Atomic emission spectroscopy in high electric fields

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 timeand space-resolved measurements of the electric field with the Poisson equation to determine the charged particle distributions. This unique high-field c~m_figuration 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-principalquantum-number levels, and by measuring transition-probability quenching. I N T R O D U C T I O N The light-ion beam approach to inertial confinement fusionlproposes to achieve the required high energy density by accelerating a lithium ion beam to about 30 MeV using one or two acceleration stages, each driven with a high-power (~100 TW), ~30-nsec-duration pulse. Present experiments at the Particle Beam Fusion Accelerator II (PBFA/I) facility routinely generate quasi-static, ~10 MV/ cm, ~1 cm scale-length electric fields. This enables experiments studying the physics of the ion beam acceleration gap and these conditions also present a unique opportunity for extending experimental atomic physics into the 10 MV/cm regime. This paper describes our application of atomic spectroscopy to ion diode plasma physics issues, with an emphasis on the atomic physics required to understand the results. In addition, we describe preliminary experiments that illustrate the potential of using data from this device for basic atomic physics. © 1996 American Institute of Physics 245 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 132.76.50.6 On: Wed, 12 Mar 2014 07:27:49 The primary motivation for atomic emission spectroscopy measurements in ion diodes is the need to understand and control the influence of the acceleration gap charged-particle dynamics on the ion beam brightness. Stark-shift measurements of the electric field distribution can be combined with the Poisson equation to determine key features of the charged-particle behavior 2"3. Understanding the charged-particle distributions is of fundamental importance as we seek to increase the ion beam brightness because they largely control the enhancement of the ion current above the nominal Child-Langmuir space-charge limit 4"5 and because nonuniformities in the charge density earl induce beam microdivergenee or steering errors that reduce the focussed intensity 6"9. Our results 3 show that theory and experiment are in reasonable agreement for the first 5 nsec of the ion beam pulse, but as the ion current grows significant discrepancies arise. The measurements provide evidence for new diode phenomena, including field-limited rather than space-charge-limited ion emission, a region with zero net-charge density near the anode, localized positive net-charge in the middle of the gap, and persistent azimuthal asymmetries. Our recent work has emphasized measurements of the small electric field component perpendicular to the direction of beam acceleration and on measurements of emission from Ba II dopants, both with the aim of improving understanding of ion beam divergence. A new opportunity for atomic physics measurements is provided by the relatively long duration (tens of nsec) and large volume (~1000 cm 3) of the highfield region. Previous emission spectroscopy Stark effect investigations 2 were limited to electric fields below 1 MV/cm, about an order of magnitude below the fields in the PBFA II acceleration gap. Higher fields are certainly generated in short-pulse laser 10, plasma wakefield acceleration ll, and micro-needle experiments 12. Also, the effect of high electric fields on hydrogen has been investigated 13 using the motional Stark effect to reach ~3 MV/cm. However, the short duration and/or small volume of the high-field regions has prevented acquisition of atomic spectra from these experiments. The conditions in present PBFA II experiments thus enable studies of the high-field Stark effect that were previously impossible. Comparison of independent calculations and results from different species supply confidence that theoretical predictions of the Stark pattern are accurate to within + 5-10% at fields up l0 MV/cm. Preliminary results for transitions from the tightly-bound Li 13d level are consistent with predictions of the electric field ionization threshold, and provide the in'st experimental evidence for transition probability quenching of these transitions. E X P E R I M E N T A L METHODS The experiments described in this paper werepefformed using a cylindricallysymmetric applied-magnetic-field ion diode 14"16. PBFA II supplies a 20 TW,