Technical Issues: Approach: Construct a system that will allow high frequency, high voltage switching to monitor the recovery rate and jitter of different gases and gas mixtures from atmospheric to high pressures (1000 psi) Construct a parallel test system for material lifetime and geometry evaluation Payoff: High rep-rate low loss switch for pulsed ring- down applications. End Goals: Allow accurate switching for a pulsed ring down phased array antenna that has both good recovery rate and low jitter Accomplishments: -Completed project design and construction -Integration and improvement of project subsystems -Basic diagnostics setup and initial testing -Triggered repetitive operation (100Hz, 65 kV, 400 psi nitrogen) -Performed initial lifetime testing JITTER AND RECOVERY RATE OF A TRIGGERED SPARK GAP WITH HIGH PRESSURE GAS MIXTURES James Dickens, Use hermetically sealed high pressure spark gap design -Introduce a simple effective gas mixing subsystem -Fast diagnostics and data acquisition techniques -Modular design for both simple system integration and minimal corona and breakdown possibilities -System integrity at high voltages and high pressures

PROJECT DESIGN IMAGES Diagnostics Trigger High pressure gases Charge Line Switch Vacuum Charge resistor 6, 300 Ω HV resistors Load Containment Chamber HV Charger  Hermetically sealed  >300 psi  RG 220 (10m)  50 Ohm, 100 ns pulse, ~1 nF  >50kV, 25mA  Safety containment  Gas backfill accessible  SOS pulser  100 kV, 10 ns rise-time 1kHz in burst mode  >400V, 1.5 A power supply  >10V trigger  dry air, N2, H2, SF6  various gas mixtures 1” Lexan Cover Gas mix output Exhaust Gas Mix Chamber  Hold >1500 psi  Provide simple gas mixing Pressure monitor

Gas flow Copper tungsten electrode Kel-F lining G-10 housing Gas input RG220 fitting Set screw Switch Design

Spark Gap G-10 Housing Al Connecting Pieces CuW Electrodes KEL-F Liner Al Baffle

Polished CuW Electrodes

Eroded CuW Electrodes Electrode wear after ~10 4 shots Example of minimal erosion Ablation measurements indicate negligible material loss

PROJECT IMAGES HV Charge Line 125 KΩ Charging Resistor Feed-through for seal and corona reduction 50 Ω Load XHR DC Power Supply BNC 565 Pulse/Delay Generator

Project wave forms BNC trigger to capture 10 th pulse Rep-rated Self Break (30 kV, 30psi Nitrogen) Externally triggered 35 kV, 10Hz operation Signal from Capacitive V- probe Integral of Capacitive V- probe signal Triggered 35kV, 10Hz pulses

Lifetime Test Setup Main and peaking gaps pressurized to ~500psig Charging voltage = 90kVDC Trigger pulse is created by peaking gap self-break Voltage probes on the load side of peaking and main gap record pulse

FY07-FY08 SCHEDULE  Improve system connections for enhanced power transfer and corona reduction  Test with higher voltage and pressure to improve rise-time and jitter  Compare rise-time and jitter of different gasses  Introduce gas mixtures and record effects on jitter and rise-time

Technical Issues: Initial condition integration into model. Accurately accounting for material properties and effects. Proper modeling of a closing switch and the effects of jitter. Approach: Construct an accurate model of a single element pulsed ring-down antenna using the Comsol Multi-physics software package allowing exotic antenna structures to be evaluated before they are physically constructed. Payoff: Far field energy deposition for neutralization of Improvised Explosive Devices (IEDs) at long range distances. End Goals: Be able to accurately model and simulate various multi-element antenna structures and the effects upon the performance of a pulsed ring-down phased array. Accomplishments: Achieved accurate results of multiple antenna structures in a 2-D and 3-D regime using transient analysis. Constructed a two element array to demonstrate beam steer and the effect of high switch jitter. Achieved numerical results for energy density and magnitude at various far field points. Pulsed Ring-down Multi-Element Antenna

2-D and 3-D Modeling Monoconical Antenna 2-D Dual Dipole Array 3-D Electric Field 2-D Electric Field 3-D

Beam Steering

Far Field Results

PRDS array Example: radiated electric field for four dipole sources (spaced ½ wavelength apart), with no switch jitter Simulated single source radiated electric field waveform: Peak electric field vs. direction, measured relative to that received from a single source:

PRDS array Example: radiated electric field for four dipole sources (spaced ½ wavelength apart), with uniformly distributed switch jitter from 0 to ½ period (1 single shot) Simulated single source radiated electric field waveform: Peak electric field vs. direction, measured relative to that received from a single source:

PRDS array – Monte Carlo simulation Difficult to solve analytically for output variable statistical distributions given switch jitter distributions Use Monte Carlo method: simulate many firings of an array to build up output statistics Inputs: array parameters, simulated or experimentally measured switch jitter distributions Status: basic simulation is functional

PRDS array – advanced concept Sources mounted on multiple vehicles Firing controlled using GPS timing, coordinated to place “hot spot” on desired location High rep-rate sources could be controlled to rapidly scan an area Modeling to include GPS timing and position errors in addition to individual switch jitter

FY07-FY08 SCHEDULE Complete the Comsol model that accounts for material properties, initial charging conditions, and closing switch characteristics. Compare model to experimental results and adjust accordingly to match. Design and model various antenna structures along with the performance results when in an array. Examine the affect of jitter on a compact array (2 ft- 5ft antenna distance) and a large mobile array (2 m – 15 m antenna distance)

Technical Issues: Scaling laws and physics of ultra-fast switching are unknown Approach: Empirical analysis of fast switching gas Pulses: <150 ps rise, <300 ps FWHM V(t), I(t) with 50 ps sampling rate X-ray analysis through fast PMT Streak-camera luminosity analysis FEM analysis of geometric gap transition Distributed Monte-Carlo electron motion / amplification simulations Payoff: Scaling laws and design criteria for ultra- fast switching. End Goals: Improve transmission line switching for antenna coupling. Accomplishments: Empirical results –Gap currents determined through lumped parameter modeling –Formative delay times quantified –Runaway electron analysis –Ultra-fast luminosity imaging Monte-Carlo Analysis –Determination of electron multiplication rates –Direct calculation of space charge formation –Results support empirical analysis Ultra-Fast Gas Switching

PROJECT IMAGES 1) Experimental Setup 2) Essential Experimental Results Formative delay times as a function of pressure for different voltage amplitudes from kV. Streak-Camera results show breakdown structure as a function of time. The images show a region of high ionization near the cathode. The slope in the luminosity shows the transit time for the gap. Background gases are Argon and Dry Air with pressures from high vacuum to atmosphere. Rexolite lens between coaxial to biconical geometric transition limits wave distortion. FEM simulation of open gap for line characterization (time not to scale).

PROJECT IMAGES 3) Monte Carlo Simulation 4) Simulation Results Cathode Anode Electron amplification rates for varying pressures and field amplitudes can be combined with models to predict delay times. Space charges in the vicinity of the cathode lead to local fields on the order of the applied field. Ionization mapping shows a high ionization region near the cathode similar to the empirical results. Past this region electrons tend to accelerate to runaway velocities limiting further ionization. Simulations run on 32 node Beowulf cluster. Capable of > 5 Gflop/s Efficient internode communication using the standard message passing interface (MPI) Simulation based off null-collision method for determining collision type.