The proposed CFA design is a linear format with a meander microstrip line for the slow wave circuit. GFEAs in conjunction with hop funnels were proposed to implement the controllable distributed cathode. GFEAs provide a simple way to have a controllable
distributed cathode. Hop funnels provide the protection for the GFEAs from the high electric fields and current densities of the interaction region and provide a way to control the energies of the injected electrons separately from the sole potential. Two different configurations are proposed in this work: an injected beam and a distributed beam.
3.1.1 Injected Beam Configuration Experiment
The injected beam CFA pictorial schematic is shown in Fig. 3.1 along with the dimensions. The electron trajectory is shown as the red cycloidal line. The electrons are emitted from the GFEA and follow the cycloidal trajectory due to the crossed magnetic and electric field. The cycloidal trajectory in the figure is a pictorial representation and not representative of the actual trajectory. The electrons enter the interaction region, interact with the RF wave on the circuit, and collect either on the slow wave circuit or the end collector. The electric field in the interaction region is controlled by the potentials on the sole and slow wave circuit. The magnetic field is controlled by external Helmholtz configuration. An RF wave is input on the slow wave circuit on the left, and if the electron velocity is close to the phase velocity in the interaction region, the RF wave will be amplified at the RF output on the right. Note that the GFEA cathode is below the sole electrode in some parts; this is because the GFEAs available to the group were large (9.5×12.5 cm), and this was the best way to fit the cathode in the CFA chamber. The GFEA cathode and slow wave circuit are discussed in the next sections.
Figure 3.1: Schematic representation of the injected beam CFA design with dimensions, not to scale.
3.1.2 Meander Line
A meander line microstrip circuit is used as the slow wave circuit. The meander line circuit is used because of its ease of manufacture and ease of impedance matching. The practical use of meander lines is limited by the inability of the circuit to dissipate power and by dielectric charging. Because of the lower power operation of this CFA, the meander line is sufficient.
Figure 3.2 shows the geometry and dimensions of a generic meandering microstrip circuit. The exact dimensions and parameters of operation of the circuits used in this work are listed in Table 3.1. The circuits were designed to be at least 6 slow wave wavelengths long and to fit in the chamber available to our group. Two circuits were designed and used. The first circuit, SW1, experimentally demonstrated undesirable phase velocities, so a new design, SW2, was developed. Much of the results are redundant between the circuits, so the experimental focus is on the circuit called SW2.
Figure 3.2: The diagram showing the meander line microstrip. A metal line meanders over a dielectric with thickness Hd over a ground plane.
Table 3.1: Slow wave specifications
Name Period (P) [mm] Width (W) [mm] Line Width [mm] Line Height [mm] Dielectric Thickness [mm] Effective dielectric (r) Estimated Retardation (R) Operating Frequency [MHz] SW1 8 50 1.5 1.8 0.5 1.796 18.09 800-1000 SW2 7 74 1.2 1.8 0.33 1.815 29.83 400-600 3.1.3 Cathode
The proposed cathode for this CFA is a GFEA. The GFEA available to the group was a Spindt type gated field emitter array [23] obtained from PixTech Field Emission Displays fabricated in 2001 [108]. The cathode unit was 9.5×12.5 cm and the CFA configuration was designed around this constraint. The emission area is about 4 cm2, and the desired current was on the order of 100−200 mA. GFEAs at the time this CFA was designed (2011) had demonstrated current densities of 20 A/cm2, which would theoretically allow for80 Aof current from the emission area, but space charge
limits the current with the electric fields used in the CFA to currents on the order of
100−200 mA. At the time of this writing, current densities of 100 A/cm2 have been achieved by GFEAs developed by Guerra et. al. at MIT [25–27].
3.1.4 Distributed Cathode
The distributed cathode configuration includes the same meander line circuit and electron source as the injected beam configuration. The difference is the sole design. The distributed cathode CFA pictorial schematic is shown in Fig. 3.3. In this configuration, electrons are emitted up into the hop funnel, ’hop’ up the dielectric wall, and enter the interaction region. There are multiple injection points in this design, and electron current at each injection point can be controlled by the GFEA. The potentials between the sole electrode and the slow wave circuit control the electric field in the interaction region while the hop electrode controls the electron energy of the electrons.
Figure 3.3: Schematic representation of the distributed cathode CFA design, not to scale. Electrons injected into the hop funnels are extracted though slits in the sole electrode.
3.1.5 Sole/Hop Funnels
The hop funnels were fabricated out of Low Temperature Co-Fired Ceramic (LTCC) [109]. A schematic of the hop funnels/sole is included in Fig. 3.3. The LTCC spans the width and length of the interaction region. Two layers of metal are on the surface of the LTCC structure separated by a dielectric layer. The two metal layers are called the hop and sole electrodes. As explained in Chapter 2, the hop electrode is used to control the energy at which the electrons are born, and the sole electrode is biased more negative than the hop electrode in order to prevent cycloidal electrons from collecting on the sole.