Preparations of QWR superconducting cavities for beam commissioning

Quarter-wave resonator (QWR) cavities are prepared for beam commissioning. RF conditioning is performed for each QWR cavity. The total heat load, including static and dynamic heat loads, is measured for each cavity. The helium pressure fluctuation is reduced by changing the flow rate, supply pressure, return pressure, cryogenic valve control, etc. The cavity pressure is monitored during RF preparation. The amplitude and phase of the QWR cavity are stably controlled for beam commissioning.


Introduction
A superconducting cavity is widely used to accelerate electrons and heavy ions in the accelerator.Electron emission and generalized field emission were investigated [1][2][3].The performance of the superconducting cavities was measured and studied with the field emission effect [4,5].The performance of the cryomodule was measured with a horizontal test [6][7][8].The quarter-wave resonator (QWR) and half-wave resonator (HWR) cryomodules are installed in the superconducting linear accelerator 3 (SCL3) site.The RAON SCL3 accelerator is constructed and under beam commissioning.In this research, we show the piping and instrument diagram (P&ID) for quarter-wave resonator (QWR) cryomodules, the heat load measurement of the QWR cavity and the vacuum pressure of the cavity during RF preparations.The helium pressure fluctuation can be reduced in many ways.The amplitude and phase for the QWR cavities are stably controlled for beam commissioning.

Experiment
The performance of QWR superconducting cavities, which includes quality factor, resonance frequency, pressure sensitivity, Lorentz force detuning (LFD), and x-ray emission, is measured through a vertical test.The total number of the quarter-wave resonator (QWR) cavities is 22, and all of them are passed through the vertical test [9].The required quality factor for the cavities is 2.3x10 8 at 6.1 MV/m.The Q slope for the QWR cavities decreases as the peak magnetic field is increased, which shows the magnetic heating effect [9].
The cryomodules, which include a superconducting cavity, are installed in RAON superconducting linear accelerator 3 (SCL3) site to accelerate heavy-ion beams.The cryomodules and warm section magnets are installed and aligned in the tunnel.The superconducting cavities are pumped with a dry pump and a turbomolecular pump (TMP).Once the vacuum pressure of the superconducting cavity is low and leak-tight, the ion pump and non-evaporable getter (NEG) pump are used to make the cavity vacuum.
Figure 1 shows the piping and instrument diagram (P&ID) for the QWR cryomodules.The P&ID shows the supercritical helium supply, gas helium return, thermal shield helium supply, thermal shield helium return, helium pressure, liquid helium level in the reservoir, cavity vacuum pressure, etc.The helium pressure, vessel pressure and cavity vacuum pressure are monitored for each cavity.The cryogenic valve control and the liquid helium level in the reservoir are monitored as well.
The static heat load decreases with time because it goes to thermal equilibrium.The thermal shields of the cryomodules are cooled down first.The two QWR cryomodules are cooled down by sharing the valve box to supply the liquid helium and return the gas helium.The total number of the QWR cryomodules are 22 in RAON SCL3 linac.
The pressure in the helium reservoir is around 1.28 bars.The helium pressure fluctuation goes down by changing the flow rate, supply pressure, return pressure, cryogenic valve control speed, etc.The liquid helium level in the helium reservoir is kept between 40 and 60 %.The QWR cavities have different pressure sensitivities, so the cavities with low pressure sensitivity can be controlled better for the same helium pressure fluctuation compared to the cavities with high pressure sensitivity.The majority of the pressure sensitivity for the QWR cavities is from 3 to 7 Hz/mbar.Most of the Lorentz force detuning (LFD) for the QWR cavities is from -9 to -20 Hz/(MV/m) 2 .Multipacting (MP) conditioning is performed for all QWR cavities.First, the accelerating field for the QWR cavities increases up to 2 MV/m for multipacting (MP) conditioning.Second, the accelerating field increases up to 6.1 MV/m to prepare for beam commissioning.The total heat load, which includes static and dynamic heat loads, is measured for each cavity.Figure 3 shows the total heat load measurement for the QWR 17 cavity.The total heat load for the QWR 17 cavity is 10.5 W at 6.1 MV/m.The dynamic heat load is almost constant once RF conditioning is finished, even if the static heat load goes down in order to reach thermal equilibrium.The total heat load of static and dynamic heat is measured.Most of the total heat load for the QWR cavities is around 12 W.The total heat load for each cavity determines how much helium flow is needed.
Figure 4 shows the vacuum pressure in the QWR cavities as a function of time.The vacuum pressures in the QWR cavities are monitored.The majority of the vacuum pressures for the QWR cavities are below 5x10 -10 mbar.

