A.L. Carlson

Observation of K α. X-ray satellites from a target heated by an intense ion beam

We have made the first observation of K α X-ray satellites from a target heated by an intense ion beam. The satellites are produced when thermal ionization due to beam heating is accompanied by inner-shell ionization from beam ion impact. The Particle Beam Fusion Accelerator II was used to irradiate a conical aluminum target with a proton beam. The nominal beam parameters were 50–75 kJ in a 1-cm spot, 15–20-ns pulse length, and 4–5-MeV protons at peak power. An elliptical crystal X-ray spectrograph inside a 1000-kg tungsten shield was used to record the spectra. The peak ion stage reached by the aluminum target was +8. Collisional radiative calculations were performed, which indicate a peak electron temperature of 20–60 eV.

Visible spectroscopy measurements in the PBFA II ion diode (invited)

We describe a new visible spectroscopy diagnostic system for measuring plasma properties in the PBFA II applied‐B ion diode. The system transports light from the ion diode to a remote screen room where it is recorded by a spectrograph coupled to a streak camera. We developed extensive calibration techniques for measuring the collection efficiency into the fiber link, the effects of the background bremsstrahlung radiation on the fibers, the fiber transmission as a function of wavelength, and the absolute streaked‐spectrograph sensitivity as a function of wavelength. We have recorded time‐dependent spectral line profiles and intensities from the PBFA II plasma opening switch, the beam‐transport gas cell, and the anode plasma. The Stark shift of the LiI 2s‐2p transition observed on LiF‐anode shots shows that the time‐resolved electric field peaks at 7–8 MV/cm, the highest field ever measured using the Stark effect. The potential of these measurements to expand our knowledge of ion‐diode physics is being expl...

Current scaling of radiated power for 40-mm diameter single wire arrays on Z

In order to estimate the radiated power that can be expected from the next-generation Z-pinch driver such as ZR at 28 MA, current-scaling experiments have been conducted on the 20 MA driver Z. We report on the current scaling of single 40 mm diameter tungsten 240 wire arrays with a fixed 110 ns implosion time. The wire diameter is decreased in proportion to the load current. Reducing the charge voltage on the Marx banks reduces the load current. On one shot, firing only three of the four levels of the Z machine further reduced the load current. The radiated energy scaled as the current squared as expected but the radiated power scaled as the current to the 3.52±0.42 power due to increased x-ray pulse width at lower current. As the current is reduced, the rise time of the x-ray pulse increases and at the lowest current value of 10.4 MA, a shoulder appears on the leading edge of the x-ray pulse. In order to determine the nature of the plasma producing the leading edge of the x-ray pulse at low currents furt...

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 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.

LEVIS active anode lithium ion source development on PBFA-II

Summary form only given. Experiments are ongoing on the PBFA-II accelerator to optimize an active lithium ion source in a 15-cm focusing applied-B ion diode using the LEVIS (Laser Evaporation Ion Source) process. Spectroscopic observations indicate the presence of a preformed anode plasma prior to the power pulse. However, the time between application of high voltage and the initiation of significant ion current on the LEVIS diode is roughly comparable to that seen with a passive diode using LiF as the ion source. This is consistent with time-dependent diode modeling, which shows that both LEVIS and LiF are achieving enhanced diode current within a few nsec of the expected time determined from ideal diode theory. The LEVIS-generated beam has attained focal spot sizes of 6-8 mm (horizontal focusing), comparable to that seen with the LiF source operated with similar parameters. There are preliminary indications that diodes with the LEVIS source experience less impedance decay for comparable ion beam currents than those with the LiF source.

Current-scaling of radiated power for 40-mm diameter single wire arrays on Z

Summary form only given, as follows. In order to estimate the radiated power that can be expected from the next-generation z-pinch driver such as ZR at 28 MA, current-scaling experiments have been conducted on the 18-MA driver Z. We report on the current scaling of single 40-mm diameter tungsten 240-wire arrays with a fixed 110-ns implosion time. The wire diameter is decreased in proportion to the load current. The load current is reduced by reducing the charge voltage on the Marx banks. On one shot firing only 3 of the 4 levels of the Z machine further reduced the load current. The radiated energy scales as the current squared as expected but the radiated power scales as the current to the 3.5 power due to increased pinch instability at lower current. As the current is reduced the rise-time of the x-ray pulse increases and at the lowest current value of 10.4 MA a shoulder appears on the leading edge of the x-ray pulse. We will report on experiments in February 2002 which will attempt to image the pinch along the axis to determine the nature of the reduced stability at lower currents.

