Multipactor discharge simulations

These results show that the gun back wall, the cathode plug, and the coaxial antenna nose undergo interaction with high energy electrons (up to 6.4 MeV). Therefore these places most prone to damage in case of high dark current. This demands high accuracy in design, fabrication, installation, and manipulation with these parts.

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Figure 4.12: Multipacting simulations for the cathode vicinity with watchband RF contact spring design . The number of particles in the gun is presented as a function of time for the solenoid currents of 370 A (part a) and 200 A (part b), respectively.

The multipacting simulations of the cathode vicinity with the watchband contact design indicated the gradual decrease of the number of electrons over time at all investigated gradients and for both external magnetic field configurations (Fig. 4.12).

The simulations also showed some growth in the number of particles at some RF field levels for a short time period, and this behavior implicate that resonant conditions for secondary emission can appear, but it does not lead to multipacting because there is no synchronization between the field and particle motion.

The investigation of particle trajectories showed that the major part of the secondary electrons is concentrated in the gap between the cathode and the cathode vicinity at the place close to the cavity area. Some of these secondaries can be captured by the accelerating fields and transported downstream the cathode, where they are considered as dark current electrons. An example of this behavior is presented in Fig. 4.13. The example shows the simulations for the 60 MV/m accelerating gradient at the cathode and the external magnetic field corresponding to the main solenoid current of 370 A.

The dependence of the particle count vs. time (see Fig. 4.13a) shows that no conditions for multipacting are visible.

Multipacting simulations for the gun cavity and the gun coupler were performed in a way similar to the simulations of the cathode vicinity with watchband spring design but with the difference that the surfaces of the gun cavity and RF coupler were used as the source of the initial particles.

The simulations revealed that multipacting trajectories can appear at an accelerat-ing gradient on the cathode surface of 60 MV/m (∼6.5 MW power in the gun). The stable multipacting trajectories are located inside the coaxial waveguide (between walls of outer conductor) and at the outer cylinder of the gun full cell. Examples of the trajectories and number of particles versus time are shown in Figs. 4.14 and 4.15 for the coaxial waveguide and gun full cell, respectively.

The simulations also revealed places where stable (but not multipacting) trajectories are observed for an accelerating gradient at the cathode of 60 MV/m (∼6.5 MW power in the gun). The locations of the stable trajectories are the cone and the its surrounding parts of the gun coupler (see Fig. 4.16), the inner conductor of the coaxial waveguide (see Fig. 4.17) and the gun cavity (outer cylinders of the cells ) (see Fig. 4.18). A typical plot of the number of particles versus time for this kind of trajectories is presented in Fig. 4.19.

Multipacting trajectories were found at an accelerating gradient at the cathode of 1 MV/m (∼2 kW power in the gun) that corresponds to the residual high voltage level of the RF modulator present in the gun when the RF pulse is not yet present or is already stopped. In other words, a power level of ∼2 kW, which corresponds to an accelerating gradient at the cathode of 1 MV/m, can be associated with the baseline of the RF signal. The schematic presentation of this power level is shown in Fig. 4.20.

The trajectory locations are the back wall of the first cell (see Fig. 4.21), the space between the inner conductor of the coaxial coupler and the back wall of the rectangular

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Figure 4.13: Multipacting simulations of the cathode vicinity with watchband RF contact spring design. The number of particles in the gun as a function of time for 60 MV/m at the cathode (part a) and secondary electron trajectories in the cathode area (part b).

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Figure 4.14: Number of particles in the gun as a function of time for 60 MV/m and 370 A current of the main solenoid (part a) and secondary electron trajectories in the coaxial coupler (part b).

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Figure 4.15: Number of particles in the gun as a function of time for 60 MV/m and 370 A current of the main solenoid (part a) and secondary electron trajectories in the gun cavity (part b).

Figure 4.16: Stable multipacting trajectories at the cone part and cone surrounding part of the gun coupler.

Figure 4.17: Stable multipacting trajectories at the outer side of the inner conductor of the coaxial waveguide.

Figure 4.18: Stable multipacting trajectories in the gun cavity.

Figure 4.19: Number of particles in the gun as a function of time.

waveguide (vacuum pump port location)(see Fig. 4.22) and the surface of the second gun iris (to coaxial coupler) (see Fig. 4.23).

Figure 4.20: The schematic presentation of RF power level.

Figure 4.21: Multipacting trajectories at the back wall of the gun half cell.

Multipacting simulations for the cathode vicinity with the contact stripe design, which was applied at the gun prototypes 4.2, 4.3 and 4.4, were performed by means of the PIC solver, keeping settings (like particle source parameters) from the simulations of the cathode vicinity with the watch band spring design.

The simulations of the cathode vicinity with contact stripes design did not reveal any gun field conditions when multipacting appears inside the cathode vicinity. The results

Figure 4.22: Multipacting trajectories in the RF coupler.

Figure 4.23: Multipacting trajectories at the iris between the coaxial coupler and the full cell.

Figure 4.24: Particle distributions in the free space volume between cathode plug and cathode vicinity.

are similar to the simulations of the cathode vicinity with watch band spring design.

An example of the particle distribution in free space volume between the cathode plug and the cathode vicinity is presented in Fig. 4.24.

The studies of the dark current and multipactor discharge by the simulations in the CST Particle studio showed that the high dark current can be especially dangerous for the coaxial antenna nose, the gun back wall, and the cathode plug. These areas undergo interactions with electrons of energies up to 6.4 MeV. Simultaneously, the multipactor discharge was not found in the cathode region, but it was observed at the gun cavity walls and the coaxial coupler. The found multipacting trajectories can also contribute to the dark current.

The results show that the dark current must be minimized in order to prevent the sensitive cathode surface and the cathode vicinity with the RF contact spring from damage.