A. Krasnykh

A solid state Marx type modulator for driving a TWT

This paper describes a solid state Marx type modulator design delivering an 11 kilovolt, 2-4 /spl mu/sec pulse to the cathode of an X-band driver TWT. Insulated gate bipolar transistors (IGBTs) are used as on/off switches to operate the Marx circuit in the energy storage capacitor partial discharge mode. With the aid of a passive compensation circuit, a very flat TWT cathode driver pulse is obtained. The 2 /spl mu/sec, 11 kV pulse amplitude is flat to 0.06%.

Pulse transformer R&D for NLC klystron pulse modulator

The authors have studied a conventional pulse transformer for the NLC klystron pulse modulator. The transformer has been analyzed using a simplified lumped circuit model. It is found that a fast rise time requires low leakage inductance and low distributed capacitance and can be realized by reducing the number of secondary turns, but it produces larger pulse droop and requires a larger core size. After making a tradeoff among these parameters carefully, a conventional pulse transformer with a rise time of 250 ns and a pulse droop of 3.6% has been designed and built. The transmission characteristics and pulse time-response were measured. The data were compared with the model. The agreement with the model was good when the measured values were used in the model simulation. The results of the high voltage tests using a klystron load are also presented.

Confined flow multiple beam guns for high power RF applications

Summary form only given. Multiple beam (MB) guns are finding increased application in different types of microwave tubes. The advantages of NIB guns in a microwave tube, for example, in a klystron (MBK), is a decrease of beam voltage, reduced power densities, and lower voltage gradients. Another advantage is increased bandwidth. Double convergent MB guns are capable of providing lower cathode loading and higher Rf power at a given frequency. At present all known MBKs use Brillouin type focusing, where the cathode is shielded from the magnetic focusing field. This type focusing is seldom used in single beam, high power klystrons due to inferior beam transmission as compared to the confined flow type focusing. This program is focused on developing confined flow multiple beam guns for RF power levels in excess of 50 MW at X-Band. A number of advanced, computational tools are used to model and refine the electric and magnetic field configural ions and transport electrons through the 3-D structure. A multiple beam gun configuration was achieved that provided confined flow beam transport through a simulated device without beam interception. Efforts are now focused on refining the modeling capability and improving the beam quality. Progress to date will be reported.

Development of the pulse transformer for NLC klystron pulse modulator

We have studied a conventional pulse transformer for the NLC klystron pulse modulator. The transformer has been analyzed using a simplified lumped circuit model. It is found that a fast rise time requires low leakage inductance and low distributed capacitance and can be realized by reducing the number of secondary turns, but it produces larger pulse droop and core size. After making a tradeoff among these parameters carefully, a conventional pulse transformer with a rise time of 250 ns and a pulse droop of 3.6% has been designed and built. The transmission characteristics and pulse time-response were measured. The data were compared with the model. The agreement with the model was good when the measured values were used in the model simulation. The results of the high voltage tests are also presented.

Developments in the NLC modulator R&D program at SLAC

The NLC (Next Linear Collider) development effort continues to move forward from its initial Zero Design Report (ZDR). A major component of the NLC is the high power RF source, which is designed around the technology of the klystron. The NLC is conceptualized around 75 MW, PPM (periodic permanent magnet) focused klystrons and will use approximately 3300 klystrons in each of the two main linacs. Simple analysis has shown that operating two klystrons per modulator has both economic and configuration advantages. The physical design of the modulators is an integral part of the design of the tunnels, etc. Klystron spacing is dictated by the pulse compression regime. This paper presents the to-date results of the investigation efforts in energy storage, pulse transformers and efficiency. Future plans, including new technologies to pursue are discussed. SLAC is also continuously looking for new and novel approaches to either the modulator components or the overall modulator approach. SLAC and the DOE are attempting to use the SBIR program to help with industry development of the needed components and new ideas.

High dielectric constant materials for pulsed energy storage capacitors

A high dielectric constant coupled with a high dielectric strength is essential in producing high energy density, low inductance capacitors used in pulsed energy systems. The NLC (Next Linear Collider) which is envisioned as the next step in high energy physics research tools will employ several thousand pulsed klystron RF sources. The modulators for these klystrons require efficient, reliable energy storage capacitors. Collaboration with Russian scientists has made available to us information and capacitor samples based on the Tungsten-Bronze family which exhibit dielectric constants of over 1,000, and working voltage gradients in excess of 60 kV/cm. We are currently testing 10 nanofarad, 40 kV capacitor samples in pulsed modulator service manufactured at the GIRIKOND Institute in St. Petersburg, Russia. Dr. Galina Blokhina is the designer of these capacitors. The material is a solid solution of complex compounds of niobate dielectric. The Tungsten-Bronze-Type Crystals are bound in amorphous glass. The paper summarizes the theoretical studies of this material with a Russian and English bibliography, describes Dr. Blokhinas manufacturing processes, and presents the results of pulsed energy storage testing carried out at SLAG for the NLC R&D effort. It is the intent of this paper to spark interest in transferring this technology to the world community so that R&D in this area will expand and provide this technology to many users including the Parallel NLC project.

