Novel High Power Sources for the Physics of Ionospheric Modification*

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)