Parasitic Oscillations in Smooth-Wall Circular Symmetric Gyrotron Beam Ducts

Abstract

In order to study parasitic oscillation that may occur in a realistic beam duct upstream to the gyrotron cavity, the self-consistent linear and spectral code TWANGlinspec has been modified. The large inhomogeneities in the smooth-wall beam duct geometry or in the magnetic field profile required the implementation of a numerical approach using a hybrid finite element method. The new model permits to characterize a large number of potentially spurious TE modes. Compared to previous studies on gyrotron beam duct instabilities, an extended interaction space including also the gyrotron cavity has been considered. The role of the connecting part between the beam duct and the cavity, called spacer, is highlighted and it is shown that the gyro backward-wave TE modes excited in this region generally have their minimum starting current. The sensitivity of the minimum starting current on electron beam velocity spread is also evaluated.

Experimental Classification and Enhanced Suppression of Parasitic Oscillations in Gyrotron Beam Tunnels

“…In addition, the theoretical results of [13] have been confirmed, since the optimized low-permittivity, low-loss-tangent ceramic material led to higher starting currents of the parasitic signals, which for some operating points have been increased by about 40% giving a parasitic free operation with beam currents up to 60 A and higher. Finally, for the parasitic modes that were not affected by the optimized ceramics, it was shown, by extending the stacked ceramic-loaded section that they originate from an interaction at the area of the smooth metallic spacer confirming the findings of [22]. The final configuration led to an increase of the parasitic-modes starting currents for all operating points by more than 10%.…”

“…Considering that the cyclotron frequency at the border between the beam tunnel and the spacer is around 150 GHz, it is reasonable to assume that the 151 GHz -159 GHz signals are excited mostly in the spacer area. The strong possibility of parasitic excitation in the spacer area has been shown by means of numerical simulations in [22]. It should be noted that for the operating points (-1º, 9.40 mm) and (-1º, 9.35 mm) both types of parasitic signals are noted in Fig.…”

Overview of recent gyrotron R&D towards DEMO within EUROfusion Work Package Heating and Current Drive

“…Preliminary calculations show significant increase in the starting current of resonant parasitic modes, if a metallic smooth-wall beam duct is coated with BeOSiC or SiC. In parallel to those investigations, the possibility of localized parasitic oscillations, emerging by gyro-backward-wave interaction in the metallic spacer used to connect the beam tunnel to the gyrotron cavity, has been identified [59]. Following this finding, studies on possible guidelines for the spacer design have been initiated.…”

Linear studies of spurious backward-wave instabilities in a dielectric-loaded smooth-wall gyrotron beam ducts

“…It has been derived from the non-linear code TWANG 23 , following a moment approach and using a spectral formulation 24 . Recently, the model was adapted to study the interaction in an extended interaction region including the beam duct, the spacer and the cavity 25 .…”

“…As it was the case for the metallic beam duct studies 25 , the strategy was to start from a simple symmetric situation of a cylindrical beam duct with a downtaper and an uptaper, as shown in Figure 6. From this situation, the metallic wall was progressively adjusted to simulate a smooth-wall beam duct with a realistic profile, as shown with the dashed line in Figure 6.…”