An electron gun for a sheet beam klystron

Summary form only given. Calabazas Creek Research, Inc. (CCR) is developing a rectangular, gridded, thermionic, dispenser-cathode gun for sheet beam devices. The first application is expected to be klystrons for advanced particle accelerators and colliders. The current generation of accelerators typically use klystrons with a cylindrical beam generated by a Pierce-type electron gun. As RF power is pushed to higher levels, space charge forces in the electron beam limit the amount of current that can be transmitted at a given voltage. The options are to increase the beam voltage leading to problems with X-Ray shielding and modulator and power supply design, or to develop new techniques for lowering the space charge forces in the electron beam. The current program addresses issues related to beam formation at the emitter surface, design and implementation of shadow and control grids in a rectangular geometry, and is directed toward a robust, cost-effective, and reliable mechanical design. A prototype device will be developed that will operate at 415 kV, 250 A for an 80 MW, X-Band, sheet-beam klystron being developed by Stanford Linear Accelerator Center. The cathode will have 100 cm/sup 2/ of cathode area with an average cathode current loading of 2.5 A/cm/sup 2/. For short pulse formation, the use of a grid was chosen. We will report the electrostatic and beam optics design in both 2- and 3-D as well as a thermal-mechanical analysis of the cathode region. The 2-D calculation gives the basis for the 3-D simulation, which, particularly with the grid structure, is expected to take a great deal of time per run. The modeling was done using Trak, a code from Field Precision. This code allows the use of variable mesh size, a feature that is essential for accurately including the grid structure. To minimize the power lost to the grid, the cathode has non-emitting segments in line with the grid. These segments are in the form of sections of arcs to help focus the beam around the grid, which is formed of 1.25 mm, wire. With careful adjustment of this geometry and the grid potential, an emittance close to that of an ungridded gun can be achieved. At a beam current of 256A, the emittance was 7 10/sup -5/ /spl pi/ m rad.

A gridded electron gun for a sheet beam klystron

In this paper we propose a gridded electron gun(dispenser-cathode gun) for sheet beam devices. The current generation of accelerators typically use klystrons with a cylindrical beam generated by a pierce-type electron gun. The gun design were used to determine the starting point for the electrodes to produce the compression. The use of grid was chosen for short pulse formation. The result showed that a very quality beam could be achieved even in the presence of the shadow grid.Calabazas Creek Research Inc. (CCR) is developing rectangular, gridded, thermionic dispenser-cathode guns for sheet beam devices. The first application is expected to be klystrons for advanced particle accelerators and colliders. The current generation of accelerators typically use klystrons with a cylindrical beam generated by a Pierce-type electron gun. As RF power is pushed to higher levels, space charge forces in the electron beam limit the amount of current that can be transmitted at a given voltage. The options are to increase the beam voltage, leading to problems with X-ray shielding and modulator and power supply design, or to develop new techniques for lowering the space charge forces in the electron beam. The current development program addresses issues related to beam formation at the emitter surface, design and implementation of shadow and control grids in a rectangular geometry, and the high voltage insulator.

Development of Multiple Beam Guns for High Power RF Sources for Accelerators and Colliders

There is a need for high power RF sources for the next generation of accelerators and colliders. Sources that operate at reduced beam voltage allow solid state power supplies with significant cost reduction over conventional pulse modulators. Multiple beam RF sources provide reduced beam voltage by using a multiplicity of beamlets that traverse the RF circuit through individual beam tunnels, reducing the space charge forces that drive the voltage requirement. The current generation of multiple beam devices typically use Brillouin focusing, which limits high power operation. The devices reported here utilize confined flow focusing which allows much tighter control of the electron beamlets and consequently, higher power operation. Progress in the development of a 100 MW multiple beam electron gun with confined flow focusing is reported.

Fabrication technology for multi megawatt peak RF absorption layer

An RF absorber capable of stable absorption of multi MW peak and several kW average powers is needed for traveling wave mode high gradient accelerating structures. At the end of structure an RF load is used to absorb residual RF power left in the system. Typically, a liquid media (for example, pure water) is used as an effective power absorber of this residual power. However most large scale accelerators require a metal interface to separate the high RF vacuum envelope and any liquids used to carry away heat from RF absorption. This requirement comes from a desire to minimize the possibility of contaminating the vacuum envelope. The metal/liquid interface is a weak component for a high peak power mode operation. Our presentation we will address is the development of a multi MW RF absorber that is compatible with ultra-high vacuum operation while reliably and stably absorbing MW level peak power. Load prototypes developed at SLAC using this technology have proven capable of absorbing >30 MW peak, >4 kW average, and 120 Hz RF pulse with an attenuation rate 5.5 dB per meter for S-Band case.