Development of a Commissioning Plan for the Apt Linac

The Accelerator Production of Tritium (APT) facility [1,2] is based on a linac which incorporates both normalconducting and superconducting RF technology and accelerates a 100-mA cw proton beam to an energy of 1030 MeV or higher, depending on the desired production rate. Commissioning plans to achieve full power operation with minimum beam-induced activation of components have been evolving [3]. This paper presents the main issues and the basic approaches that are now being discussed. 1 COMMISSIONING OBJECTIVES By commissioning of the accelerator, we refer to the process whereby the components are brought into operation with beam for the first time as a functioning integrated accelerator system. Commissioning need not await the complete installation and alignment of all the accelerator components, but may be done in stages. Two main activities make up the commissioning process. First is the initial setting of parameters, which includes the focusing and steering fields, and cavity-field amplitudes and phases, to values determined by the physics and engineering design. Second are the measurements to characterize the functioning of the integrated system, especially the beam and the RF system. These data will be compared with the predictions of simulation code. Discrepancies that are outside of error tolerances must be understood and, if appropriate, used to update the codes. The commissioning process is the time to detect and resolve any unanticipated performance problems that might arise. Also needed for evaluation of the overall performance are tests of different operating modes. For APT this includes operation with some superconducting cavities turned off, simulating fault conditions, where the rest of the linac is reset to continue beam operation. 2 SPECIAL CONSIDERATIONS FOR COMMISSIONING APT The experience gained from operating and restarting the LANSCE proton linac, a pulsed machine with multiple beam operation that includes a 6%-duty-factor, 1-mA average current, and 800-MeV final energy, will be our reference point. LANSCE experience has shown that minimizing beam loss during normal operation is important, because beam loss produces radioactivity that restricts hands-on maintenance, and can cause equipment damage. Beam loss could be a concern during commissioning because the parameters are adjusted over a wide range and at times can deviate substantially from the design values. Many steps, including alignment, polarity setting, and calibration of focusing quadrupoles and steering dipoles, alignment and calibration of beamdiagnostic components, and relative phasing of multiple power feeds should be carried out prior to the start of beam tests. Also, commissioning operations that require beam should be done using pulsed beam with as small a beam duty factor and as small a peak current as is practicable, consistent with the capabilities of the beam diagnostics and the ion source. Beam-pulse lengths should be long compared with the transients in the low-energy beam transport and the RF system. We anticipate using cw RF power and pulsed beams with approximately 200μs pulse length and a repetition rate from 1 to 10 Hz. Depending on the beam current, an RF-system settling time of 30 to 100-μs is expected before steady state is reached. The choice of peak current depends on the procedure. We expect a peak current of about 1 mA to be adequate for measuring the transverse beam-centroid alignment and setting the transverse beam steering. The 1mA peak current, for which the space-charge forces are small, is suitable for phase scans used to set the phases and amplitudes of the cavity fields. After the 1-mA procedures are completed, the peak current will be increased to 100 mA to evaluate beam and system performance at full space charge and with full beam loading. To limit the beam losses during the commissioning, LANSCE experience suggests that the commissioning procedures should be as simple as possible, and should be done one section at a time, where each section to be commissioned consists of one of more accelerating modules. After all the parameters have been set, beam measurements will be made to characterize the output beam from that section, using a commissioning beamdiagnostic package placed at the output. After the diagnostic package, a beam stop is installed, which prevents the commissioning beam from inducing radioactivity downstream of the section being commissioned. The beam stops must have sufficient cooling capability to absorb the beam power at 100-mA peak current with materials chosen to minimize longlifetime activation. After the commissioning of a given section is completed, the diagnostic package and the commissioning beam stop are removed, making room for installation of the next section. Although dividing the linac into sections helps to locate and fix problems, to keep the commissioning time within reasonable bounds, it is desirable to commission the linac in no more separate sections than is necessary, consistent