One class of modern pulse power generators use deionized water as an energy storage, switching and transmission dielectric. Water is chosen for its high dielectric constant and relatively high resistivity, which allows reasonably sized and efficient low-impedance high-voltage pulse lines where pulse durations are less than 100 µs. Water/ethylene glycol mixtures are being researched, so that rotating machinery, rather than the usual Marx generator, can be used as the primary energy store. The high resistivity and high dielectric constant of these mixtures at low temperature permit low-loss operation on millisecond time scales. Simple design criteria linking load parameters and charging circuit characteristics to the liquid dielectric are developed which show that the dielectric constant, breakdown strength, and relaxation time are the primary properties of interest to the pulse power engineer. On time scales greater than 100 µs, injection of space charge, with density q and mobility µ, affects the charging and discharging circuit characteristics, introduces the time constant of the time of flight for injected charge to migrate between electrodes, and increases the effective ohmic conductivity σ to σ + qµ. A drift-dominated conduction model is used to describe measured space-charge effects. Kerr electrooptic field mapping measurements show strong space-charge effects with significant distortions in the electric field distribution a few hundred microseconds after high voltage is applied. The injected charge magnitude and sign depends on the electrode material. Thus by appropriate choice of electrode material combinations and voltage polarity, it is possible to have uncharged liquid, unipolar-charged negative or positive, or bipolar-charged liquid. An important case is that of bipolar injection, which has allowed up to a 40 percent higher applied voltage without breakdown than with no charge injection, and thus a doubling of stored energy due to the space-charge shielding which lowers the electric field strengths at both electrodes. Although injected space charge increases the stored electric energy over the capacitive space-charge-free energy, (1/2)CV2, more energy is required from a source during charging and the energy delivered to a resistive load is reduced because of internal dissipation in the capacitor as the charge is conducted to the electrodes. However, it appears that this extra dissipation due to injected charge can be made negligibly small and well worth the price if the space charge allows higher voltage operation for long charging time or repetitively operated machines.

Dielectric properties of water and water/ethylene glycol mixtures for use in pulsed power system design

Exploding wire loads have been used for several years to generate a hot dense plasma. For a generator with a rise time of tens of nsec and an impedance of 1 Ω or less, the inductance of the wire load and the tendency of current to flow outside the wire limits the energy that can be transferred to the wire. An array of several wires has now been used to lower the inductance and improve the energy transfer.

Multiple‐wire array load for high‐power pulsed generators

Transmission lines with distributed parameters are often used as electrical energy storage units and high-voltage (high-current) pulse shapers in electrophysical facilities. To increase the pulse power and reduce the dimensions of the equipment, deionized water, purified from mechanical impurities and gas, is used as an insulator in lines. Water has a high breakdown strength ((BS) ∼ 130 kV/cm) at pulses with a duration of ∼1 µs and electrode areas of up to tens of square meters, a large value of the relative permittivity (∼80), and a small loss tangent in the frequency range 0–1 GHz. Water also possesses a number of characteristics needed to achieve the above aims. We review here the data on the electrical properties of water and the studies of it published over the last three decades; the methods for increasing the water BS and the influence of numerous factors on the BS; the physical mechanism of water breakdown and the basic regularities of the BS in strong electric fields; the BS in the most frequently used electrode systems; formulas for calculating the water BS under different conditions and over the surfaces of solid dielectrics; the characteristics of mixtures of water and ethylene glycol or methanol; water purification systems for electrophysical facilities; the comparative parameters of coaxial energy storage units having various insulators, etc. The summary generalizes the brief and predominantly reference information on the electrical properties of water and its applications for researchers, engineers, and the users of the equipment having water insulation; it can help to solve related problems more quickly and reliably.

Water as an insulator in pulsed facilities ( Review )

Various aspects of the magnetically self‐pinched electron flow patterns in megavolt megampere diodes have been investigated. The onset of pinching has been correlated with a transition from Child‐Langmuir to parapotential voltage‐current characteristics. The average velocity at which the pinch sweeps towards the axis has been measured and is found to be ∼2 mm/nsec. The invariance of the current under geometrical scaling is demonstrated, and the connection between this invariance and plasma motion is discussed. Measurements of total diode current are compared with existing theories and agreement is found with the parapotential model.

Experimental investigation of high-current relativistic electron flow in diodes

We report experimental and theoretical studies of an intense relativistic electron beam (REB) injection system designed for tokamak current drive experiments. The injection system uses a standard high‐voltage pulsed REB generator and a magnetically insulated transmission line (MITL) to drive an REB‐accelerating diode in plasma. A series of preliminary experiments has been carried out to test the system by injecting REBs into a test chamber with preformed plasma and applied magnetic field. REBs were accelerated from two types of diodes: a conventional vacuum diode with foil anode, and a plasma diode, i.e., an REB cathode immersed in the plasma. REB current was in the range of 50 to 100 kA and REB particle energy ranged from 0.1 to 1.0 MeV. MITL power density exceeded 10 GW/cm2. Performance of the injection system and REB transport properties is documented for plasma densities from 5×1012 to 2×1014 cm−3. Injection system data are compared with numerical calculations of the performance of the coupled system ...

Intense relativistic electron beam injector system for tokamak current drive

OWL is a water‐dielectric line‐type pulse generator capable of producing ∠100‐nsec electron beams with total energies of as much as 150 kJ. In its present form, OWL II consists of an oil‐immersed, 1/3‐MJ Marx generator charging a 3.9‐Ω coaxial pulse‐forming line which is series switched into either a two‐ or three‐stage impedance transformer. The output impedance is 1.9 or 1.1 Ω, depending upon which transformer is used, and nominal beam outputs in the two cases are 1.3 MV at 0.8 MA and 0.9 MV at 1.2 MA, respectively. The pulse width can be selected at either 60 or 120 nsec (FWHM of power) by interchanging two available pulse lines. Shot‐to‐shot reproducibility in output of ±2% with a mean electron energy ∠1.0 MeV is obtained with the 1.9‐Ω final transformer. A triggered multichannel water switch minimizes the risetime of pulses injected into the transformer section.

G. B. Frazier

Papers

OWL II pulsed‐electron‐beam generator