Advanced radiography capabilities can be provided by diodes in which a < 1 mm diameter cathode is immersed in a -60 T magnetic field and pulsed to -16 Mv. An electron current of -50 kA is constrained by the magnetic field to -1 mm at the anode, and produces v 1 krad at 1 m. A 16 MV, 50 ns flat-top voltage adder has been designed that is optimum to develop such &odes. The adder approach is chosen to give a relatively short risetime and low prepulse. It also has the advantage of being re-configurable to provide two 8 MV pulses. The pulser uses standard Marxes, intermediate stores, and water P E S . Novel features include an oil prepulse switch, an induction cell that is fed from one pain< and a blocking network to couple two pulses to one cell. Based on detailed simulations, a design has been completed through detailed drawings and prototype hardware will be tested next year. Figure 1 shows an "immersed diode" designed for advanced radiography applications [ 11. The negative inner conductor of a coaxial MlTL enters a strong axial magnetic field produced by a pulsed solenoid, and tapers to a cathode less than 1 mm in diameter. The magnetic field prevents the electron beam from expanding in radius sigmficantly, thereby forming a -1 mm x-ray source at the anode. The spacecharge-limited current in such an "immersed diode" system is -V/300 ohms, and at 16 MV, for example, a current of 50 kA impinging on a high-Z anode should be able to create a radiation source of > 1 kradat 1 min tens of ns. I. RADIOGRAPHY REQUIREMENTS AND THE RITS CONFIGURATIONSExperiments at up to 50 T on Hermes I11 [2] and Sabre [3] have had considerable success in demonstrating such sources, producing as much as 114 rads at 1 m from a 1.5 mm FWHM diameter spot. Experimental progress has been limited by the availability of these accelerators and the fact that they were not designed for this application. The Radiographic Integrated Test Stand (RITS) has been designed as a 16 MV source optimized for immersed diode radiography. It can also reconfigured to deliver two pulses at 8 MV or more. Figure 1. High B, rdographic diode.Figure 2 shows the three different configurations of RITS. It is an induction voltage adder like Hermes and Sabre and uses similar drive technology. M a n generators (two) charge coaxial water transfer capacitors (TCs--two per Marx, four in all). Each TC is switched out by a laser triggered gas switch (LTGS) to fastcharge water pulse forming lines (PES --three per TC, twelve in all) that have synchronized self-brealang water output switches. *Bechtel Nevada Subcontract 18719; **JFC, Castro Valley, CA &77803-549&u99/slO.~l999 IEEE. 403Single adder configuration producing 16 MV pulse.Double adder configuration producing separate 8 MV pulses on two cathodes in common B, magnet.
We have already demonstrated the utility of inductive voltage adder accelerators for production of small-size electron beams. In our approach, the inductive voltage adder drives a magnetically immersed foilless diode to produce high-energy (10-20 MeV), high-brightness pencil electron beams. This concept was first demonstrated with the successful experiments which converted the linear induction accelerator RADLAC 11' into an WA fitted with a small l-cm radius cathode magnetically immersed foilless diode (FUDLAC II/SIVULE).~ We present here first validations of extending this idea to mm-scale electron beams using the SABRE3 and HEMS-II14 inductive voltage adders as test beds. The SABRE experiments are already completed and have produced 30-kA, 9-MeV electron beams with envelope diameter of 1.5-mm FWHM. The H E W S -I I I experiments are currently underway.
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 highbrightness pulsed x-ray radiographic sources are needed from sub-megavolt to 16-MeV endpoints with greater source brightness (dose/spotz) than presently available. We are applying WA systems to produce very intense (up.to 75 TW/cma) electron beams for these flash radiographic applications.
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