Design of a cw, low-energy, high-power superconducting linac for environmental applications

The treatment of flue gases from power plants and municipal or industrial wastewater using electron beam irradiation technology has been successfully demonstrated in small-scale pilot plants. The beam energy requirement is rather modest, on the order of a few MeV; however, the adoption of the technology at an industrial scale requires the availability of high beam power, of the order of 1 MW, in a cost effective way. In this article we present the design of a compact superconducting accelerator capable of delivering a cw electron beam with a current of 1 A and an energy of 1 MeV. The main components are an rf-gridded thermionic gun and a conduction cooled $\beta =0.5$ elliptical ${\mathrm{Nb}}_3\mathrm{Sn}$ cavity with dual coaxial power couplers. An engineering and cost analysis shows that the proposed design would result in a processing cost competitive with alternative treatment methods.

Figure 1

Schematic layout of the 1 MeV, 1 MW, cw SRF electron accelerator for wastewater and flue gas treatment. The estimated overall length of the accelerator is 6 m.

Figure 2

Close-up views of the injector (a) and the cryomodule assembly (b).

Figure 3

On-axis longitudinal field profile as a function of distance from the cathode of the simulated electron gun.

Figure 4

Beam evolution through the accelerator: average kinetic energy (a), transverse rms bunch size (b), longitudinal rms bunch size (c), and rms energy spread (d).

Figure 5

Longitudinal phase space at 2.4 m from the cathode (a) and associated histogram (b). The transverse configuration space after the cavity is shown in (c).

Figure 6

Geometry of the $\beta =0.5$ SRF cavity. Dimensions are in centimeters.

Figure 7

Cavity dipole impedance spectra (solid lines denote wakefield calculations with beam traversing off axis with a radial offset $r$ either in horizontal or vertical direction; green dots denote complex Eigenmode computations, either polarization). The horizontal black line is the single-pass BBU impedance threshold impedance for a 1 A average beam current.

Figure 8

Cavity monopole impedance spectra (solid line denotes wakefield calculation with beam traversing along the axis; green dots denote complex Eigenmode computations, either mode polarization). Note that the accelerating mode is not resolved in the wakefield simulation. The external $Q$ is dominated by the FPC couplers.

Figure 9

Real impedance of the monopole modes (red line) and beam current spectral lines (green vertical lines) for an average current of 1 A. See text for detailed explanation.

Figure 10

3D model used for the thermal analysis. Each color represents a different material: light blue indicates ${\mathrm{Nb}}_3\mathrm{Sn}$, the dark blue indicates stainless steel, orange represents Ti45Nb, red is the copper first stage intercept, and fuchsia represents the copper plating on niobium. The nomenclature “Stage 1-3” means first stage of cryocooler number 3. The fourth cryocooler is symmetric to cryocooler number 2.

Figure 11

Temperature-dependent thermal conductivity of the materials used in the thermal analysis.

Figure 12

Temperature-dependent surface resistance of ${\mathrm{Nb}}_3\mathrm{Sn}$ used in the thermal analysis.

Figure 13

Cooling power as a function of temperature for the first and second stages of a commercial cryocooler (SRDK-415D, Sumitomo Cryogenics of America) [32].

Figure 14

Temperature distribution for the single cavity operating at a gradient of $5.6\mathrm{MV}/\mathrm{m}$ and with 600 kW rf power into each coaxial FPC.

Figure 15

Temperature distribution along the cavity surface.

Figure 16

Temperature dependence of the surface resistance of YBCO films scaled to 750 MHz from values measured at 8 GHz [33]. The solid symbol is the surface resistance of Cu at 90 K.

Figure 17

3D model of the cavity after electroforming a 6 mm thick Cu layer on the outer surface.

Figure 18

3D model of the components comprising the hermetic string to be assembled in the clean room.

Figure 19

MW-class, 748 MHz cw coaxial FPC manufactured by CPI (model VWP1185/1186) [37].

Figure 20

Hermetic string inserted into the vacuum vessel.

Figure 21

Cryomodule with cryocoolers, FPC top hats and inner magnetic shield installed.

Figure 22

Cryomodule after installation of thermal shield (a) and outer magnetic shield (b).

Figure 23

3D views of the fully assembled cryomodule.

Figure 24

MW-class, 700 MHz CW commercial klystron manufactured by CPI (model VKP-7952) [42].

Figure 25

Effective dose rate as a function of distance from the target computed with FLUKA for stainless steel and aluminum.

Figure 26

Interior layouts of the commercial facility with CPI klystron (a) and MBIOTs (b).

Figure 27

Simplified energy flow diagram.

Figure 28

Breakdown of the processing cost.

Figure 29

Processing cost dependence on rf driver efficiency (a) and annual operation time (b).