Jiquan Guo

A Non-Invasive Magnetic Momentum Monitor Using a TE011 Cavity

The Jefferson Lab Electron-Ion Collider (JLEIC) design relies on cooling of the ion beam with bunched electron beam. The bunched beam cooler complex consists of a high current magnetized electron source, an energy recovery linac, a circulating ring, and a pair of long solenoids where the cooling takes place. A noninvasive real time monitoring system is highly desired to quantify electron beam magnetization. The authors propose to use a passive copper RF cavity in TE011 mode as such a monitor. In this paper, we present the mechanism and scaling law of this device, as well as the design of the prototype cavity which will be tested at Jlab Gun Test-Stand (GTS). INTRODUCTION Non-invasive measurement of the magnetic moment of a charged particle beam has long been on the wish-list of beam physicists. The previous efforts were mainly focused on measuring the beam polarization [1, 2, 3], which is in the order of ħ/2 per electron or proton. Enhanced by the Stern-Gerlach polarimetry, the RF signal in the cavity generated by the beam is still extremely hard to measure. The magnetic moment per particle of the magnetized beam is typically a few orders of magnitude higher. As a demonstration of the source for the JLEIC e-cooler, the magnetized beam generated at JLab GTS [4] can have a magnetic moment M=200 neV-s or 3.0×108 ħ. The JLab GTS beam also has a typical energy of 300 keV and a low γ, as well as a beam current of 5mA. These parameters make the magnetic moment more likely to be detected with an RF cavity. INTERACTION BETWEEN PILLBOX TE011 MODE AND MAGNETIZED BEAM The angular momentum and magnetic momentum of a charged particle is determined by its motion in azimuthal direction, as shown in Fig. 1, left. == (1) In a perfect pillbox RF cavity, the electric field of TE011 mode has only azimuthal component, and will be zero in other directions (radial or longitudinal), as shown in Fig. 1, right. For a pillbox with thickness d and radius a, when ρ/a<0.3, the TE011 mode azimuthal E-field’s amplitude can be approximated (within 1% error) as = sin ⁄ 2 ⁄ (2) TE011 mode will only have energy exchanging interaction with the azimuthal motion of a particle, making it an ideal candidate for magnetic moment measurement. To estimate the excited RF power analytically, we assume that the beam-cavity interaction has negligible perturbation on beam trajectory. By integrating E-field tangential to the particle trajectory, the cavity transverse R/Q can be calculated as

Polarized positrons in Jefferson lab electron ion collider (JLEIC)

The Jefferson Lab Electron Ion Collider (JLEIC) is designed to provide collisions of electron and ion beams with high luminosity and high polarization to reach new frontier in exploration of nuclear structure. The luminosity, exceeding 1033 cm−2s−1 in a broad range of the center-of-mass (CM) energy and maximum luminosity above 1034 cm−2s−1, is achieved by high-rate collisions of short small-emittance low-charge bunches with proper cooling of the ion beam and synchrotron radiation damping of the electron beam. The polarization of light ion species (p, d, 3He) and electron can be easily preserved, manipulated and maintained by taking advantage of the unique figure-8 shape rings. With a growing physics interest, polarized positron-ion collisions are considered to be carried out in the JLEIC to offer an additional probe to study the substructure of nucleons and nuclei. However, the creation of polarized positrons with sufficient intensity is particularly challenging. We propose a dedicated scheme to generate ...

