Successful Production of KEKB-type Superconducting Radio Frequency Modules for Taiwan Photon Source

A new 3-GeV, 500-mA, synchrotron light source named Taiwan Photon Source (TPS) at National Synchrotron Radiation Research Centre (NSRRC) is now entering its final stage of construction with partial occupancy of accelerator components. The commissioning of the new concentric machine is scheduled for 2014. The beam power consumption for the TPS storage ring will exceed 600 kW at its design beam current 500 mA after the machine is fully occupied with the insertion devices. How to operate the SRF modules highly reliably on consumption of such huge RF power but with an optimal amount of total RF gap voltage from 2.8 MV to 3.5 MV is obviously the most critical challenge in a successful construction and operation of the RF systems for TPS. The 500-MHz SRF module of KEKB type has been selected for TPS because of its highly reliable operational record at a RF power rating up to 350 kW at KEKB and the availability of an external quality factor (Q_ext) down to 7x10⁴ satisfying the SRF operation for TPS. Following an agreement of technology transfer from KEK to NSRRC at the end of 2009, a commercial contract was awarded to Mitsubishi Heavy Industries Ltd. (MHI) in 2010 June for the mechanical fabrication of 500-MHz SRF modules of KEKB type in three (3) sets for TPS. MHI had delivered SRF modules of several types to KEK but for the first time were manufacturing the 500-MHz SRF modules of KEKB type for routine operation.

The four individual parts of the niobium cavity, i.e., small beam pipe (SBP), two half-cavity cells, and large beam pipe (LBP), were made of pure niobium with RRR of about 300 from Tokyo Denkai and manufactured by deep drawing, with welding together with an electron beam at MHI. The standard procedure for surface treatment developed for the 508-MHz niobium cavity of KEKB type was applied to the niobium cavities after their receipt from MHI, undertaken by the SC group of KEK with joint manpower from MHI and NSRRC. The surface treatment includes the heavy electropolishing (EP1, 100μ), vacuum annealing (700oC, 1.5 h) by MTC, frequency tuning, light electropolishing (EP2, 20μ), ozonated pure water (3 ppm) rinsing, ultrasonic rinsing with hot pure water (50oC), and baking (90-120oC, 24 h). Even though without application of high-pressure rinsing to the niobium cavity, the vertical test proceeded smoothly without requirement of an extra pulse or helium processing, but with ramping up the accelerating voltage directly to 3 MV without difficulty. In parallel to the completion of RF processing of high-power input couplers (up to 300 kW, CW) and LBP/SBP HOM dampers (up to 7 kW and 5 kW, CW, respectively) in KEKB’s test stands, the liquid-helium test of the MHI-assembled cryo-modules were undertaken at KEK’s horizontal test area to verify the vacuum tightness through full thermal cycling. Finally, the high-power input couplers were dismantled for delivery of the cryo-modules to NSRRC.

The cryo-modules and associated end groups of room-temperature vacuum beamline components were then delivered to NSRRC for final assembly, system integration with the cryogenic regulation system, SRF electronics and diagnostic system, and RF plant, high-power RF conditioning of the coupler (up to 300 kW, CW), high RF power performance (horizontal) test (up to 2.4 MV or 300 kW, CW), and the long-term reliability test at 1.6 MV. Figure 1 snapped the moment of completion of cryo-module assembly for one of three SRF modules by MHI at KEK. The results of the vertical tests and horizontal tests of three 500-MHz niobium cavities and SRF modules of KEKB type for TPS are shown in Fig. 2.

<Fig1> Completion of cryo-module assembly of #1 SRF module by MHI at KEK.

<Fig2> Results of vertical test (V/T) and horizontal test (H/T) of the 500-MHz niobium cavities and SRF modules of KEKB type for Taiwan Photon Source (TPS). The #1 Nb cavity experienced two vertical tests for extra frequency tuning. The target RF performance is Q0 of 1E9 and 5E8 at 1.6 MV and 2.4 MV, respectively.

The gas load will be a critical challenge for the SRF operation with a target to operate with higher RF power, like the SRF modules for TPS. Hydrogen, water molecule and carbon monoxide are the dominant residual gases in measured cavity vacuum of a fresh SRF module. The water molecules appear to be the most problematic for reliable SRF operation. During initial years of the SRF module for machine operation, the absorbed water molecules are released and may bring heavy gas load to the cold surface of the SRF module, eventually resulting in multipacting on the high-power input coupler, extra dynamic cryogenic heat loss and radiation dose, vacuum burst from the niobium cavity, etc. We propose to install the SRF modules into the storage ring of TPS after the machine is commissioned and vacuum-cleaned. The 5-cell copper (Petra) cavities will be used for the machine commissioning up to 100 mA of storage-ring current as originally planed for vacuum cleaning. We expect that our measures will minimize the impacts of heavy gas loads on the reliability and availability of SRF operation after routine operation.

Following the TPS progressing into mature operation a few years later, beam power more than 250 kW will be delivered to the electron beam from each SRF module. The high-power input coupler for the 508-MHz SRF module of KEKB type had been RF-conditioned up to 500 kW in the test stand at KEK. It is promising for a single SRF module of KEKB type to deliver a beam power up to 400 kW, or even more. Combining the available RF power from the existing 300-kW klystron-based RF transmitter with a 180-kW solid-state RF transmitter for each SRF module provides an economical solution for TPS in its mature operational stage. If the SRF operational reliability becomes an issue at such a high rating of RF power, the #3 SRF module will be installed into the storage-ring complex, which is now a stand-alone spare module. In parallel, we contracted MHI to produce a spare niobium cavity including thermal-transition beam pipes ready for insertion into the cryostat for cryo-module assembly. A complex of this kind will decrease the duration of repair from a failure of a SRF module when a stand-alone spare SRF module becomes unavailable.

〜 Author : 王兆恩 Chaoen Wang - Radio Frequency Group, NSRRC 〜

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