Development of a high average current rf linac thermionic injector

Thermionic electron guns are capable of operating at high average currents in a variety of vacuum electronic applications, including conventional microwave tubes, but have been replaced by laser photocathode injectors for most applications requiring high-brightness electron beams. However, while laser photocathode guns are capable of providing the very high-brightness beams, they provide an increased level of system complexity and do not extrapolate well to injectors for high average current applications requiring high beam quality. We are developing a 714 MHz injector based on a gridded thermionic electron gun for these applications. This paper presents an experimental study, computer simulations, and analysis of the performance of an existing gridded thermionic electron gun as an injector prototype, and a design concept for an improved injector configuration based on these results.

Figure 1

Schematic diagram of the gridded thermionic electron gun.

Figure 2

Schematic diagram of the experimental setup for bunch length measurements.

Figure 3

Measured beam current versus $V_{\mathrm{grid}}$ for $V_{\mathrm{cathode}}=-31\mathrm{kV}$. The solid line is from a MICHELLE simulation of the electron gun, with emission only from the face of the cathode. The simulation model takes into consideration temperature-limited emission from the cathode at 1095 K. The dashed line includes emission from both the face of the cathode and the edge surface of the central hole through the cathode.

Figure 4

Measured Faraday cup current ($I_{\mathrm{FC}}$) for single-frequency 714 MHz (left) and 2142 MHz (right) grid drive. For the left-hand data set, the parameters are $V_{\mathrm{cathode}}=-24\mathrm{kV}$, $V_{\mathrm{grid}}=-90\mathrm{V}$, and $\sim 100\mathrm{W}$ at 714 MHz producing a 408 ps FWHM ($\sim 170\mathrm{ps}$ rms) micropulse with peak current $\sim 2.2\mathrm{A}$ and 0.9 nC per bunch. For the right-hand data set, the parameters are $V_{\mathrm{cathode}}=-24\mathrm{kV}$, $V_{\mathrm{grid}}=-63\mathrm{V}$, and $\sim 300\mathrm{W}$ at 2142 MHz, producing a peak current of $\sim 110\mathrm{mA}$. The colored dot on the left axis associated with each waveform shows the associated baseline of the trace. Also shown are the incident and reflected rf waveforms, ${\mathrm{rf}}_{\mathrm{in}}$ and ${\mathrm{rf}}_{\mathrm{out}}$.

Figure 5

Measured Faraday cup current, $I_{\mathrm{FC}}$, for combined first and third harmonic modulation of the electron gun. The parameters are $V_{\mathrm{cathode}}=-36\mathrm{kV}$, $V_{\mathrm{grid}}=-90\mathrm{V}$, with $\sim 540\mathrm{W}$ at 714 MHz and $\sim 450\mathrm{W}$ at 2142 MHz. The microbunches are 325 ps FWHM (135 ps rms), with 2.81 A peak current and $\sim 0.91\mathrm{nC}$ per bunch. Also shown are ${\mathrm{rf}}_{\mathrm{in}}$ and ${\mathrm{rf}}_{\mathrm{out}}$.

Figure 6

MICHELLE simulation of the bunched beam for operation corresponding to Fig. 5. (top) Emitted current from the cathode; (center) current reaching the Faraday cup; (bottom) comparison of simulation and experimental data from Fig. 5.

Figure 7

Faraday cup data for combined first and third harmonic modulation of the electron gun and higher negative grid bias for $V_{\mathrm{cathode}}=-36\mathrm{kV}$, $\sim 200\mathrm{W}$ at 714 MHz and $\sim 450\mathrm{W}$ at 2142 MHz. The left traces correspond to $V_{\mathrm{grid}}=-100\mathrm{V}$; the right traces correspond to $V_{\mathrm{grid}}=-125\mathrm{V}$.

Figure 8

The rms bunch length and first harmonic power versus third harmonic power at 0.6 A peak microbunch current.

Figure 9

Peak current and microbunch length versus power at 714 MHz.

Figure 10

Schematic of experimental setup for emittance measurements (not to scale). The vacuum enclosure is not shown.

Figure 11

Simulation of the emission measurements at $V_{\mathrm{cathode}}=-32\mathrm{kV}$ and effect of the emission from the central hole. Electron trajectories for: (top) $I_{\mathrm{lens}}=1.65\mathrm{A}$ and (center) $I_{\mathrm{lens}}=0\mathrm{A}$ include emission from the central hole; (bottom) $I_{\mathrm{lens}}=0\mathrm{A}$ is with emission from the central hole suppressed.

Figure 12

Simulation of the emission measurements at $V_{\mathrm{cathode}}=-32\mathrm{kV}$. Normalized rms transverse emittance (top) and bunch charge (bottom) as a function of axial position for $I_{\mathrm{lens}}=1.65\mathrm{A}$ and $I_{\mathrm{lens}}=0\mathrm{A}$. Particles begin hitting the wall at the point labeled 1 and the magnetic field begins to affect the emittance at the point labeled 2. The normalized magnetic field profile of the lens is also shown.

Figure 13

Comparison of electron bunches at the gun exit with emission from the central hole (top) and without emission from the central hole (bottom).

Figure 14

Measured emittance versus solenoid current for $V_{\mathrm{cathode}}=-32\mathrm{kV}$, $V_{\mathrm{grid}}=-90\mathrm{V}$, and $\sim 250\mathrm{W}$ of rf drive at 714 MHz.

Figure 15

Image of YAG plate at $I_{\mathrm{lens}}=1.15\mathrm{A}$ with a $100\mathrm{\text{−}}\mu \mathrm{m}$ Ta slit on the beam axis, showing the circular beam pattern resulting from emission from the central hole in the cathode. The faint horizontal line is produced by the residual electrons emitted from the entire cathode surface and passing through the slit.

Figure 16

(Left) Cut-away view of the schematic of a 700 MHz rf-gated thermionic electron gun integrated into a cylindrical rf accelerating cavity. The power port is not shown. (Right) Close-up view of the central region of the cavity, showing the cathode and the holeless grid (inset), and the transit of a electron bunch.

Figure 17

Beginning stages of bunch formation.