JLEIC SRF cavity RF Design

The initial design of a low higher order modes (HOM) impedance superconducting RF (SRF) cavity is presented in this paper. The design of this SRF cavity is for the proposed Jefferson Lab Electron Ion Collider (JLEIC). The electron ring of JLEIC will operate with electrons of 3 to 10 GeV energy. The ion ring of JLEIC will operate with protons of up to 100 GeV energy. The bunch lengths in both rings are ~12 mm (RMS). In order to maintain the short bunch length in the ion ring, SRF cavities are adopted to provide large enough gradient. In the first phase of JLEIC, the PEP II RF cavities will be reused in the electron ring to lower the initial cost. The frequency of the SRF cavities is chosen to be the second harmonic of PEP II cavities, 952.6 MHz. In the second phase of JLEIC, the same frequency SRF cavities may replace the normal conducting PEP II cavities to achieve higher luminosity at high energy. At low energies, the synchrotron radiation damping effect is quite weak, to avoid the coupled bunch instability caused by the intense closely-spaced electron bunches, low HOM impedance of the SRF cavities combined with longitudinal feedback system will be necessary. INTRODUCTION In order to achieve high luminosity in the electron ion collider, shorter bunches and higher beam current are needed. In the ion collider ring of JLEIC [1], after a series of acceleration, the last step of manipulation before colliding is bunching. Since there is no significant synchrotron radiation the ion bunches are located at the zero crossing of RF voltage. The resulting bunch length is inversely proportional to the RF cavity frequency and square root of RF voltage. So, higher frequency is preferred and 952.6 MHz was chosen as mentioned above. With this frequency, the required total RF peak voltage can be derived to be around 43 MV to obtain a bunch length of 12 mm, so SRF cavities are preferred for the ion collider ring. In the electron ring of JLEIC, the synchrotron radiation power loss becomes more significant when beam energy is higher. CEBAF is used as injector for the electron collider ring. Initial energy at injection will be up to 10 GeV with an option to upgrade to 12 GeV in the future. The number of normal conducting PEP II RF cavities available won’t be sufficient at this higher energy, so SRF cavities of the same type as used in ion ring may be added to the electron ring in the second phase of JLEIC. Eventually the PEP-II NC cavities may be phased out and replaced by 952.6 MHz SRF cavities, enabling a higher bunch rate in both collider rings. In the electron collider ring the beam current at the high energy end will be limited by the RF power supply available, so the beam current at high energy end could be increased in the future by adding more klystrons if desired. At the low energy end, the synchrotron radiation damping effect becomes too weak to suppress the coupled bunch instability of the nominal 3 A beam current without proper HOM impedance control. Two strategies are used to ensure the beam stability: low HOM impedance cavities and bunch-to-bunch feedback systems. LOW HOM IMPEDANCE CAVITY In order to lower HOM impedance, it is proposed to use three on-cell dampers to extract HOM power, just as in the PEP-II RF cavity [2]. A one third model of the present concept with electric and magnetic field on surface is shown in Fig. 1. In ion ring application, the SRF cavity will see the max gradient, about 8 MV/m, the corresponding Bmax is 71 mT. Figure 1: One-third model of the cavity. Left: electric field; right: magnetic field. All three damper waveguides are located on one side of the cavity and symmetrically located around the central axis of the cavity. We compared two, three and four damper waveguides in different configurations; this is the one that gives lowest overall impedance. The cut-off frequency of the damper waveguide is chosen to be half way between the fundamental mode and the lowest HOM, ~1 GHz. In this way HOMs can be extracted well without too much leakage of fundamental mode. Good coupling between the damper waveguides and cavity is essential for the low impedance design, however too large of an opening may cause unacceptable field concentration around the aperture. A ridged waveguide iris with a dumbbell shaped cross-section is added be___________________________________________ * Authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177 and DE-AC02-06CH11357. The U.S. Government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce this manuscript for U.S. Government purposes. † email address: wang@jlab.org WEPMW039 Proceedings of IPAC2016, Busan, Korea ISBN 978-3-95450-147-2 2522 C op yr ig ht

High Power Co-Axial Coupler

A very high power Coax RF Coupler (MW-Level) is very desirable for a number of accelerator and commercial applications. For example, the development of such a coupler operating at 1.5 GHz may permit the construction of a higher-luminosity version of the Electron-Ion Collider (EIC) being planned at JLab. Muons, Inc. is currently funded by a DOE STTR grant to develop a 1.5-GHz high-power doublewindowcoax coupler with JLab (about 150 kW). Excellent progress has been made on this R&D project, so we propose an extension of this development to build a very high power coax coupler (MW level peak power and a max duty factor of about 4%). The dimensions of the current coax coupler will be scaled up to provide higher power capability.