## Spark Sujitch

The spark gap switches the energy from the capacitor into the inductor where a resonant tank is momentarily set up (see Figure 25-2). The current rise time occurs over the period pi/2 X (LC) ‘.The gap separation distance is set to fire at the desired breakdown voltage. The impedance of the spark switch is determined by the equation ZSP = (к X l)/Q, where к equals.8 X 10 I equals the spark gap distance in centimeters, and Q equals ihe amps per second (coulombs) of discharge. The gap is self-firing and requires no external triggering. The gap assembly is an integral part of the discharge path and must be constructed to minimize inductance and resistance.

before V reaches 50,000 volts. Once fired, a peak current rise cf di/dt=V/L occurs. The period of circuit response is functional of 16 x (LC)S The capacitor now discharges into ihe circuit inductance in 1/4t with the peak current now causing Ihe wire to explode and imerrupting lhis current just before it peaks. The inductive energy (LI2) is released in an explosive burst of broadband electromagnetic radiation. The peak power is derived via the following and rs m excess of many megawatts!’!!! |

1. Charging Cycle – dv=ldt/C (Expresses the voitage charging on Ihe capecitor as a f(t) with I constant current)

2. Storage energy in С as a f(v): E=.5Cv2 (Expresses energy in joules as the voltage increases)

3 Response time 1/4 cycle current peak: 1 57(LC) fi (Expresses the time for the first resonant current peaking when ihe spark switch fires)

4. Peak current in 1/4 cycle. V(C/L)S (Expresses the peak current)

5. Initial response as a (1)1. Ldi/dt+iR+1 /С+1 /Cint idt = 0 [Expresses voltages as a f(t)l 6 Energy joules in inductor E= ,5Li2

7. Response when circuit is disrupted at max current through L:

Ld2 i/dt2 + Rdi/dt + it/C = dv/dt. One now sees the explosive effects of the first term of this simpte equation as the energy in the inductor must go somewhere in a very short time, resulting in an explosive E X В field energy release.

An appreciable pulse of many megawatts in the upper RF energy spectrum can be obtained by destabilizing the LCR circuit as shown above. The only limiting factor is the intrinsic real resistance that is always present in several forms, such as leads, skin effect, dielectric and switching losses, etc. These losses must be minimized for optimum resuits. The RF output can be coupled to a parabolic microwave dish or tuned hom. The Q of the output will depend to an extent on the geometry of the wire switch. Longer lengths will produce more "B" field characteristics while short more "E" field. These perameters will enter into the coupling equations regarding the radiation efficiency of the antenna. Experimenting is the besi approach using your math skills only for approximating key parameters Damage to circuitry usuallly is the result of very high di/dt (B field) pulse properties. This is a point of discussion!!

Figure 25-2 EMPpitlser schematic

a threaded rod and locking jam nut scheme. The top ball also uses a threaded rod that fits into the 74-inch PVC tubing used for the structural support of the wire disruption scheme. |

The fabrication of our lab test unit is shown and you may deviate with your own ideas, but the objective must be minimal circuit impedance.

You will note that the bottom ball of the spark gap switch is at a high potential and is made adjustable by

Foci вгва ot parabolic алмпла |

COMMOWGRD |

Retaining blocks to keep bottom of С1 secured |

Figure 25-3 Front view of pulser showing spark switch |

Low-induclanee extension pieces are used to lengthen the capacitor terminals and are fabricated from ‘/4-inch brass plates with mating holes to the existing block terminals of the capacitor. The edges are rounded and smoothed to prevent corona.

Dimension of spark switch and output section will depend on the maximum value of the charging voltage used

Tungsten/cerium electrodes are recommended The brass spheres provide a frictional press fit as well as some cooling.

Partition plate—’ Nut & bolt |

The method we used to attach pillars to partition plates using fist-faced end caps attached with 1/4-20 nuts and bolts Drill holes through cap and pillar for tie wraps to secure together Note there are 24 of these attachment points!

I he open-air, self-triggering spark switch is intended as a low-cost approach to this very strategic part. A system of this type requires the fastest switching times possible. Gas-filled, triggered gaps will switch faster ihan open air. and isotope doped electrodes will further enhance performance.

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