# Bandwidth Enhancement of a V-Band Klystron with Stagger-Tuned Multiple Radial Re-Entrant Cavities
## 3 Results and Analysis  
The RF structure of the V-band Klystron shown in Figure 2 has been modeled and simulated using a CST PIC solver. The PIC simulation has been run for 100 ns with a 0.5 W input RF signal. Using the PIC solver, different parameters of the RF section, like the number of cavities and distance between cavities, have been optimized, and finally, a 27.14 dB gain has been achieved by using 11 numbers of cavities, including the input and output cavities. A combined view of port signals of the beam-wave interaction is shown in Figure 5. Table 3 presents the design parameters of the RF section for the V-band Klystron and the PIC simulation results. Beam-wave interaction has been performed for different frequencies, and we have obtained different gains for different frequencies. For the estimation of bandwidth, the gain of the RF section has been plotted for different frequencies  
energies. The structure shown in Figure 2 has been designed and simulated using a PIC solver for 100 ns, and the output extracted from the output cavity is shown in Figure 9.  
To increase the operating bandwidth, tuning has been started from the last intermediate cavity (IC9) by changing frequency. The frequency tuning has been achieved by changing the transverse dimensions of the particular cavity under tuning. For determining the bandwidth, PIC simulation has been performed by tuning the last intermediate cavity  
Figure 8: The electron trajectory along the drift tube shows bunching of the same near the output cavity section. Figure 7: Phase space plot of the particles plotted in z-direction. Figure 9: The input signal at the input port and amplified RF signal at the RF output port (stagger tuning). (IC9) from 59.7 GHz to 60.5 GHz with a step range of 100 MHz. The frequencies of other ICs (from IC9 to IC2 and input-output cavities for this instance) were fine-tuned to the earlier configuration, where maximum gain and stability were achieved. Figure 10a shows the bandwidth estimation plot with the optimized frequency of IC9. From Figure 10a, it can be observed that the \(-3\) dB bandwidth is 340 MHz. This tuning process has been performed for all other intermediate cavities (IC). Tuning has been performed for ICS, IC7, IC6, IC5, IC4, IC3, IC2, and IC1, respectively. For each and every tuning process, PIC simulation has been performed from 59.7 GHz to 60.5 GHz with a step range of 100 MHz. Frequency vs. gain has been plotted for bandwidth estimation after tuning each of the intermediate cavities. The details of cavity parameters and tuning frequency of all the cavities of the optimized device with a bandwidth of 540 MHz are mentioned in Table 2.  
Figure 10b shows the bandwidth estimation plot after tuning IC8, and it can be observed that the \(-3\) dB bandwidth is 334 MHz. From Figure 10c, the estimated \(-3\) dB bandwidth, after tuning IC7, comes out to be 348 MHz. Similarly, Figure 10d-h show the bandwidth plots of the device after tuning the cavities from IC6 to IC2, which are 347 MHz, 351 MHz, 381 MHz, 372 MHz, and 376 MHz, respectively. Figure 11b shows the bandwidth estimation plot after tuning IC1, and it can be observed that the \(-3\) dB bandwidth is 540 MHz, and Figure 11a shows that the corresponding \(-1\) dB bandwidth is 272 MHz. A moderate bandwidth of 540 MHz has been achieved after tuning the first intermediate cavity, and 260% bandwidth enhancement has been achieved using the stagger tuning process. The Fourier transform (FT) of the output RF signal is shown in Figure 12. The maximum amplitude of the output signal is achieved at the 60.1 GHz operating frequency.