The first Heavy Ion Medical Machine (HIMMs) constructed by the Institute of Modern Physics, China^[1-3] has passed the registration tests and entered clinical treatment in Wuwei city. A compact cyclotron is designed as the injector of a synchrotron which forms the HIMM, and it accelerates the ¹²C⁵⁺ from the ion source to 7.0 MeV/u; meanwhile, the extracted beam current is more than 10 µA^[4].

The radial probe detector plays an important role during the beam commissioning as well as the turn measurement, especially in monitoring the beam current varying under the condition of setting different physical parameters to estimate the machine characteristic and its acceptance^[5-7]. As the compact structure of the cyclotron, two radial probes are installed on the hill and in the valley respectively. The one installed in the valley is used to monitor the beam current during the injection period, and the other one installed between the extraction deflector and extraction dipole is used to monitor the extraction beam current. Furthermore, the beam center measurement can be implemented with the two targets during the acceleration period. The radial probes installed in the cyclotron are shown in Fig.1.

The radial probe system is mainly composed of the physical and water-cooling optimization design, mechanical structure, interference shielding of weak signal measurement, data acquisition system, and motion control system.

The radial probe target tips made up of one integral block and three differential fingers, which are distributed concurrently in the horizontal direction, can get the beam distribution in axial and radial directions by blocking the beam^[8]. The three differential fingers are distributed uniformly in the vertical direction with a gap of 5 mm. The sizes of the integral and differential target tips are listed in Table 1. The three differential target tips protrude beyond the integral one by 0.5 mm to intercept the beam current for acquiring the beam distribution in the vertical direction. The radial probe target tips are made of copper. The top view of the radial probe is displayed in Fig.2, and the target probe structure of the radial probe is shown in Fig.3. Each radial probe is driven by a servo motor for closed-loop control of the position.

The extraction beam energy of the cyclotron is 7 MeV/u, and the beam current is 10 µA corresponding to the average power of 400 W. The Gaussian beam distribution with the size of 10 mm is used in the thermal structural analysis by COMSOL. The structures of the radial probe without and with water cooling are analyzed.

The target of the radial probe is made of oxygen-free copper. It can only achieve external heat transfer through the thermal radiation of the material surface to the environment without a water-cooling structure. From the simulation results shown in Fig.4, it can be seen that the temperature of the radial probes’ target tips is more than 1400 ℃, which exceeds the melting point of the oxygen-free copper and can directly cause damage to the target. As a result, a water-cooled structure is a necessary design.

Because of the water-cooling structural design of the detector, where the support tube and the target are insulated by ceramics, the target transfers heat to the outside through the water pipe, then the heat convection conducts between the water pipe and the air in the water-cooling structure of the radial probe. While simulating the thermal characteristic of the water-cooling structure, the water-flow rate is set to 4 m/s, and the water pressure is set to 5 kg/cm². Meanwhile, the radiation heat transfer from a body (e.g., a black body) to its surroundings is proportional to the fourth power of the absolute temperature and can be expressed by equation (1)^[9]

$ q=\varepsilon \sigma {T}^{4} $ (1)

where $ \sigma $ is a fundamental physical constant, T is the temperature, the emissivity $ \varepsilon $ is the effectiveness of a material’s surface in emitting energy as thermal radiation and varies between 0.0 and 1.0. Here the emissivity coefficient of the oxygen-free copper is assumed as 0.1.

The result of the thermal analysis during the scanning process of the detector is shown in Fig.5. Fig.5 shows that the water-cooling structure gives a maximum surface temperature of less than 200 ℃, which is much lower than the melting point of oxygen-free copper of 1083 ℃. Therefore, the water cooling structure with these designed parameters is in the range of thermal tolerance. What’s more, a layer of heat-conducting ceramic made of aluminum nitride is between the integral and the differential tips which can enhance the effect of water cooling^[8].

During the scanning process with the probe from the outside towards the beam center of the cyclotron, in the first case, the differential target tips receive the highest beam power and reach the maximum temperature of 100 ℃ when the beam center is at the front edge of the detector, while the temperature of the integral target tip is 124 ℃ at this moment as shown in Fig.5(a).

Similarly, when the beam is completely intercepted by the integral target tip, the temperature of the entire detector is the highest that reaching 182 ℃, as shown in Fig.5(b). Meanwhile, the water temperature rises 3.2 ℃ when the maximum water flow velocity is 5 m/s. From the temperature equilibrium curve of the target, it can be seen that the temperature of the target tends to stabilize after about 3 min, as illustrated in Fig.5(c).

The radial probes are placed on the center plane of the magnetic pole gap of the cyclotron. It is mainly used to measure the radial and axial profiles of the beam and further get the information of radial and axial distribution of beam intensity inside the accelerator. The detectors are inserted from the central plane of symmetry in the valley or hill area of the cyclotron while measuring the beam information.

