The baseline RF-driven H- ion source configuration at the Spallation Neutron Source (SNS) facility uses a continuous wave (CW) 600 W 13 MHz RF system to ignite and maintain a low-density plasma inside the ion source vacuum chamber. After the continuous lowdensity 13 MHz plasma has been established, a pulsed (typical 1 ms pulse width and 60 Hz pulse repetition rate) 80 kW 2 MHz RF system is used to increase the plasma density to produce the pulsed H- ion beam. Incremental upgrades and improvements to the SNS ion source systems have resulted in the ability to reliably operate an H- ion source for SNS neutron production run cycles that can last up to four months. Conditions inside the SNS H- ion source evolve throughout a four-month run cycle due to changes in impurity levels, sputtering, and erosion. As the internal ion source conditions change during the run cycle, there can also be changes in plasma stability and the 13 MHz RF power level required to ignite the plasma. This paper presents the preliminary results of testing performed on the SNS Ion Source Test Stand (ISTS) system where we looked at plasma ignition and plasma stability using 27 MHz RF in place of the baseline 13 MHz RF system.

Abstract The baseline RF-driven H− ion source configuration at the Spallation Neutron Source (SNS) facility uses a continuous wave (CW) 600 W 13 MHz RF system to ignite and maintain a low-density plasma inside the ion source vacuum chamber. After the continuous low-density 13 MHz plasma has been established, a pulsed (typical 1 ms pulse width and 60 Hz pulse repetition rate) 80 kW 2 MHz RF system is used to increase the plasma density to produce the pulsed H− ion beam. Incremental upgrades and improvements to the SNS ion source systems have resulted in the ability to reliably operate an H− ion source for SNS neutron production run cycles that can last up to four months. Conditions inside the SNS H− ion source evolve throughout a four-month run cycle due to changes in impurity levels, sputtering, and erosion. As the internal ion source conditions change during the run cycle, there can also be changes in plasma stability and the 13 MHz RF power level required to ignite the plasma. This paper presents the preliminary results of testing performed on the SNS Ion Source Test Stand (ISTS) system where we looked at plasma ignition and plasma stability using 27 MHz RF in place of the baseline 13 MHz RF system.

We consider the development of a highly efficient, gridless tetrode as a megawatt-level RF source in the 3 to 10 MHz range for application in mobile ionospheric heaters. Such a heater has potential advantages over the stationary facilities, such as High-Frequency Active Auroral Research Program, found at high latitudes. The considered device operates in class D mode with an annular electron beam allowing realization of high efficiency. The present study, based on numerical simulations using the Particle in Cell code Michelle [Petillo et al., IEEE Trans. Electron Devices Sci. 52, 742 (2005)], examines the optimization of device geometry. In particular, the dependence of efficiency on spacing between electrodes is studied. In addition, the role of secondary electrons emitted at the collector is examined. Both static and time dependent operations are simulated. In the time dependent case, it is found that during the portion of the RF cycle when the beam current is on, secondaries emitted from the collector are driven back into the collector by the incoming primary beam. When the beam is switched off, secondaries can stream back into the tetrode and have a small negative impact on efficiency. We present a design in which the secondary electrons are eventually absorbed at the collector rather than at the cathode or anode.

The ionosphere plays a prominent role in the performance of critical civilian and military communication systems. The properties of the ionosphere can be affected by Ionospheric Modification (IM). The key instrument in IM research is a powerful, ground-based, high frequency source of electromagnetic waves known as a heater. Existing heaters operate with large, fixed location antenna arrays. With a mobile heater, investigators would be able to conduct IM research at different latitudes without building a costly permanent installation. For developing a mobile heater with a much smaller antenna array, a new highly efficient megawatt-class Radio Frequency (RF) source is required to reduce the overall power demands on a fully deployable system. The concept of such a source has been described previously [Beaudoin et al., J. Electromagn. Waves Appl. 31(17), 1786–1801 (2017)]. Here, experimental results using an electron beam produced by a gridded thermionic electron gun to drive an external lumped element circuit for a high efficiency RF generation are reported. The gun produces an electron beam bunched at the driving frequency with a narrow phase angle spread that is then collected by an external circuit for resonant impedance matching to the load. The results showed that effects, such as the internal resistance of the inductor and deflection of the beam electrons by the induced RF voltages on the beam collector, are important considerations to be included in the design of a practical device using this configuration for high efficiency RF generation.

