# Development of a Radio-Frequency Quadrupole Accelerator for the HL-2A/2M Tokamak Diagnostic System
## 3 RF Conditioning
### High-Power Tests  
The main purpose of the high-power tests is to improve the cavity vacuum through an outgassing process when the cavity is heated after the RF power is loaded and to eliminate the burrs on the inner surface of the cavity through the controlled sparks. Additionally, we also need to calibrate how much RF power can achieve the design inter-vane voltage for the beam commissioning.  
The forward and backward power was measured by the directional coupler. We can monitor the spark through the reflected signal and the vacuum gauge, and adjust the frequency of the pulse modulated RF source to minimize the backward power to track the resonance frequency of the cavity.  
The RFQ uses 99.9% oxygen-free copper produced by Chinalco Luoyang Copper Processing Co., Ltd. Luoyang, China [14]. The RF conditioning started when the vacuum pressure reached a level below \(1.5\times 10^{-5}\) Pa. The main difficulty met during the primary RF conditioning is that: the vacuum pressure rises significantly as the cavity power increases, and the worst cavity vacuum reached \(9.5\times 10^{-4}\) Pa. At first, we suspected that the cavity was not well sealed. This may cause the RF spark and irreversible pollution to the cavity if we continue to increase the cavity power. We performed a leak test on the cavity while maintaining a low level of cavity power, and no leakage of the cavity was found.  
As a result of our investigation, we found that this 99.9% oxygen-free copper has the problem of hydrogen adsorption [15]. The most effective way to degas hydrogen on the metal surface is to anneal the metal. Fiberglass heating belts were used to heat the RFQ cavity. Additionally, we also increased the inlet temperature of the RFQ cooling water from 16 \({}^{\circ}\)C to 20 \({}^{\circ}\)C. During the consecutive seven days, the cavity vacuum slowly improved with RF conditioning at a power of 55 kW. In addition, the reflected signal on the scope was stable and there was almost no spark in the cavity. The log of a 6-h long operation at 55 kW is shown in Figure 7. The cavity power trace was almost constant and the cavity vacuum trace was slow to fall during that time.  
Another problem encountered during RF conditioning came from the cooling system of the RFQ. The inlet water temperature of the cooling water cannot be maintained at a certain value, and the inlet water temperature difference reached 3 \({}^{\circ}\)C, as shown in Figure 7 (blue trace). Since the RFQ operates at a 1% duty cycle, the heat loss caused by RF power is relatively small. During RF conditioning with a power of 55 kW, we recorded the resonant frequency of the cavity under different cooling water temperatures. The influence of the inlet water temperature on the frequency of the cavity is shown in Figure 8. A new cooling system will be introduced into the RFQ in the future to reduce frequency instability during high-power operations.  
After 80 net hours of RF conditioning, a vacuum pressure of \(7.5\times 10^{-6}\) Pa was achieved without RF power. Although the cavity vacuum was reduced to \(3.5\times 10^{-5}\) Pa during high-power operation, it still met the requirements of subsequent beam commissioning.  
Figure 6: Coupling factor for setup.