ANL, Argonne, Illinois
The Advanced Photon Source (APS) is a 7-GeV storage ring light source that has been in operation for over a decade. In order to make revolutionary improvements in the performance of the existing APS ring, we are exploring the addition of a 7-GeV energy recovery linac (ERL) to the APS complex. In this paper, we show the possible configuration of such a system, taking into account details of the APS site and the requirement that stored beam capability be preserved. We exhibit a possible configuration for the single-pass, 7-GeV linac. We discuss optical solutions for transport from 10 MeV to 7 GeV and back, including a large turn-around arc that would support 48 additional user beamlines. Tracking results are shown that include incoherent and coherent synchrotron radiation, resulting in predictions of the beamline performance.
ANL, Argonne, Illinois
SLAC, Menlo Park, California
In recent years, we have explored application to the Advanced Photon Source (APS) of Zholents'* crab-cavity-based scheme for production of short x-ray pulses. Work concentrated on using superconducting (SC) cavities in order to have a continuous stream of crabbed bunches and flexibility of operating modes. The challenges of the SC approach are related to the size, cost, and development time of the cavities and associated systems. A good case can be made for a pulsed system** using room-temperature cavities. APS has elected to pursue such a system in the near term, with the SC-based system planned for a later date. This paper describes the motivation for the pulsed system and gives an overview of the planned implementation and issues. Among these are overall configuration options and constraints, cavity design options, frequency choice, cavity design challenges, tolerances, instability issues, and diagnostics plans.
*A. Zholents et al., NIM A 425, 385 (1999).**P. Anfinrud, private communication.
ANL, Argonne, Illinois
The Intense Pulsed Neutron Source (IPNS) Rapid Cycling Synchrotron (RCS) accelerates 3.2x 1012 protons from 50 MeV to 450 MeV at 30 Hz. During the 14.2 ms acceleration period, the RF frequency varies from 2.21 MHz to 5.14 MHz. The beam current is limited by a vertical instability. By analyzing turn-by-turn Beam Position Monitor (BPM) data, large amplitude mode 0 and mode 1 vertical beam centroid oscillations were observed in the later part of the acceleration cycle. The oscillations develop in the tail of the bunch, build up and remain localized in the later part of the bunch. This vertical instability was compared with a head-tail instability that was intentionally induced in the RCS by adjusting the trim-sextupoles to make the horizontal chromaticity positive (below transition). It appears that our vertical instability is not typical head-tail instability. More data analysis and experiments were performed to characterize the instability.
ANL, Argonne, Illinois
The Phase I Advanced Photon Source Deflecting Cavity System for producing short X-ray pulses uses one multi-cell, S-band cavity to apply a deflecting voltage to the stored electron beam ahead of an undulator that supports a beamline utilizing short picosecond X-rays. Two additional multi-cell cavities are then used to cancel out the perturbation and redirect the electron beam along the path of its nominal orbit. The pulsed rf system driving the deflecting cavities is described. Design tradeoffs are discussed with emphasis on topology considerations and digital control loops making use of sampling technology in a manner consistent with the present state of the art.
ANL, Argonne, Illinois
The Intense Pulsed Neutron Source (IPNS) rapid cycling synchrotron (RCS) is used to accelerate protons from 50 MeV to 450 MeV, at a repetition rate of 30 Hz. The original ring design included two identical RF systems, each consisting of an accelerating cavity, cavity bias supply, power amplifiers and low level analog electronics. The original cavities are located 180 degrees apart in the ring, and provide a total peak accelerating voltage of ~21 kV over the 2.21 MHz to 5.14 MHz revolution frequency sweep. A third RF system has been constructed and installed in the RCS. The third RF system is capable of operating at the fundamental revolution frequency for the entire acceleration cycle, providing an additional peak accelerating voltage of up to ~11kV, or at the second harmonic of the revolution frequency for the first ~4 ms of the acceleration cycle, providing an additional peak voltage of up to ~11kV for bunch shape control, resulting in a modest increase in bunch length. We describe here to date, the hardware implementation and operation of the third RF cavity in the second harmonic mode.
ANL, Argonne, Illinois
In the IPNS RCS, a single proton bunch (h=1) is accelerated from 50 MeV to 450 MeV in 14.2 ms. The bunch experiences an instability shortly after injection (<1 ms). During the first 1 ms, the beam is bunched but little acceleration takes place; thus, this period of operation is similar to that of a storage ring. Natural vertical oscillations (assumed to be tune lines) show the vertical tune to be rising toward the bare tune value, suggesting neutralization of space charge and a reduction of its detuning effects. Neutralization time near injection ranges from 0.25 ms - 0.5 ms, depending on the background gas pressure. Oscillations move from the LSB to the USB before disappearing. Measurements made with a recently installed pinger system show the horizontal chromaticity to be positive early but approaching zero later in the cycle. The vertical chromaticity is negative throughout the cycle. During pinger studies, two lines are observed, suggesting the formation of islands. Neutralization of the beam space charge implies the generation of plasma in the beam volume early in the cycle which may then dissipate as the time-varying electric fields of the beam become stronger.
ANL, Argonne, Illinois
The Advanced Photon Source storage ring is normally operated with 100 mA of beam current. A number of high-current studies were carried out to determine the multibunch instability limits. The longitudinal multibunch instability is dominated by the rf cavity higher-order modes (HOMs), and the coupled-bunch instability (CBI) threshold is bunch-pattern dependent. We can stably store 200 mA with 324 bunches, and the CBI threshold is 245 mA. With 24 bunches, several components are approaching temperature limits above 160 mA, including the HOM dampers. We do not see any CBI at this current. The transverse multibunch instabilities are most likely driven by the resistive wall impedance; there is little evidence that the dipole HOMs contribute. Presently, we rely on the chromaticity to stabilize the transverse multibunch instabilities. When we stored beam up to 245 mA, we used high chromaticity, and the beam was transversely stable. The stabilizing chromaticity was studied as a function of current. We can use these experimental results to predict multibunch instability thresholds for various upgrade options, such as smaller-gap or longer ID chambers and the associated increased impedance.