The ionosphere plays a prominent role in the performance of critical civilian and military communication systems. The properties of the ionosphere can be modified by Ionospheric Modification (IM). The key instrument in IM research is a powerful, ground-based, High Frequency (HF) source of electromagnetic waves known as a heater. Existing heaters operate with large, fixed location antenna arrays. With a mobile heater, investigators would be able to conduct IM research at different latitudes without building a costly permanent installation. A mobile heater for ionospheric modification studies will require a new megawatt class RF source. Our design incorporates a grid-less electron gun in which the beam is turned on and off by a mod-anode in class-D mode [1].

The power extraction circuit in an lOT (Inductive Output Tube) is a resonant circuit made of actual inductors and capacitors at low frequencies (3–10 MHz) due to the size limitation of a cavity. To build a mobile, low frequency range, high power sources for ionospheric heating with high efficiency and tunability, we need a constant impedance to decelerate the beam at all frequencies [1]. This paper discusses the design and construction of pi-circuit for two different gun designs: 70kV - 30A & 20 kV - 1A guns [2]. While the tunability is achieved by variable capacitors, the high efficiency is limited by the internal resistance of the inductor in the pi-circuit.

Results of a scaled experiment using the electron beam produced by a gridded thermionic cathode from an Inductive Output Tube electron gun to drive an external lumped element circuit for high efficiency radio frequency generation are reported. The IOT gun produces an electron beam bunched at the driving frequency that is then collected by an external circuit for resonant impedance matching to the load. Results showed that effects such as the internal resistance of the inductor and deflection of beam electrons by the induced RF voltages on the beam collector are important considerations to be included in the design of a practical device using this configuration for high efficiency, MW-class RF sources for Mobile Ionospheric Heaters (MIHs).

We report progress on the development of a high-power, high-efficiency source for a moveable ionospheric heater. The source is based on the Inductive Output Tube concept. The application requires operation in the 3-10 MHz range, thus opening the possibility to achieve true class - D operation and with that high efficiency. Our designs incorporate a solid-state driver, a grid-less gun, and a tunable output circuit. Preliminary results on a scaled device will be presented.

A mobile heater for ionospheric modification studies will require a new megawatt class RF source operating with an antenna array that is 1/20 the area of HAARP [1]. In order to deliver an effective power density comparable to that of HAARP, the total source power must be in the range of 16 MW, thus demanding highly efficient sources [2]. The source design we are currently pursuing utilizes a grid-less electron gun that uses a mod-anode to turn a thin annular beam on and off in class D operation. The beam is passed through a decelerating gap and its kinetic energy is extracted using a tunable resonant circuit that presents a constant impedance over the frequency range of 3–10 MHz, such that the beam is almost decelerated at all frequencies.

For mobile ionospheric heaters, it is necessary to develop highly efficient RF sources capable of delivering radiation in the frequency range from 3 to 10MHz with an average power at a megawatt level. A promising source, which is capable of offering these parameters, is a grid-less version of the inductive output tube (IOT), also known as a klystrode. In this paper, studies analyzing the efficiency of grid-less IOTs are described. The basic trade-offs needed to reach high efficiency are investigated. In particular, the trade-off between the peak current and the duration of the current micro-pulse is analyzed. A particle in the cell code is used to self-consistently calculate the distribution in axial and transverse momentum and in total electron energy from the cathode to the collector. The efficiency of IOTs with collectors of various configurations is examined. It is shown that the efficiency of IOTs can be in the 90% range even without using depressed collectors. Published by AIP Publishing.

