Ion extraction capabilities of two-grid accelerator systems

An experimental investigation into the ion extraction capabilities of two-grid accelerator systems common to electrostatic ion thrusters is described. This work resulted in a large body of experimental data which facilitates the selection of the accelerator system geometries and operating parameters necessary to maximize the extracted ion current. Results suggest that the impingement-limited perveance is not dramatically affected by reductions in screen hole diameter to 0.5 mm. Impingement-limited performance is shown to depend most strongly on grid separation distance, accelerator hole diameter ratio, the discharge-to-total accelerating voltage ratio, and the net-to-total accelerating voltage ratio. Results obtained at small grid separation ratios suggest a new grid operating condition where high beam current per hole levels are achieved at a specified net accelerating voltage. It is shown that this operating condition is realized at an optimum ratio of net-to-total accelerating voltage ratio which is typically quite high. The apparatus developed for this study is also shown to be well suited measuring the electron backstreaming and electrical breakdown characteristics of two-grid accelerator systems

Pencil-like mm-size electron beams produced with linear inductive voltage adders (LIVA)

This paper presents design, analysis, and first results of the high brightness electron beam experiments currently under investigation at Sandia. Anticipated beam parameters are: energy 12 MeV, current 35-40 kA, rms radius 0.5 mm, pulse duration 40 ns FWHM. The accelerator is SABRE, a pulsed LIVA modified to higher impedance, and the electron source is a magnetically immersed foilless electron diode. 20 to 30 Tesla solenoidal magnets are required to insulate the diode and contain the beam to its extremely small sized (1 mm) envelope. These experiments are designed to push the technology to produce the highest possible electron current in a submillimeter radius beam. Design, numercial simulations, and first experimental results are presented

Lawson criterion for ignition exceeded in an inertial fusion experiment

For more than half a century, researchers around the world have been engaged in attempts to achieve fusion ignition as a proof of principle of various fusion concepts. Following the Lawson criterion, an ignited plasma is one where the fusion heating power is high enough to overcome all the physical processes that cool the fusion plasma, creating a positive thermodynamic feedback loop with rapidly increasing temperature. In inertially confined fusion, ignition is a state where the fusion plasma can begin "burn propagation" into surrounding cold fuel, enabling the possibility of high energy gain. While "scientific breakeven" (i.e., unity target gain) has not yet been achieved (here target gain is 0.72, 1.37 MJ of fusion for 1.92 MJ of laser energy), this Letter reports the first controlled fusion experiment, using laser indirect drive, on the National Ignition Facility to produce capsule gain (here 5.8) and reach ignition by nine different formulations of the Lawson criterion

Inductive Voltage Adder Driven X-Ray Sources for Hydrodynamic Radiography

Inductive Voltage Adder (IVA) accelerators were developed to provide high-current (100s of kA) power pulses at high voltage (up to 20 MV) using robust modular components. This architecture simultaneously resolves problems found in conventional pulsed and linear induction accelerators. A variety of high-brightness pulsed x-ray radiographic sources are needed from sub-megavolt to 16-MeV endpoints with greater source brightness (dose/spot) than presently available. We are applying IVA systems to produce very intense (up to 75 TW/cm{sup 2}) electron beams for these flash radiographic applications. The accelerator electromagnetic pulse is converted to a directed electron beam at the end of a self-magnetically insulated vacuum transmission line. The cantilevered cathode threading the accelerator cavities terminates in a small (1-mm diameter) needle, producing the electron beam which is transported to a grounded bremsstrahlung converter within a strong ({approximately}50-T) axial magnetic field. These systems produce mm-sized stable electron beams, yielding very intense x-ray sources. Detailed simulations of the electron beam generation, transport, and target interaction are presented along with scaling laws for the radiation production and x-ray spot size. Experimental studies confirm these simulations and show this reliable, compact, and inexpensive technology scales to 1000-R doses a meter from a mm-diameter source in 50 ns

Experimental Comparison of 2-3MV X-Ray Sources for Flash Radiography

High-brightness flash x-ray sources are needed for penetrating dynamic radiography for a variety of applications. Various bremsstrahlung source experiments have been conducted on the TriMeV accelerator (3MV, 60 {Omega}, 20 ns) to determine the best diode and focusing configuration in the 2-3 MV range. Three classes of candidate diodes were examined: gas cell focusing, magnetically immersed, and rod pinch. The best result for the gas cell diode was 6 rad at 1 meter from the source with a 5 mm diameter x-ray spot. Using a 0.5 mm diameter cathode immersed in a 17 T solenoidal magnetic field, the best shot produced 4.1 rad with a 2.9 mm spot. The rod pinch diode demonstrated very reproducible radiographic spots between 0.75 and 0.8 mm in diameter, producing 1.2 rad. This represents a factor of eight improvement in the TriMeV flash radiographic capability above the original gas cell diode to a figure of merit (dose/spot diameter) > 1.8 rad/mm. These results clearly show the rod pinch diode to be the choice x-ray source for flash radiography at 2-3 M V endpoint