∼ 6 T) fields in the diode gap. Large Stark shifts within 0.5 mm of the anode indicate the LiF emits with a finite field threshold rather than with Child-Langmuir-type emission, and the small slope in the electric field indicates an unexpected build-up of electrons near the anode. In the diode gap, the authors aim to unfold fields to quantify the time-dependent ion and electron space-charge distributions that determine the ion beam properties. Observed electric field non-uniformities give local beam deflections that can be comparable to the total beam microdivergence. The authors are implementing active laser absorption and laser-induced fluorescence spectroscopy on low-density Na atoms injected into the diode gap prior to the power pulse. The small Doppler broadening in the Na spectra should allow simultaneous electric and magnetic field mapping with improved spatial resolution. (author). 4 figs., 13 refs