Automated High-power Conditioning of Medical Accelerators

Medical accelerators require arc-free operation. Due to high-field emission, arcing and outgassing can occur in high-power accelerators. Therefore, the accelerator’s inner surfaces have to be conditioned before its use at high gradient levels in Radiation Therapy machines. At Siemens, we have developed a technique to automatically condition accelerator structures by continually inspecting the accelerator running conditions (arcing and vacuum) and stepping up the pulse width, pulse repetition frequency (PRF), and RF power until reaching the maximum power rating. The implemented program also reads, displays, and archives the data it collects along the entire process of conditioning. NEED FOR RF CONDITIONING OF MEDICAL ACCLERATORS Uninterrupted delivery of x-ray or electron treatment requires arc-free operation of the medical accelerator. Siemens Radiation Therapy machines deliver up to 21 MeV of electron radiation and up to 23MV of x-ray radiation. Operation at such energies requires RF conditioning during the manufacturing of the accelerators structure in the factory prior to their medical use [1]. In an RF conditioning process, the pulse width, pulse repetition frequency (PRF), and peak RF power are automatically stepped up to a level higher than the maximum operational power rating [2]. This is done under the control of an optimizing technique. A Proportional, Integral, Derivative (PID) algorithm is employed where the accelerator running conditions (arcing and vacuum) are being continually inspected and the above operating parameters are changed until reaching maximum programmed power. Arcing rates and vacuum levels are decisive control variables. MANUFACTURING WORK-FLOW AND PRODUCTIVITY REQIURMENT At Siemens Medical Solutions USA, Inc., Oncology Care Systems Group, we have been implementing various actions and measures to streamline and optimize workflow of manufacturing and testing medical accelerators [3,4]. One process we focused on optimizing was the accelerator conditioning and testing. RF conditioning used to be carried out as part of the “Beam Test” and performed in a concrete test cell. We have designed and constructed the Accelerator Conditioning System (ACS), a separate RF conditioning process that is integrated into the accelerator manufacturing and testing workflow to increase the accelerator manufacturing capacity and productivity. As shown in Fig.1, the ACS is physically located before the concrete test cell where the “Beam Test” is conducted. Figure 1: Accelerator Conditioning System (ACS) located within the accelerator manufacturing workflow. ACCELERATOR CONDITIONING SYSTEM (ACS) The main constituent sub-systems of the ACS are shown in schematically in Fig. 2. These are: RF Source: This subsystem includes a solid-state RF driver, a 150 KVmodulator, 7 MW klystron. RF Transmissions Subsystem: This includes a three-port circulator, high-power water load, a directional coupler, and sampling RF probes with crystal detectors measuring reflected power. Water Cooling System: This provides cooling to the klystron/modulator subsystem, circulator, highpower load, and the accelerator structure. Shielding Enclosure: Lead structure was built to enclose the accelerator in order to provide the necessary x-ray shielding. Ion-Pump: Power Supply current is measured to monitor gas activity. Interlocks: Fifteen safety and operational interlocks to power off the system or remove triggers to the low-level drive of the klystrons. Radiation Monitor: A calibrated radiation meter is used to monitor radiation leakage. Computer Control: A computer program based on LabVIEW controlled the processing according to an algorithm described in the next section. Final Assembly Gun Test A C S Beam Test ___________________________________________ *E-mail: [email protected]. Proceedings of EPAC 2004, Lucerne, Switzerland