Physical mechanisms of power limitation for coaxial gyrotrons :: SPE RESST

Physical mechanisms of power limitation for coaxial gyrotrons


Cavities in the form of a weakly inhomogeneous coaxial waveguide with a corrugated inner conductor are used in powerful continuous gyrotrons for thermonuclear fusion applications. Such gyrotron also find expanding applications in spectroscopy, plasma diagnostics and material processing.

Investigation of microwave power attenuation in a coaxial cavity with arbitrary-shaped corrugations of the inner conductor

	At present, the most promising applications of the coaxial gyrotrons are the NMR spectroscopy of biological molecules and the plasma diagnostics based on the collective Thompson scattering. The main technological obstacle to the high-efficient operation of coaxial gyrotrons is the microwave power attenuation in the inner conductor. The ohmic losses in the coaxial cavity can reach up to 80% of electron beam power. One of the most effective methods for reducing the microwave power attenuation in the conductor is to use surface corrugations of special shape. For this reason, an investigation of the effect of the corrugation shape on the attenuation and the transverse wavenumbers of cavity modes has been performed.

	We have considered the cases of rectangular and wedge-shaped corrugations with the various radiuses of edge rounding (see Fig. 1).

Figure 1 – Cross-section of a coaxial cavity with a corrugated inner conductor: (a) - wedge-shaped grooves (the groove sides are directed along the radial lines) with sharp edges; (b) - rectangular grooves with rounded edges

	For rectangular and radial grooves with rounded edges, the averaged density of the ohmic losses in the corrugated inner conductor coincides with the values obtained by the singular integral equation technique with an accuracy of about 5%. The transverse wavenumber of the operating mode depends only slightly on the edge rounding and the shape of corrugations, if the radius of such rounding is within the range between 0.001 mm to 0.08 mm. In this case, the rounding has no effect on the frequency and the quality factor of the operating mode and just modifies the local field distribution near the edges (see Figure 2).

Figure 2 – Distribution of the field component \| Hz\| inside and near the grooves of a rectangular shape: (a) r₀ = 0.01 mm, (b) r₀ = 0.025 mm, (с) r₀ = 0.1 mm

	The corrugation with the width increasing towards the bottom has been consdered for the central cross-section of the coaxial gyrotron cavity with the operating TE34,19 mode. It has been shown that such corrugations reduce attenuation in the inner conductor and this effect depends on corrugation geometry. As a result, the ohmic losses dissipated in the inner conductor can be of about 40% of the generated power.

Figure 3 – Groove with the width increasing towards the bottom

Figure 4 – Distribution of the field component \| Hz \| inside and near the groove, which expands with the angle γ: (а) γ = 1.35 rad, (b) γ = 1.46 rad, (c) γ = 90° (rectangular groove), (d) radial groove

	For grooves having width increasing towards the bottom, the transverse wavenumbers of the cavity modes change only slightly as compared to the case of rectangular grooves. This indicates that the selective properties of the coaxial gyrotron cavity are insensitive to the corrugation shape and makes it possible to optimize the corrugations with the aim of reducing the ohmic losses, while leaving competition between the operating and the parasitic modes unaltered.

Investigation of the plasma influence on the electromagnetic properties of coaxial gyrotron cavities

Background plasma appears in the cavities of powerful gyrotrons due to impact ionization of the background gas, which is initiated by an electron beam. This is confirmed by the experimental observations of the beam-space-charge neutralization. Background plasma can change the electromagnetic properties of the gyrotron cavity. The modification of cavity eigenfrequencies and eigenfields can, in its turn, affect the beam-wave interaction and thereby change the gyrotron efficiency. Therefore, for improving and optimizing gyrotron performances, it is necessary to take into account the presence of background plasma in the gyrotron cavity.

	A novel theory describing the effect of background plasma on the electromagnetic properties of gyrotron cavity has been developed. The theory remains valid for arbitrary plasma density.

	It has been shown that the resulting cold-cavity equations have the same form as conventional ones and can be incorporated into both the linear and the nonlinear theories of plasma-filled gyrotron. Besides, they can be also extended to the case of space-charge waves propagated inside vacuum gyrotron cavity.

	A dispersion equation describing the effect of background plasma on the resonance frequencies has been obtained for the coaxial gyrotron cavity with a smooth insert.

	It has been shown that the plasma reduces the operating frequency of coaxial gyrotron (see Fig. 5).

Figure 5 – Normalized critical frequency versus plasma density

	An analytical expression of the wave power per unit length has been obtained and studied numerically for the coaxial gyrotron cavity filled with magnetoactive plasma.

	The effect of plasma on thermal loading of the cavity walls has been investigated. It has been shown that such loading decreases with increasing plasma density inside the cavity. As a result, the probability of RF breakdown also reduces in the process.