RF STATUS OF SUPERCONDUCTING MODULE DEVELOPMENT SUITABLE FOR CW OPERATION: ELBE CRYOSTATS

Transcription

1 RF STATUS OF SUPERCONDUCTING MODULE DEVELOPMENT SUITABLE FOR CW OPERATION: ELBE CRYOSTATS J. Teichert, A. Büchner, H. Büttig, F. Gabriel, P. Michel, K. Möller, U. Lehnert, Ch. Schneider, J. Stephan, A. Winter Forschungszentrum Rossendorf P.O. Box , Dresden, Germany Corresponding author: Jochen Teichert Forschungszentrum Rossendorf P.O. Box Dresden, Germany Phone: FAX: ABSTRACT For the ELBE electron linear accelerator a superconducting accelerating module was developed and is now in routine operation. The cryostat contains two TESLA cavities (1.3 GHz) and is designed for continuous-wave (CW) operation with an accelerating gradient of 10 MV/m and a maximum average beam current of 1 ma. For the RF power two 10 kw klystrons are used. Special tuners, power couplers, low level RF control, cryogenic control systems and safety systems were developed. Engineering design, operation parameters and experience with the module are discussed. PACS: Bd, Cr

2 Keywords: Energy Recovery, Cryomodule, Niobium cavity, Radio frequency, Superconductivity, Linear accelerator 2

3 1. INTRODUCTION A 40 MeV superconducting electron linear accelerator (ELBE) operating in the continuous-wave (CW) mode [1] has been built at the Research Center Rossendorf (FZR). The LINAC is intended to produce electromagnetic radiation and secondary particles for various applications. At present, three beam lines are being used: gamma rays are produced by means of a bremsstrahlung target for nuclear photo excitation, x-rays are generated by electron channeling for radiation physics experiments, and a free electron laser produces infra-red radiation between 3 and 20 µm wave length. For the accelerator, a cryomodule was designed based on TESLA-type nine-cell cavities [2] and can be operated in CW mode. The module contains two cavities and is able to accelerate a 1 ma beam up to 20 MeV. The niobium cavities were produced by ACCEL and then treated and tested at DESY [3]. After commissioning in the year 2000, the accelerator was operated with a single cryomodule. During the shut down at the end of 2004 a second cryomodule was installed and put into operation. In the paper the design of the ELBE module will be presented and the operational experience with respect to a future use in energy recovery linacs will be discussed. 2. CRYOMODULE In Fig. 1 the ELBE cryomodule is shown. The two cavities are assembled in such a way that the main power couplers are in the center of the module. The tuners for each cavity are at both ends. The two tanks containing the superfluid helium are made of titanium. A titanium bellows at one side of the tank allows the cavity to be tuned. Each helium tank is connected via two chimneys with the two-phase helium supply tube above the cavities. The 80 K shielding uses liquid nitrogen. The cavities are passively shielded against ambient magnetic fields by means of a cylindrically shaped mu-metal sheet, closed at both sides, 3

4 which is placed between the 80 K liquid nitrogen shielding and the stainless steel vacuum vessel. The use of thin titanium spokes for the cavity support, thin stainless steel bellows in the beam lines and RF-couplers, as well as the multi-layer superinsulation foils ensures low thermal losses of the cryostat. Helium support lines connect the cryomodules via a distribution box with the helium refrigerator. It has a capacity of 220 W at 1.8 K. Usually the cryomodules are operated at 2 K (30 mbar), although 1.8 K is possible. The pressure control is performed by two cold compressors. In the commissioning phase two problems appeared: The helium pressure stability was unsatisfying (> 0.1 mbar) and an interference between pressure and level control was observed. The pressure stability was improved by increasing the helium mass flow in the cold compressors using an existing bypass in the helium plant. At the beginning, the helium level in the cryomodule was controlled with the Thomson valve in the helium supply line. Later, an electric heater was installed in the cryomodule. The second problem we solved by using the heater instead of the input valve for the helium level control. The installation of the second cryomodule did not cause new problems with respect to the helium system. The helium pressure stability is very good for both modules (< 0.1 mbar) although only the pressure value of the first module is included in the control loop. 3. TUNER The requirements of the tuner with respect to the applied forces, tuning range and accuracy are known from TESLA cavity studies at TTF [2]. But some of the demands as concerns the required space and tuning speed are less restrictive in our case. The CW operation with permanently applied RF power does not require fast Lorentz force compensation. Fig. 2 shows the ELBE module tuner design. The tuner is a spindle-lever system where the two levers act on the movable endplate of the cavity whereas the support is 4