Measurements of charged particle dynamics in high accelerating fields

Summary form only given. A high-voltage power pulse applied to a magnetically-insulated ion accelerating gap results in a complex evolution of the electron and ion distributions within the gap. Divergence-inducing electromagnetic instabilities and the ion current both depend on the charged-particle distributions. Thus, understanding and control of these distributions is extremely important for applications, such as inertial confinement fusion, that require high-brightness ion beams. This work describes time- and space-resolved spectroscopic measurements of the charged particle distributions in the acceleration gap of an applied-B ion diode. The Particle Beam Fusion Accelerator II supplies a 20 TW, 40 nsec power pulse to a 15 cm radius, cylinder-symmetry axis insulates the anode-cathode (AC) gap against electron losses. The Li/sup +/ ion beam is accelerated radially from a LiF source to /spl sim/10 MV across the /spl sim/2-cm-wide AC gap. We determine the charged-particle distributions using measurements to the electric field, obtained from Stark-shifted Li I 2s-2p emission that arises from Li neutrals injected into the AC gap when Li ions undergo charge exchange near the anode surface. Our diagnostic system is presently capable of measuring the field to within about /spl plusmn/4% with /spl sim/1 nsec time resolution and /spl sim/2 mm spatial resolution at 18 locations within the AC gap. The electric field and measured Li/sup +/ ion current density are used to determine the space- and time-resolved ion and electron densities in the AC gap.

Comprehensive Diagnostic Set For Intense Lithium Ion Hohlraum Experiments On Pbfa II

Development of Transient Internal Probe (TIP) Magnetic Field Diagnostic. J.P. Galambos, MA. Bohnet, T.R. Jarboe, A.T. Mattick, t h d j

Time dependent measurements and calculations of field profile in the PBFA-II ion diode

%&+ The Transient Internal Probe (TIP) is designed to permit measurement of internal magnetic fields in hot, high density plasmas. The concept consists of accelerating a probe to high velocities (2.2 Km/s) in order to minimize probe exposure time to plasma. Faraday rotation within the probe is used to measure the local magnetic field. An Argon laser illuminates the probe consisting of a Faraday-rotator material with a retro-reflector that r e m s the incident light to the detection system. Performance results of the light gas gun and optical detection system will shown. To date, the gas gun has been extensively tested consistently achieving velocities between 2 and 3 Ms. The probe and detection scheme have been tested by dropping the probe through a static magnetic field. Magnetic field resolution of 20 gauss and spatial resolution of 5 mm has been achieved. System frequency response is 10Mhz. Work is currently being conducted to integrate the diagnostic system with laboratory plasma experiments. Specifically a gas interfaced system has been developed to prevent helium muzzle gas from entering the plasma chamber with the probe. Additionally the probe must be separated from the sabot which protects the probe during acceleration in the gas gun. Data will be presented showing the results of various separation techniques. Results of pressure measurements illustrating the effectiveness of the gas interface system will also be presented. The diagnostic is scheduled to make measurements on the Helicity Injected Toms (HIT) at the University of Washington. 4P29

Spectroscopic measurements of ion source divergence in a high-power applied-B ion diode

Summary form only given. The reduction of the effective anode cathode (AK) gap during an applied-B ion diode pulse is driven by the diamagnetic effect of E/spl times/B drifting electrons in the sheath between the virtual cathode and the anode, and may be assisted by fluctuations that enable electrons to cross magnetic field lines. The dynamics of the sheath and the resulting AK gap are manifested in the time and space dependence of the electric field across the gap. The authors are studying the electric field profile in the PBFA-II (particle beam fusion accelerator II) ion diode with the goal of improving the understanding of acceleration gap dynamics, leading to improved power coupling to the ion beam. The field profile measurements have been compared with calculations performed using two different computer codes QUICKSILVER and TWO-QUICK.