During the cyclotron commissioning period, it is required to localize the turns at the azimuth of the probes. The radius gains allow reconstructing the energy gain as a function of radius^[6]. Simultaneously, the beam intensity signal from the radial probe tips is measured when it moves from the center to the extraction or vice versa. As a result, the control hardware of the radial probe is based on Field Programmable Gate Arrays (FPGAs), and a real-time (RT) operating system implements the requirements.

The application of FPGA can realize precise timing, control, synchronization, and user-defined high-speed communication protocol requirements. The real-time operating system can ensure the realization of deterministic control, data processing and data logging^[10-11]. Furthermore, the whole control system is based on the National Instrument(NI) CompactRIO hardware. The hardware architecture of this control and data acquisition system are shown in Fig.6.

The NI CompactRIO 9075^[12] used in Wuwei city, which was upgraded to NI 9065^[13] in Lanzhou city, is in charge of acquiring the beam current from the picoammeters where the beam current is converted to voltage linearly. In detail, the data acquisition and digital input/output functions were implemented on FPGA, where the raw analog data was first gathered by picoammeter and then further acquired by NI 9215 that performed 4-channel differential and simultaneous analog input (AI) at the maximum sample rate 100 kS·s⁻¹·ch⁻¹ with 16-bit resolution^[14]. The position information was input into the NI 9411^[15] which worked with industrial logic levels and signals for direct connection to the motor driver whose resolution is 24 bits^[16].

The software framework is designed and implemented with modularity technology organized by the producer-consumer design pattern (shown in Fig.7). The modularity design makes it fast to transplant the modules to the other similar projects of beam diagnostics front-end control system of the heavy ion accelerator transparently^[17]. The automated variables publication of the OPC UA communication protocol^[18] is implemented based on the XML configuration files shown in Fig.8^[19]. Furthermore, it is validated the feasibility of accessing the database by OPC UA variables. The control information is sent into the beam diagnostics database developed with MariaDB^[20]. Moreover, there are three tables designed for the target: the motion table, the DAQ table, and a static table consisting of the detector’s installation, connection cables, and the network connection.

Based on the described hardware, the AI card can execute all channels of one radial probe detector simultaneously. Meanwhile, the position of the radial probe detector is read by the resolver, sampled simultaneously with the beam current signal at the FPGA and then transferred to RT by First In First Output (FIFO). Accordingly, the analog and digital data are finally parsed and converted into beam current and the position information at the GUI. Furthermore, the mechanical position precision is less than 1mm (nearly 0.5 mm) which means the whole control system can reach the 0.5 mm position precision compared with the encoder position resolution.

The OPC UA server was developed and ran in the real-time operating system (VxWorks or Linux-RT); meanwhile the control logic was based on the state-machine design pattern executed at this terminal.

The control system of this detector is mainly composed of three modules, which are described in the following.

(1) Parameters setting mainly includes three items: motor motion control parameters, front-end electronics control parameters, sampling mode and period, which can be set or controlled by these parameters.

(2) Real-time display and data storage mainly include beam current, position, and motion status information from the servo motor encoder readback. These data are further processed and indicated in different formats according to the requirement. During the measurement process, it is also necessary to save the beam current and the corresponding motor position in a file for offline processing and analysis.

(3) Motion control is used to control the system movement and monitor the motion status such as the limits, encoder position and potentiometer position, and so on.

The offline assembly and test were implemented in the laboratory, including the mechanical and control systems.

During the alignment and calibration of the radial probe, the laser tracker is used to calibrate the mechanical structure of the detector. The mounting flange of the detector is used as the benchmark; the flange circle is used as the zero point. The stability and moving distance were tested first. The moving distance of the radial probe is 796 mm, and the repeated positioning accuracy of the probe is less than 0.1 mm. The probe moves smoothly and the position fluctuation of the target during the movement process is less than ±0.5 mm. The insulation resistance between the probes and their outer casing is more than 1000 MΩ, which can keep excellent insulation measured in the laboratory. The pressure resistance of the cooling system is 1.8 MPa, and the leakage rate of the detector’s weld is less than 5.0×10⁻⁸ Pa·l·s⁻¹. All the above parameters were measured and recorded before the on-site installation.

（1）Nonlinearity error verification

In the lab testing, the nonlinearity error verification of the front-end electronics of the radial probe is implemented with the current source, the CompactRIO control system, and the radial probe. Under the specified conditions, the maximum deviation for the calibration curve of the sensor is defined as the ratio between the fitted straight line (ΔY_max) and the percentage of the full-scale output (Y), which is called linearity (linearity is also called “non-linear error”). The smaller the value, the better the linearity characteristics. It is expressed as the following equation (2).

