JITTER AND RECOVERY RATE OF A TRIGGERED SPARK

JITTER AND RECOVERY RATE OF A TRIGGERED SPARK GAP WITH HIGH PRESSURE GAS MIXTURES Technical Issues: Payoff: High rep-rate low loss switch for pulsed ringdown applications. End Goals: Allow accurate switching for a pulsed ring down phased array antenna that has both good recovery rate and low jitter 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 -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 Accomplishments: - Completed project design and construction Integration and improvement of project subsystems Basic diagnostics setup and initial testing Triggered repetitive operation (100 Hz, 65 k. V, 400 psi nitrogen) Performed initial lifetime testing James Dickens, james. dickens@TTU. edu, 806 -742 -1254

PROJECT DESIGN IMAGES Charge Line 1” Lexan Cover Pr 50 mix ovide s 0 psi ing imp le g as Diagnostics Gas mix output Sw ØH itc e Ø> rm h e 300 tic psi ally sea le n kf tai ill nm ac en ce t ss ib le 6, res 300 Ω ist ors HV re ac ad Exhaust Co Ch nt a Ø am in S m Ø af G et be en as y r c t b o Lo iou , N s g 2, as H 2 m , S ixt F 6 ur es Pressure monitor ØRG 220 (10 m) Ø 50 Ohm, 100 ns pulse, ~1 n. F Hi ga gh pr Ø se es Øv dry s su a ar ir Charge resistor Gas Cha Mix ØH mbe old Ø >1 r d Ø>400 V, 1. 5 A power supply Ø>10 V trigger Trigger ØSOS pulser Ø 100 k. V, 10 ns rise-time 1 k. Hz in burst mode Vacuum HV Charger Ø>50 k. V, 25 m. A

Switch Design Gas input Copper tungsten electrode Gas flow Set screw RG 220 fitting Kel-F lining G-10 housing

Spark Gap G-10 Housing Al Baffle Cu. W Electrodes Al Connecting Pieces KEL-F Liner

Polished Cu. W Electrodes

Eroded Cu. W Electrodes • • • Electrode wear after ~104 shots Example of minimal erosion Ablation measurements indicate negligible material loss

PROJECT IMAGES BNC 565 Pulse/Delay Generator XHR 600 1. 7 DC Power Supply 50 Ω Load Feed-through for seal and corona reduction HV 125 KΩ Charging Resistor Charge Line

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

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

FY 07 -FY 08 SCHEDULE ØImprove system connections for enhanced power transfer and corona reduction ØTest with higher voltage and pressure to improve rise-time and jitter ØCompare rise-time and jitter of different gasses ØIntroduce gas mixtures and record effects on jitter and rise-time

Pulsed Ring-down Multi-Element Antenna 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. 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. 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. 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.

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

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

FY 07 -FY 08 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 5 ft antenna distance) and a large mobile array (2 m – 15 m antenna distance)

Ultra-Fast Gas Switching Technical Issues: • Scaling laws and physics of ultra-fast switching are unknown Payoff: Scaling laws and design criteria for ultrafast switching. End Goals: Improve transmission line switching for antenna coupling. 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 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

PROJECT IMAGES 1) Experimental Setup 2) Essential Experimental Results Formative delay times as a function of pressure for different voltage amplitudes from 40 -150 k. V. • 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). 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.

PROJECT IMAGES 3) Monte Carlo Simulation • 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. 4) Simulation Results • 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. Cathode Anode