Ya. S. Derbenev

Advanced optical concepts for electron cooling

Abstract The results of explorations of non-traditional solutions of beam transport which could raise the electron cooling rates and efficiency are presented. The proposed optical elements, methods, and conceptual designs are summarized in the following. (1) Magnetized electron beam acceleration and transport with discontinuous solenoid to provide matching between the electron gun and solenoid of the cooling section. These concepts allow the possibility to design and build economical, high beam quality accelerators for electron cooling over a wide energy range, up to that suited for hadron colliders. (2) A special beam adapter (skew quadrupole block) to transform between a magnetized and a flat beam state. This element meets a variety of uses in electron cooling trends. (3) Injectors with ring-shaped cathodes and resonance concentrators of hollow beams involving (optionally) beam adapters. (4) An isochronous (at no RF) electron recirculator ring with a solenoid in the cooling section and beam adapters. (5) Electron storage rings incorporating strong wigglers, solenoid in cooling section with beam adapters, non-coupled focusing outside the cooling section, and (optionally) a strong longitudinal optics for beam compression in wigglers. (6) Hadron beam optics in the cooling section with non-extended beams and dispersion introduced in order to maximize the transverse cooling rate. (7) Low-energy cooling with matched electron and hadron beams. (8) Low-energy cooling with hollow beams.

Electron cooling of positrons

Abstract This article deals with the relaxation of a positron beam in electron gas moving in a magnetic field. A situation is considered where the electron longitudinal temperature is lower than the transverse one. Expressions for the friction force and scattering coefficients of positron momenta have been found, with plasma shielding taken into account. In a strong magnetic field the effective cross section of the cooling process of the longitudinal motion of positrons is shown to be determined by their longitudinal velocity relative to the electron gas. The similarity of frequencies of Larmor rotation of electrons and positrons sharply increases in its turn the cooling intensity of the transverse degree of freedom of positrons. These circumstances can be used for multiple acceleration of high-temperature positron beam cooling.

Siberian snake experiments at the IUCF Cooler Ring

Recent polarized proton beam experiments in the IUCF Cooler Ring found an evidence for a second-order snake depolarizing resonance, when the vertical betatron tune was inadvertently set near a quarter-integer. We have also studied the possibility of spin-flipping the beam polarization in the presence of a full Siberian snake using an RF solenoid. By varying the rf solenoids ramp time and frequency range, we reached a spin-flip efficiency of about 97%.

Radio-frequency polarimetry

A method of fast non-destructive absolute spin monitoring for a bunched beam in an accelerator ring based on use of the RF techniques is considered. The coherent spin of the beam is driven by RF magnets in the spin echo regime. A passive superconducting resonator is proposed to respond to the flipping spin. A possibility is established to enhance the spin-related excitation of the resonator using the charge-resonator dipole interaction and spin-orbit coupling induced by the quadrupoles. It is shown that the spin impedance can be gauged via measurements of the beam dipole impedance. The noise demands are evaluated. Numerical examples are given.

RF-resonance beam polarimeter Part I.Fundamental concepts

Abstract The possibility of an RF-resonance polarimeter (RFP) for fast non-destructive measurement of beam polarization in an accelerator ring is considered. In order to accumulate the transition radiation from the free oscillating coherent spin of the beam, a passive superconducting cavity is proposed. The increase of effective voltage in the cavity with time (related to beam polarization) is calculated here. The efficiency of the polarimeter does not decrease with beam energy and is proportional to the average beam current. A possible scheme of measurement of the accumulated voltage is presented. The noise limitations are taken into account and evaluated. Siberian snakes can be used in order to provide a sufficiently small value for the spin tune spread. Numerical examples are given.

On possibilities of fast cooling of heavy particle beams

Two methods of fast cooling of intensive beams are described. The first one, coherent electron cooling, is based on enhancement of friction effect in the electron cooling method using a microwave instability of electron beam specially arranged in the cooling section. This method is effective for cooling of high‐temperature circulating beams. The second one, self‐cooling, is based on use of the intrabeam Coulomb scattering of particles during the adiabatic processes of beam acceleration and transverse compression. This method allows frequent decrease emittance of an intensive beam issued by a low‐temperature source.

Spin-flipping with an rf-dipole and a full Siberian snake

We recently used a vertical-field rf-dipole magnet to study the spin-flipping of a 120 MeV horizontally polarized proton beam stored in the presence of a nearly-full Siberian snake in the IUCF Cooler Ring. The spin was flipped by ramping the rf-dipole’s frequency through an rf-induced depolarizing resonance. After optimizing the frequency ramp parameters, we used multiple spin-flips to measure a maximum spin-flip efficiency of 86.5±0.5% in April 2000, and 92.5±0.5% in June 2000. The spin-flip efficiency was apparently limited by the maximum achievable current in the rf-dipole. This result indicates that spin-flipping a stored polarized proton beam should be possible in high energy rings such as RHIC (and perhaps HERA in the future), where Siberian snakes are utilized and the dipole rf-flipper-magnets should be quite practical. During the June 2000 run, a new faster technique of locating the rf depolarizing resonance frequency was developed.

rf driven stable spin-flipping motion of a stored polarized beam

The traditional method of spin-flipping a stored polarized beam is based on slowly crossing an rf induced depolarizing resonance. This paper discusses a novel approach where the polarization reversal is achieved by trapping the beam polarization into a stable spin-flipping motion on top of the rf induced resonance at a half-revolution frequency.

A novel two‐beam accelerator (twobetron)

A new configuration is analyzed wherein a low current beam is accelerated to high energies (10’s of amps, 10’s of MeV) by a driver beam of high current and low energy (a few kiloamps, <1 MeV). The annular driver beam excites the TM020 cavity mode of an accelerating structure which transfers its rf power to the on‐axis secondary beam. Systematic variation of the driver beam radius provides the secondary beam with phase focusing and adjustable acceleration gradient. A proof‐of‐principle experiment is suggested. Various issues, such as the scaling laws, transverse and longitudinal instabilities, rf coupling among cavities, etc., are examined.

The stimulated Stern‐Gerlach effect in charged particle storage rings

The results of the study of possibilities to use spin‐orbital force in order to split a circulating beam to two polarized beams are presented in this report. It is shown that the original spin‐splitter idea which is based on using intrinsic spin‐orbital resonance is not sufficient for splitting, in principle, because the spin’s long lasting effect on particle betatron oscillations is reduced just to a small tune shift. The theorem on the conservation of the sum or difference of quantum orbital and spin numbers, i.e. the combined spin‐orbital invariance, is established for this case. The resonant RF magnetic field parallel to the plane of splitting is introduced in order to stabilize spin in the plane of its precession and remove the combined invariance. The new double‐resonance invariants are established, which describe the spin dynamics and the splitting process.