Discussion
It is very important to reduce microphonics to control the superconducting cavity with RF.The liquid helium pressure of the QWR cryomodule is around 1.28 bars in the superconducting cavity at 4.5 K.The microphonics mainly come from the helium pressure fluctuation and external vibrations.The helium pressure fluctuation goes down by changing the flow rate, supply pressure, return pressure, cryogenic valve control speed, etc. Figure 5 shows the cavity vacuum pressure during RF preparations.The cavity pressure can increase when RF power is on and the cavity RF mode is changed.First, the cavity pressure is not changed.Second, the cavity pressure increases and quickly decreases.Third, the cavity pressure increases and slowly decreases.Fourth, the cavity pressure increases, decreases, and then slowly increases and decreases.In the fourth case, the warm section pressure goes up and then down, which causes increased cavity vacuum pressure.Gas shows molecular flow in this low pressure range [10].Gases mainly collide with the cavity wall and beam tubes.The gases are generated by a cavity and a fundamental power coupler.The cavity can be contaminated if the gases generated by the fundamental power coupler move to the cavity surface.Therefore, the RF power needs to be applied once the cavity pressure goes down.
Figure 6 shows the amplitude and phase of the QWR cavity as a function of time.The amplitude and phase of the QWR cavity are stably controlled.The helium pressure, the accelerating field amplitude, the phase, and the frequency are shown as functions of time.The amplitude fluctuation is less than 1%, and the phase fluctuation is also less than 1 degree for an hour.The QWR cavities in QWR cryomodules in the RAON SCL3 tunnel can be controlled for beam commissioning.The amplitude and the phase are set for each QWR cavity for Ar+9 beam.The beam commissioning is performed for all QWR cryomodules from 1 through 22 in RAON SCL3, in which the energy of Ar+9 is increased from 0.5 to 2.45 MeV/u with 30 µA.The beam current is small, so the amplitude of the accelerating field is not changed much by the beam.The beam energy is determined by the time of flight measurement.For each QWR cavity in the QWR cryomodule, the accelerating amplitude and phase are determined one by one during beam commissioning.The accelerating field and phase for each cavity are kept during the beam commissioning.

Conclusion
We have shown the RF beam commissioning of the QWR cavities in the RAON SCL3 tunnel.Multipacting (MP) conditioning is performed for each QWR cavity installed in the RAON SCL3 tunnel.The total heat load, including static and dynamic heat loads, is measured for each cavity.The pressure of the QWR cavities are monitored as a function of time during RF conditioning.The helium pressure fluctuation goes down by changing the flow rate, supply pressure, return pressure, cryogenic valve control, etc.The accelerating field and phase for each cavity are stable during the beam commissioning.The beam commissioning is successfully performed for the QWR cavities in RAON SCL3.

Figure 2
Figure 2 shows the loaded Q measurement of the QWR cavity.The tuner is used to change the resonance frequency to 81.25 MHz.The loaded Q is measured for each cavity at 4.5 K. From the loaded Q and the measured frequency of 81.28 MHz, the cavity bandwidth becomes 156 Hz.The S11, S21 and S22 are also measured for each cavity.The Qext of the pickup for the QWR is 1.36x10 11 , which comes from S11, S21, and S22.The Qext of the QWR cavities obtained from the vertical test is almost the same as that of the loaded Q measurement in the QWR cryomodule.The average value of the Qext for the QWR cavities is about 1x10 11 .

Figure 2 .
Figure 2. Loaded Q measurement for the QWR cavity.S11, S12, S21 and S22 are measured.From S21, S11 and S22, the bandwidth and loaded Q are measured.The S11 and S22 represent the input return loss and pickup return loss, respectively.

Figure 3 .
Figure 3.Total heat load measurement for the QWR 17 cavity.

Figure 4 .
Figure 4. Vacuum pressure in the QWR cavities as a function of time.

Figure 6 .
Figure 6.Stable amplitude and phase of the QWR cavity as a function of time.

Figure 7
Figure 7 shows the change in amplitude and phase of the QWR cavity as a function of time.The accelerating field amplitude, phase, and frequency are well controlled for 26 minutes, and then they are not controlled after that.External vibration can cause the change in amplitude, phase, forward power and frequency of the QWR cavity shown in Figure 7. Pressure fluctuation also causes increased RF forward power, amplitude fluctuation, phase fluctuation, and frequency fluctuation.It is important to control the amplitude and phase of the cavities by reducing the pressure fluctuations, which include low frequency vibration, impulse, acoustic noise, helium pressure, etc.

Figure 7 .
Figure 7. Change in amplitude and phase of the QWR cavity as a function of time.