EMITTANCE GROWTH AND BEAM LOSSES IN LANSCE LINEAR ACCELERATOR

The LANSCE Accelerator facility currently utilizes four 800-MeV Hbeams and one 100-MeV proton beam. Multibeam operation requires careful control of the accelerator tune to minimize beam losses. The most powerful 80-kW Hbeam is accumulated in the Proton Storage Ring and is extracted to the Lujan Neutron Scattering Center facility for production of moderated neutrons with meVkeV energy. Another Hbeam is delivered to the Weapon Neutron Research facility to create un-moderated neutrons in the keV MeV energy range. The third Hbeam is shared between the Proton Radiography Facility and the UltraCold Neutron facility. The 23-kW proton beam is used for isotope production in the fields of medicine, nuclear physics, national security, environmental science and industry. Minimization of beam losses in the linac is achieved by careful tuning of the beam in each section of the accelerator facility, imposing limitations on amplitudes and phases of RF systems, control of Hbeam stripping, and optimization of ion source operation. This paper summarizes experimental results obtained during accelerator tuning and identifies various sources of emittance growth and beam losses. LANSCE ACCELERATOR FACILITY The LANSCE Accelerator facility has been in operation for more than 40 years. Currently it delivers 800-MeV Hbeams to four experimental areas and one 100-MeV proton beam (see Fig. 1 and Table 1). The accelerator facility is equipped with two independent injectors for H+ and Hbeams, merging at the entrance of a 201.25-MHz Drift Tube Linac (DTL). The DTL accelerates the two beams to 100 MeV. After the DTL, the Transition Region (TR) beamline directs the 100-MeV proton beam to the Isotope Production Facility (IPF), while the Hbeam is accelerated up to the final energy of 800 MeV in an 805MHz Coupled Cavity Linac (CCL). The Hbeams, created by different time structures of a low-energy chopper, are distributed in the Switch Yard (SY) to four experimental areas. Minimization of beam losses is one of the main criteria of successful operation of the accelerator facility. BEAM LOSS IN ACCELERATOR Beam losses in the LANSCE accelerator are mostly determined by the two most powerful beams: the 80-kW Hbeam injected into Proton Storage Ring, and the 23-kW H+ beam, which is used at the Isotope Production Facility. The main sources of beam losses in the linac are mismatch of the beam with the accelerator structure, ________________________ *Work supported by US DOE under contract DE-AC52-06NA25396 #batygin@lanl.gov Figure 1: Layout of LANSCE Accelerator Facility. Table 1: Beam Parameters of LANSCE Accelerator Area Rep. Rate (Hz) Pulse Length (μs) Current/ bunch (mA) Average current (μA) Average power (kW) Lujan 20 625 10 100 80 IPF 100 625 4 230 23 WNR 100 625 25 4.5 3.6 pRad 1 625 10 <1 <1 UCN 20 625 10 10 8 variation and instabilities of accelerating and focusing fields, transverse-longitudinal coupling in the RF field, misalignments and random errors of accelerator channel components, field nonlinearities of focusing and acceleratirng elements, beam energy tails from un-captured particles, particle scattering on residual gas and intra-beam stripping, non-linear space-charge forces of the beam, excitation of high-order RF modes, and dark current from unchopped beams. Beam losses at LANSCE are controlled by various types of loss monitors. The main control is provided by Activation Protection (AP) detectors, which are one-pint size cans with a photomultiplier tube immersed in scintillator fluid. AP detectors integrate the signals and shut off the beam if the beam losses around an AP device exceed 100 nA of average current. The same devices are used as beam loss monitors (LM), where the signal is not integrated and therefore one can see a real-time of beam loss across the beam pulse. Another type of loss monitor are Ion Chamber (IR) detectors, which are used in the high energy transport lines (Line D, PSR, 1L, WNR). They are usually located in parallel with Gamma Detectors (GDs) that feeds into the Radiation Safety System. An advantage of the IRs is that they do not saturate at high loss rates like the AP devices. 61st ICFA ABDW on High-Intensity and High-Brightness Hadron Beams HB2018, Daejeon, Korea JACoW Publishing ISBN: 978-3-95450-202-8 doi:10.18429/JACoW-HB2018-MOP2WB03