A mobile heater for ionospheric modification studies requires a new megawatt (MW) class radio frequency (RF) source operating with an antenna array 1/20 the area of the High-Frequency Active Auroral Research Program (HAARP). To deliver an effective power density comparable to HAARP, the total source power must be in the range of 16MW, thus demanding highly efficient sources. While the development of a whole multi-megawatt system for mobile ionospheric heaters is a complex engineering problem, in the present paper we describe only the work of our group on studying main features of a prototype MW-class vacuum electronics RF source for such system. The source design we are currently pursuing assumes class D operation using a modified version of the inductive output tube. The electron beam is a thin annular beam, switched on and off by a mod-anode as opposed to a grid. The beam is then passed through a decelerating gap, and its kinetic energy is extracted using a tunable resonant circuit that presents a constant impedance in the range of 3-10MHz. With this design the beam is almost completely decelerated at all frequencies, thus achieving high efficiency.

Our research is motivated by the desire to build a transmitter for modification of the ionosphere that can operate in the mid-latitudes, be moveable, and have comparable effective radiated power to stationary, large land-based, fixed facilities such as HAARP 1. Our current design incorporates a grid-less electron gun in which the beam is turned on and off by a mod-anode. The three main electron gun models have the following key features: Model A has a uniform magnetic field (1kG) and no beam compression, Model B has a tapered guiding magnetic field (peak field of 1kG) and provides beam compression, and Model C has no guiding magnetic field. These simulations were performed with the beam optics code Michelle 2. The solenoid power consumption in the case of Model A is roughly 2kW, while for model B it is just under 5kW.

Reduced Ordered Binary Decision Diagrams (ROBDDs) have traditionally been built in a bottom-up fashion, through the recursive use of Bryant's apply procedure [4], or the ITE [2] procedure. With these methods, the intermediate peak memory utilization is often larger than the final ROBDD size. This peak memory requirement limits the complexity of the circuits which can be processed using ROBDDS. Recently it was shown in [9] that for a large number of applications, the peak memory requirement can be substantially reduced by a suitable combination of bottom-up (decomposition based) and topdown (composition based) approaches of building ROBDDs. This approach consists of selecting suitable decomposition points during the construction of the ROBDD using the apply procedure, followed by a symbolic composition to obtain the final ROBDD. In this paper, we focus on the composition process. We detail four heuristic algorithms for finding good composition orders, and compare their utility on a set of standard benchmark circuits. Our schemes offer a matrix of time-memory tradeoff points.

The resonant circuit in an Inductive Output Tube (IOT) extracts the kinetic energy of the modulated electron beam converting it to electromagnetic energy. For the application considered here, mobile low-frequency sources (5-10 MHz) for ionospheric heating, high efficiency is important, and thus class D operation is desired. The broad frequency range requires the circuit to be tunable, and the need for a constant decelerating voltage requires constant impedance. This paper discusses the design and construction of a resonant circuit with a highly coupled transformer that has the above features. The design and optimization of the circuit was performed with Orcad Spice and the transformer design was designed using Autodesk Inventor and a finite element electromagnetic field solver, Maxwell 3D and HFSS Simulator software.

In this paper, we address the issue of memory explosion in ROBDD based Boolean function manipu lation methods. To reduce the intermediate peak memory requirement, we select suitable decomposition points and follow it by a symbolic composition process. In cases where the final memory requirement itself is very large or where the intermediate explosion cannot be avoided by decomposition, we create ROBDDs representing orthogonal partitions of the function. Since these partitions are orthogonal they can be manipulated and verified independently. This results in a more efficient utilization of memory resources. We discuss two partitioning approaches, one in which the partitions are chosen apriori, and another in which this choice is made dynamically as the composition proceeds. We demonstrate the utility of our schemes on the ISCAS85 benchmark circuits as well as some industrial circuits. We are able to significantly reduce the memory requirements in most cases without paying a large time penalty. Additionally, we are often able to build the ROBDDs of circuits for which conventional methods fail. We experiment with many variable ordering schemes and get impressive memory reductions in all cases.

