Nichelle Bennett

Contribution of the backstreaming ions to the self-magnetic pinch (SMP) diode current

Summary form only given. The results presented here were obtained with an SMP diode mounted at the front high voltage end of the RITS accelerator. RITS is a Self-Magnetically Insulated Transmission Line (MITL) voltage adder that adds the voltage pulses of six 1.3 MV inductively insulated cavities. Our experiments had two objectives: first to measure the contribution of the back-streaming ion currents emitted from the anode target to the diode beam current, and second to try to evaluate the energy of those ions and hence the actual Anode-Cathode (A-K) gap actual voltage. In any very high voltage inductive voltage adder (IVA) utilizing MITLs to transmit the power to the diode load, the precise knowledge of the accelerating voltage applied on the anode-cathode (A-K) gap is problematic. The accelerating voltage quoted in the literature is from estimates based on measurements of the anode and cathode currents of the MITL far upstream from the diode and utilizing the para-potential flow theories and inductive corrections. Thus it would be interesting to have another independent measurement to evaluate the A-K voltage. The diodes anode is made of a number of high Z metals in order to produce copious and energetic flash x-rays. The backstreaming currents are a strong fraction of the anode materials and their stage of cleanness and gas adsorption. We have measured the back-streaming ion currents emitted from the anode and propagating through a hollow cathode tip for various diode configurations and different techniques of target cleaning treatments, such as heating to very high temperatures with DC and pulsed current, with RF plasma cleaning and with both plasma cleaning and heating. We have also evaluated the A-K gap voltage by ion filtering techniques.

Zeeman spectroscopy as a method for determining the magnetic field distribution in self-magnetic-pinch diodes (invited)

In the self-magnetic-pinch diode, the electron beam, produced through explosive field emission, focuses on the anode surface due to its own magnetic field. This process results in dense plasma formation on the anode surface, consisting primarily of hydrocarbons. Direct measurements of the beams current profile are necessary in order to understand the pinch dynamics and to determine x-ray source sizes, which should be minimized in radiographic applications. In this paper, the analysis of the C IV doublet (580.1 and 581.2 nm) line shapes will be discussed. The technique yields estimates of the electron density and electron temperature profiles, and the method can be highly beneficial in providing the current density distribution in such diodes.

Contribution of the backstreaming ions to the Self-Magnetic pinch (SMP) diode current

The results presented here were obtained with an SMP diode mounted at the front high voltage end of the RITS accelerator. RITS is a Self-Magnetically Insulated Transmission Line (MITL) voltage adder that adds the voltage pulses of six 1.3 MV inductively insulated cavities.

Magnetic field measurements on the self magnetic pinch diode at SNL using Zeeman splitting

Summary form only given. The RITS-6 accelerator at Sandia National Laboratories has been used to test the Self Magnetic Pinch (SMP) diode. The SMP diode is a flash x-ray radiography source with a radiation pulse length of 30-50 ns, dependent on the diode configuration. A hollow cathode emits electrons through an approximately 1 cm vacuum A-K gap onto a high Z material. The electron beam pinches from its own magnetic field as it traverses the gap1. Consequently, magnetic field measurements provide key insights into the diode physics. Presently, field profiles are predicted from LSP simulations, and direct local measurements would also help benchmark these codes.Visible spectra of the diode plasma have been taken along the anode surface using a lens coupled fiber arrays which is imaged onto a 0.3 m, high resolution spectrometer. Zeeman splitting measurements of C IV and Al III suggest magnetic fields of 3-4 T a few mm from the diode axis. These measurements yield current profiles near the target surface, and suggest that a significant fraction of the current density is outside the few mm spot region.

Back-streaming ion beam measurements in a self magnetic pinch (SMP) electron diode

Summary form only given. A self-magnetic pinch diode (SMP) is presently the electron diode of choice for high energy flash x-ray radiography utilizing pulsed power drivers. The Sandia National Laboratories RITS accelerator presently drives an SMP diode that generates small electron beam spots. RITS is a Self-Magnetically Insulated Transmission Line (MITL) voltage adder that adds the voltage pulse of six 1.3 MV inductively insulated cavities. The diodes anode is made of high Z metal in order to produce copious and energetic flash x-rays for radiographic imaging of high areal density objects. In any high voltage inductive voltage adder (IVA) utilizing MITLs to transmit the power to the diode load, the precise knowledge of the accelerating voltage applied on the anodecathode (A-K) gap is problematic. This is even more difficult in an SMP diode where the A-K gap is very small (~1cm) and the diode region very hostile. The accelerating voltage quoted in the literature is from estimates based on measurements of the anode and cathode currents of the MITL far upstream from the diode and utilizing the para-potential flow theories and inductive corrections. We are currently measuring the back-streaming ion currents emitted from the anode and propagating through a hollow cathode tip and evaluating the A-K gap voltage by energy filtering techniques. Experimental results compared with LSP simulations will be presented.