$ \mathrm{\delta }=(\Delta Y\mathrm{m}\mathrm{a}\mathrm{x}/Y)\mathrm{*}100\mathrm{{\text{%}}} $ (2)

Its verification flow is listed as follows:

① The noise voltage level of each channel of the CompactRIO system is measured without the output of the current source and saved as the noise file.

② The source outputs different current values fed into the front-end electronics. Then the output voltage of the front-end electronics is acquired with the CompactRIO control system by the NI 9215. Meanwhile, the data acquired more than 0.1 s by NI 9215 at the sample rate of 10 kS/s is averaged by using the developed average algorithms. The picoammeter has 8 ranges from 2 nA to 20 mA, while the beam current in the cyclotron is below 10 µA. As a result, the ranges below 20 µA are measured for the nonlinearity error test. The linear fitting between the set data and acquired data is plotted in Fig.9.

③ The data is subtracted by the noise and transformed into the current according to the transform coefficient.

④ The calculated data is then linearly fitted to analyze the nonlinearity error of each probe.

⑤ The control system will repeat the above flow until the nonlinearity error of the 5 ranges is measured.

According to the test results, the nonlinear error of 5 ranges for each probe is less than 1% after the data is linearly fitted.

（2）Resolution test of the control system

The resolution of the front-end electronics of the radial probe is also a required verification implemented according to the following procedures.

① Set the current source output to half of each front-end electronics range. For example, when the range is 10 μA, the set output of the current source is 5 μA, and then acquire the output value by NI 9215.

② Repeat the above measurement 100 times for each range, and save the measured data into a file.

③ Do statistics with the saved file and analyze the resolution error.

④ The control system will repeat the above flow until the resolution measurement of the four probes is finished.

The measured data is further counted with the parametric regression analysis method, assuming that the data distribution conforms to the Gaussian distribution. The statistical error for the resolution is less than 2% of the set range for each probe.

There are two radial probes installed in the cyclotron of HIMMs in Wuwei and Lanhzou city to monitor the beam current at different radial direction positions, tune measurement and turn pattern information, especially during the commissioning period of each machine. While measuring the beam turn pattern information, the start position, stop position, and motion parameters were pre-set. Then moving into the cyclotron center direction, the beam turns pattern and beam current accompanied by the position information were acquired and displayed in the GUI. Typically, the radial probe monitors the beam current at the target tips during the commissioning of the cyclotron. Fig.10 and Fig.11 are the monitoring results with the slow data acquisition system during cyclotron commissioning at the 3^rd HIMM. The beam current measured is shown in Fig.10, which reaches more than 20.88 µA at a fixed position of 100 mm from the center of the cyclotron on the sum tip, and it is about 0.55 µA on the middle tip. In comparison, the value is 0.004 µA on both up and down tips close to the noise level. The beam current was continuously measured during uniform motion by pushing in the detector from 150 mm to 900 mm and then pulling it out in the opposite direction. The results shown in Fig.11 illustrate that the sum tip intercepted most of the beam particles. Likewise, the middle tip intercepted a small portion of the particles. The reverse limit of the detector is set as position 0. The distance from the reverse limit to the cyclotron center is 900 mm, howerer, the moving distance of the detector is 750 mm. As a result, the position 150 mm from the cyclotron center corresponds to 750 mm in the abscissa axis. Furthermore, the beam turn pattern was measured for machine performance study with the fast data acquisition system, Fig.12 shows the beam pattern test result of the valley radial probe with beam in Wuwei city in March 2021. All 4 probe tips’ information is displayed and the sum information of the 4 channels is displayed simultaneously. Accelerator physicists can compare the data with that of the theoretical design and further study the machine performance.

The radial probe is one of the essential devices for measuring the beam intensity in the cyclotron. It can further accurately calibrate other measuring devices because of its absolute measurement. Moreover, it also can be used as a low-energy beam stopper. To improve the system measurement accuracy, the probe’s insulation resistance and shielding method were carefully designed and simulated on the water-cooling structure and thermal analysis to ensure the usage at high beam current. The data acquisition system is based on FPGA technology and a Keithley 6485 picoammeter, which has a high resolution of 20 fA. The mechanical structure and motion control was designed and developed based on the reliable scheme. The two radial probes installed in the HIMM cyclotron can measure the beam intensity at low energy region and get the beam turn pattern information. According to its application and the test results, the radial probe plays a vital role in the commissioning and operation period for HIMMs. As a result, it is essential to improve the efficiency of beam injection and extraction.

Acknowledgements 　The authors would like to thank the colleagues of the maintenance and operation team and beam diagnostics team from Lanzhou Ion Therapy Co., Ltd for their efforts in commissioning the target, and Li Peng, Zhang Jinquan, Wang Bing for their knowledge and instruction on the cyclotron.