Development of Mobile Ionospheric Heating sources (MIHs) would allow investigators to conduct needed research at different latitudes without building permanent and costly installations. As part of an Air Force Multi-University Research Initiative (MURI), the University of Maryland is designing a prototype of a powerful Radio Frequency (RF) source utilizing Inductive Output Tube (IOT) technology operating in class-D with a mod-anode controlled electron gun [1]. This technology was chosen because it has the potential to operate at efficiencies exceeding 90% [2].

The ionosphere plays a controlling role in the performance of critical civilian and DoD systems including the ELF-ULF communications, radars, navigation (including GPS) and geo-location systems. Ionospheric Modification (IM) is a complementary approach to passively studying the ionosphere that has intensified over the last 30 years with the construction of the High-Frequency Active Auroral Research Program (HAARP). The objective of IM is to control and exploit triggered ionospheric and magnetospheric processes to improve the performance of trans-ionospheric Command, Control, Communications and Intelligence (C3I) systems and to develop new applications that take advantage of the ionosphere as an active plasma medium. A key instrument in IM is the Ionospheric Heater (IH), a powerful High Frequency transmitter that modifies the properties of the ionospheric plasma by modulating the electron temperature at preselected altitudes. A major reason for the development of a Mobile IH source (MIHs) is that it would allow investigators to conduct the needed research at different latitudes without building permanent installations. As part of a multiuniversity research initiative (MURI), UMD will develop a powerful RF source utilizing Inductive Output Tube (IOT) technology running in class-D amplifier mode. This technology was chosen because it has the potential to operate at high efficiencies. Some of the technical challenges presented in this paper will include: a gun design that minimizes intercepted current, a compact tunable hybrid cavity operating in the 1-10 MHz range, and an efficient modulator system capable of modulating a high power electron beam. MAGNETRON INJECTION GUN (MIG) IOT DESIGN A major concern with the gridded class-D operation of an IOT device is the heating of the grid due to intercepted electrons. A design using a MIG-type cathode that produces a hollow beam avoids this complication, as a small mod-anode local to the thin annular cathode can be used to bias the beam on and off without intercepting any electrons. We already possess a MIG-type cathode with a negative injection angle for this purpose. The proposed source with a Pierce-type geometry has been characterized with the Michelle code [1] with 2D axisymmetric geometry as shown in Fig. 1a. Steady state electrostatic PIC simulations show that for 60 kV on the anode and 200 V on the mod-anode, we can expect approximately 2 A of beam current from a 4.7 cm emitter. The magnetic field design was identified by using Maxwell 2D (axisymmetric geometry) code to simulate solenoid coils/pole piece geometries. Iterations over several geometries maximized the dot product of the field lines with the unmagnetized beam trajectory (to minimize conversion of longitudinal to transverse momentum). The most recent iteration is shown below in Fig. 1b. The field lines follow the unmagnetized beam trajectory closely, except near the cathode surface. An additional set of coils or iron field shapers behind the cathode may be used to adjust the field lines in this region. As seen in simulation (Fig. 1b), with a 1.4 kGauss field (shown in Fig. 1c), the beam ripple due to transverse energy gain is minimal. This field simulation assumes ideal iron. Figure 1: (a) Geometry of cathode, focusing electrode and mod-anode of a MIG-type gun and the particle trajectories. (b) Beam trajectory with field from a solenoid (c), with a peak on-axis field of 1.4 kGauss. Steady state Michelle simulations were used to estimate the capacitance seen by the grid driver due to the focus electrode-mod anode spacing in the vicinity of the cathode. Calculations were done with and without beam in a 2D axisymmetric Michelle simulation, and the nobeam case was verified against a Maxwell 3D model of the gun assembly. Michelle and Maxwell measurements agreed to within 2 %, with the most likely discrepancy being a difference in mesh density. Additional comparisons of Michelle simulations with and without beam, predicts a 1 % increase in capacitance due to beam loading. We predict that the capacitance due to the inner surface of the mod-anode is 15.6 pF, requiring the grid driver to pull 2 A for a 5 ns rise time on a 600 V swing. (a) (b)