Parallel Operation of Multiple Closely Spaced Small Aspect Ratio Rod Pinches

A series of simulations and experiments to resolve questions about the operation of arrays of closely spaced small aspect ratio rod pinches has been performed. Design and postshot analysis of the experimental results are supported by 3-D particle-in-cell simulations. Both simulations and experiments support these conclusions. Penetration of current to the interior of the array appears to be efficient, as the current on the center rods is essentially equal to the current on the outer rods. Current loss in the feed due to the formation of magnetic nulls was avoided in these experiments by design of the feed surface of the cathode and control of the gap to keep the electric fields on the cathode below the emission threshold. Some asymmetry in the electron flow to the rod was observed, but the flow appeared to symmetrize as it reached the end of the rod. Interaction between the rod pinches can be controlled to allow the stable and consistent operation of arrays of rod pinches.

Plasma dynamics in the self-magnetic-pinch diode

The self-magnetic-pinch (SMP) diode1,2 is being developed as a source for flash x-ray radiography. The high electric field stresses applied to this diode and material heating from the intense electron beam generate electrode plasmas. While these plasmas are instrumental in beam formation and focusing, rapid plasma expansion from the electrodes can degrade radiographic performance.

Scaling of SMP diode performance with geometry and voltage

The self-magnetic pinch (SMP) electron beam diode has been studied extensively as an intense, flash x-ray, radiographic source at Sandia National Laboratories. It has been fielded across a wide range of voltages and geometric configurations on the Radiographic Integrated Test Stand (RITS-6) inductive voltage adder accelerator and the Ursa Minor 21-cavity linear transformer driver accelerator. We present the observed scaling and reproducibility behavior of the SMP diode with variations in voltage and geometry, as well as effects of various materials used in the diode. With the proper configurations, we have observed excellent performance from the SMP diode at up to 8 MV with indications of improvement at even higher voltages. Proposed physical underpinnings of this behavior, based on measurements, analysis and numerical simulation, will be presented.

Impedance behavior in the self-magnetic pinch (SMP) diode on the RITS-6 accelerator

Summary form only given. The RITS-6 accelerator (4-7.5 MeV) at Sandia National Laboratories produces high-power (TW) focused electron beams (<; 3mm diameter) for flash x-ray radiography applications. The beam is generated in a Self-Magnetic Pinch (SMP) diode, which utilizes a hollowed metal cathode to produce a pinched focus onto a high Z metal converter. Optimum radiographic performance requires a small pinch spot size. We have achieved a reproducible performance with a standard cathode diameter of ~ 1 cm. Smaller spot sizes are achievable by reducing the cathode diameter below 1 cm, but performance is compromised by the tendency of the diode to undergo premature impedance collapse in a significant number of beam generation experiments. The cause of this collapse is under active investigation. Besides power flow diagnostics (current and x-ray monitors), we are observing the anode-cathode (A-K) gap with optical diagnostics including high speed (<; 10 ns) framing cameras, optical streak cameras, and spectroscopy. Since the diode impedance decreases with the cathode diameter for a constant A-K gap, the increased current flow occurs within a smaller cathode cross-section, suggesting a possible evolving thermal instability in the A-K gap. We are planning changes to anode-cathode materials, as well as changes to the diode aspect ratio in an attempt to mitigate or eliminate the impedance collapse during the power pulse. Experiments are ongoing, and latest results will be reported.

Direct measurement of the bi-polar ion current and anode-cathode (A-K) voltage in a self-magnetic pinch (SMP) diode

Summary form only given. A self-magnetic pinch diode is currently under extensive study in the Sandia Laboratory advanced radiographic development facility. The SMP diode is utilized in conjunction with the RITS accelerator. RITS is a voltage adder adding the voltage pulse of six 1.5 MV inductively insulated cavities. A tapered MITL cathode stalk threads the cavities, adds the voltage pulses and feeds the SMP electron diode at the end of the MITL. The maximum voltage can reach 9 MV. The diodes anode is made of high Z metal in order to produce copious and energetic flash x-rays for radiographic imaging of heavy objects. In any high voltage inductive voltage adder (IVA) utilizing MITLs to transmit the power to the load, the measuring of the voltage is problematic because of the presence of vacuum electrons (sheath current). The accelerating voltage quoted in the literature is from estimates based on the anode and cathode currents of the MITL well away from the diode. We are investigating a method to measure the bi-polar ion current and the A-K gap voltage by collecting the emitted ions from the anode target utilizing filtered Faraday cups and also time of flight-techniques. The proposed design, simulations and possibly results will be presented and analyzed.