Estimation of Cryogenic Heat Loads in Cryomodule due to Thermal Radiation
Cryogenic system is one of major cost drivers in high intensity superconducting (SC) continuous wave (CW) accelerators. Thermal radiations coming through the warm-ends of cryomodule and room temperature parts of the power coupler result in additional cryogenic heat loads. Excessive heat load in 2K environment may degrade overall performance of the cavity. In this paper we present studies performed to estimate additional heat load at 2K due to thermal radiation in 650 MHz cavity cryomodule in high energy section of PIP-II SC linac.
RF LOSSES IN 1 . 3 GHz CRYOMODULE OF THE LCLS-II SUPERCONDUCTING CW LINAC *
The Linac Coherent Light Source (LCLS) is an x-ray free electron laser facility. The proposed upgrade of the LCLS facility is based on construction of a new 4 GeV superconducting (SC) linac that will operate in continuous wave (CW) mode. The major infrastructure investments and the operating cost of a SC CW linac are outlined by its cryogenic requirements. Thus, a detail understanding of RF losses in the cryogenic environment is critical for the entire project. In this paper we review RF losses in a 1.3 GHz accelerating cryomodule of the LCLS-II linac. RF losses due to various sources such untrapped higher order modes (HOMs), resonant losses etc. are addressed and presented here. INTRODUCTION The Linac Coherent Light Source -II (LCLS–II) [1] is a proposed fourth generation x-ray light source facility under construction at SLAC, California. The LCLS-II is primarily based on construction of a new 4 GeV SC RF linear accelerator (linac) that would operate in the CW regime (see Fig. 1). Figure 1: Layout of the LCLS-II SC linac. The LCLS-II SC linac is segment into several sections which are named as L0, L1, HL, L2, L3 and Lf. All the sections except L1 and HL are separated to each other by intermediate warm sections which are designed for the specific purposes such as laser heating, diagnostic and bunch compressions. Excluding HL, all sections are composed of 9-cell 1.3 GHz SC TESLA like cavities [2]. HL section consists of 9-cell, 3.9 GHz SC cavities [3]. Number of elements and their nominal operational RF parameters in each section are summarized in Table 1 and a detailed description of the LCLS-II SC linac is presented elsewhere [4]. The cryogenic efficiency is significantly lower at operating temperature of 2 K [5]. Thus, operation of the SC linac in CW mode keeps a stringent tolerance on the cryogenic load of the machine. As, one can observe from Table 1 that major portion of the LCLS-II SC linac is comprised of the 1.3 GHz cryomodules and therefore, cryogenic budget of the facility is largely outlined by the cryogenic loads in a 1.3 GHz cryomodule. In this paper, we discuss and present contribution of different sources that result in the dynamic RF losses at the operating temperature of 2 K in a 1.3 GHz cryomodule in the LCLS-II SC linac. Table 1: Configuration of Each Section in LCLS-II Linac Phase (deg) Gradient (MV/m) No. of CM’s Avail. cavities L0 ~0 16.3 1 8 L1 -12.7 13.6 2 16 HL -150 12.5 2 16 L2 -21 15.5 12 96 L3 0 15.7 18 144 Lf +/-34 15.7 2 18 1.3 GHZ CRYOMDOULE Figure 2 shows a schematic of 1.3 GHz cryomodule. It accommodates eight cavities, a magnet package that includes a quadrupole magnet, a beam position monitor (BPM) and a dipole corrector and, a beam line absorber at the end. Figure 2: 2D view of a 1.3 GHz cryomodule for the LCLSII SC linac. Operating Mode RF Loss in Cavity The major contribution to the dynamic RF losses in a cryomodule comes from dissipation of the operating mode fields on surface of the SC cavity. Power dissipated (Pd) on surface of a SC cavity is estimated as:
Results of the APT RF power coupler development for superconducting linacs
For the new baseline APT (Accelerator Production of Tritium) linac design, the power couplers are required to transmit 420 kW of CW RF power to the superconducting cavities at 700 MHz. These couplers consist of an airside waveguide-to-coax transition, an air/vacuum break made by two planar, coaxial windows, and a vacuum-side coaxial antenna section. The coaxial antenna allows adjustability of the RF matching to the superconducting cavities. Design, fabrication, and testing of the power coupler/window occurred over the last four years, and room temperature testing of the prototype design is complete. Coupler/window assemblies have transmitted power to 1 MW, CW and have handled full reflected 850 kW, CW over a limited standing-wave phase range. Couplers were tested with a portion of the outer conductor cooled by liquid nitrogen to study the effects of condensed gases. Final results, conclusions, and lessons learned about the coupler design, fabrication, and testing are discussed.