5 attached to the helium vessel. In this design the levers have a transmission ratio of about 80. Specific features of the tuner are that the lever bearing points are flexible connects without any rotational parts and that the use of two levers ensures a force-free spindle bearing point. The stepper motors and the low-vibration gears are outside the vacuum vessel. 4. MAIN POWER COUPLER AND RF SYSTEM A coaxial main power coupler for CW operation was developed in collaboration with the HEPL Stanford University. The coupler is designed for 10 kw. It has a conical ceramic window at 80 K, the design of which was adopted from the DESY TTF3 coupler. The second window in the rectangular waveguide is made of REXOLITHE. It has turned out that the main power coupler is the most critical part of the accelerator module. In order to prevent damage, sophisticated diagnostics have been installed. Now the coupler windows are monitored with respect to temperature, vacuum pressure and electric discharges. If a certain limit of one of these parameters is reached, the RF power is switched off. The coupler has a fixed coupling factor. But, the coupling factor can be matched by means of a three-stub tuner placed in the waveguide. The coupling is matched for 1 ma beam current at 10 MV/m, which corresponds to a loaded Q of 1.2x10 7. Each cavity is supplied by a 10 kw klystron CPI VKL 7811 ST. This 1.3 GHz klystron has 4 MHz bandwidth and a gain of 37 db. An analog low-level RF control system with phase and amplitude loop was developed. The system allows adjustment of gradient and phase of each cavity separately. Additionally, the input and reflected power are measured for each cavity. The cavities are tuned during gradient ramp up by the operator. 5. OPERATIONAL EXPERIENCE 5

6 The first ELBE cryomodule has been operated mainly in two shifts per day for more than 4 years without important failures. Nevertheless, we have observed some effects which will be reported in the following. After fabrication and treatment the cavities were tested in the vertical test cryostat at DESY. The results for the six cavities tested were similar to those used at DESY in the TTF [3], i.e. the measured intrinsic quality factor was about 2x10 10 and the maximum acceleration gradient between 18 and 25 MV/m. For the installed cavities a direct RF measurement of the dissipated energy is impossible due to weak coupling. But by means of the power changes of the installed heaters the power input from the cavities into the helium bath can be estimated. The results are shown in Fig. 3. It is quite evident that for all cavities the dissipated power strongly increases for acceleration gradients at 8 to 10 MV/m. These values are much lower than the maximum gradients measured in the vertical tests before helium tank welding and cryostat assembling as mentioned above. At the same time we observed a strong increase of x-ray emission. Thus, we assume that the effect comes from field emission. The strong increase of the dissipated power causes the limit of helium plant with respect to its cooling capacity and pressure stability to be reached rapidly at these gradients. Another effect observed is an increase of the beam energy with time during the first hours of operation. This seems to be connected with the CW operation of the cavities. Some of the measurements are presented in Fig. 4. The energy drift was measured to be between 0.3 and 1 MeV depending on the accelerating gradients. The beam energy was measured with a dipole magnet. At the same time the RF input power was increasing as shown in Fig. 4b, whereas the pick-up signals from the cavities remained constant. Cross-checks show thatthe effect seems to arise from the cryomodule and not from the RF control system. 6. SUMMARY 6

7 The ELBE cryomodule is suitable for CW-operation at accelerating gradients in the range of 10 MV/m and with an average current of 1 ma. Most of the module parameters are equal to or better than its desired design specifications. The two modules at ELBE operate stably using common He pressure control with cold compressors and separate He level control (heater) in each module. Analog phase and amplitude RF control for each cavity and sophisticated coupler window diagnostics exist. The acceleration gradient is restricted by field emission in all cavities. The current of 1 ma (10 kw RF power) seems to be near to the limit of the fundamental power couplers. 7

8 FIGURE CAPTIONS 1. Three-dimensional drawing of the ELBE cryomodule. 2. Three-dimensional drawing of the ELBE tuner. 3. Measured cavity losses as function of accelerating gradient in module 1 (LA1) and module 2 (LA2). 4. Measured beam energy drift versus time in module 1 (a), and corresponding RF input power (b). 8

9 REFERENCES [1] F. Gabriel, et al., NIM B (2000) [2] B. Aune, et al., Phys. Rev. ST-AB 3 (2000) [3] S. Bauer, et. a., Proc. 9th Workshop on RF superconductivity, Santa Fe (1999). [4] L. Lilje, Proc. 11th Workshop on RF superconductivity, Travemuende (2003). 9

10 Figure 1 1

11 Figure 2 1

12 60 dissipated power / W LA1-C1 LA1-C2 LA2-C3 LA2-C gradient / MVm -1 Figure 3 1

13 8,2 8,1 Energy drift of cavity RC01 after switch on beam energy / MeV 8,0 7,9 7,8 7,7 7,6 7,5 7,4 7,3 RC11 off RC11 on (8 MV/m) time / min Figure 4a 1300 rf power (forward) / W energy drift after rf on cavity RC01 slope: 1.1 W/min time / min Figure 4b 1