Positron Ring Research Articles

Development of the new method of positronium generation. Abilities and future trends

Presently the positron ring dedicated to positronium generation and called the Low Energy Positron Toroidal Accumulator (LEPTA) is under construction at JINR (Dubna). The main application of this ring is the generation of a monoenergetic and directed flux of positronium atoms and its use in experiments. Positronium is generated in interactions of positrons, circulating inside a storage ring accumulator, with free electrons of the cooling electron beam that have velocities very close to the positron velocities. This allows us to obtain a high positronium flux with small angular and velocity spreads of the atoms and provides a significant advantage for the proposed arrangement of experiments, so-called positronium-in-flight setups, as compared with traditional approaches in which positronium was generated in targets. In particular, the precision in measuring positronium parameters (lifetime, the probability of decays with momentum conservation, charge invariant violation (CPT violation), fine structure of the positronium spectrum, and Lamb-shift measurements) can be enhanced by several orders of magnitude. Moreover, some experiments that are unrealistic within traditional schemes become feasible with the proposed facility. PACS No.: 36.10Dr

陽電子加速器リングでの二次電子マルチパクタリングによる非線形圧力上昇のシミュレーション

A non-linear dependence of vacuum pressure on the beam current has been observed in the KEKB positron ring. The behaviour changes by the bunch fill pattern, the bunch current and the external magnetic field. A simple simulation on the non-linear pressure rise is proposed assuming that the behaviour occurs by the electron multipactoring. The simulation well reproduces the observed non-linear pressure behaviours. Since the beam size blow up, which is also observed in the positron ring, is similar to the non-linear pressure rise, the results would support an explanation that the beam size blow up is related to the electrons in the beam chamber.

Wake-field and fast head-tail instability caused by an electron cloud

In positron and proton storage rings, electrons produced by photoemission, ionization, and secondary emission accumulate in the vacuum chamber during multibunch operation with close spacing. A positron or proton bunch passing through this "electron cloud" experiences a force similar to a short-range wake field. This effective wake field can cause a transverse-mode-coupling instability, if the electron-cloud density exceeds a threshold value. In this report, we compute the electron-cloud induced wake in a region without external magnetic field both analytically and via computer simulation, for parameters representing the low-energy positron ring of KEKB and the LHC proton beam in the CERN SPS. We study the linearity and time dependence of the wake function and its variation with the size of the electron cloud. Using a broadband resonator model for the electron-cloud wake field, we then evaluate theoretical expressions for the transverse-mode-coupling instability based on the linearized Vlasov equation, and for the instability threshold of fast transverse blow up including its dependence on chromaticity.

Secondary electron emission yields from PEP-II accelerator materials

The PEP-II B-Factory at SLAC operates with aluminum alloy and copper vacuum chambers, having design positron and electron beam currents of 2 and 1 A, respectively. Titanium nitride coating of the aluminum vacuum chamber in the arcs of the positron ring is needed in order to reduce undesirable electron-cloud effects. The total secondary electron emission yield of TiN-coated aluminum alloy has been measured after samples of beam chamber material were exposed to air and again after electron-beam bombardment, as a function of incident electron beam angle and energy. The results may be used to simulate and better understand electron-cloud effects under actual operating conditions. We also present yield measurements for other accelerator materials because new surface effects are expected to arise as beam currents increase. Copper, in particular, is growing in popularity for its good thermal conductivity and self-radiation-shielding properties. The effect of electron bombardment, “conditioning”, on the yield of TiN and copper is shown.

Electron cooling of magnetized positrons

Electron cooling of positrons is the essential peculiarity of the method of antihydrogen and positronium fluxes generation, which is based on low-energy positron storage ring. The design of the positron ring equipped with an electron cooling system is discussed. The positron beam circulates in a longitudinal (quasitoroidal) magnetic field. Electron drift inside special “septum” coils and toroidal sections is used for superposition and separation of the cooling electron beam and the circulating positron one. The working cycle of the ring is described. Peculiarities of the electron cooling of magnetised positrons are discussed. The general parameters of the Low-Energy Positron Toroidal Accumulator (LEPTA), which is under construction in the JINR, are presented.

ＫＥＫＢ加速器初期運転時の真空特性

The commissioning of KEKB accelerator (KEKB-factory) started in December, 1998. The vacuum system is working almost satisfactorily although several troubles occurred near the interaction region. The photon stimulated gas desorption coefficient of copper beam chamber is decreasing steadily and reached to around 1×10-5 molecules photon-1 at the integrated linear photon flux of 1×1024 photons m-1. The beam lifetime is increasing with the integrated beam current but is shorter than that expected by the average pressure especially for the Low Energy Ring. A pressure burst was observed at a straight section of the positron ring. The Helicoflex vacuum sealing is stable. Bellows and gate valves with RF-shield structure showed no excess heating up to the storage current of 500 mA.

Antihydrogen beam generation using storage rings

The scheme of antihydrogen beam generation is proposed. Two storage rings should be used for effective generation of antihydrogen - an antiproton ring and a small additional one for storing and cooling of positrons. This scheme has advantages compared with those discussed earlier due to the use of a longitudinal magnetic field as the focusing system of the positron ring with the positron source immersed in this field. The electron cooling of positrons is an essential part of the project. The Low Energy Antiproton Ring (LEAR) at CERN is chosen as the basic machine for idea demonstration and all the numerical applications are made for it. The rate of antihydrogen generation is limited in the case discussed here by the positron source intensity. The simplest version of this scheme permits one to achieve a rate of about 3 × 10 4 fast (50 MeV) atoms/s and 30 slow (0.5 MeV) ones. Simultaneously such a scheme is a generator of orthopositronium with a productivity of about 2 × 10 4 and 35 “atoms”/s, correspondingly. The particle dynamics in the positron storage ring are considered.

Sputter-coated superconducting RF cavities for LEP: a technological challenge

For increasing the energy of the Large Electron Positron Ring (LEP) at CERN superconducting accelerating cavities are being installed. Most of the 216 cavities, developed at CERN and manufactured in industry, have undergone successful acceptance tests. By sputter-coating high-purity niobium onto a copper substrate accelerating fields of 6 MV m −1 with a quality factor Q 0 of 3.4 · 10 9 at 4.5 K and at a frequency of 350 Mhz were obtained. To reach these specifications it was necessary to develop surface preparation and sputtering techniques able to cover the 5.5 m 2 of a cavity with a defect-free niobium layer of 1.5 mm thickness. Assembly and all preparations have to be done in a clean room of class 100. The cavities work in the ultra-high vacuum (~ 10 −10 mb) of the LEP ring. The working temperature is 4.5 K. Higher performance for a low price has been obtained as compared to that for cavities made of niobium sheet material.

Calculations of the radiation doses to the electronic equipment and magnet coil insulation in the PEP-II tunnel

The PEP-II collaboration is a new physics project at SLAC that utilizes an existing tunnel to house two accelerator storage rings; a low-energy positron ring (4 GeV, called the LER) atop a high-energy electron ring (10 GeV, called the HER). Both rings are being designed to permit a high circulating current of 3 A, corresponding to a beam particle number of 1.38 × 10 14. Dose calculations were made to estimate the potential for radiation damages to the electronics and magnet coil insulation in the arc section of the tunnel. An analytical code, SHIELD11, was used to calculate the dose from photon and neutron radiations resulting from beam losses in various modes of operation. Several EGS4 Monte Carlo user codes were developed to calculate the dose from the synchrotron radiation escaping the vacuum chamber in stored beam mode. Two different LER vacuum chamber designs were studied: a 0.35-cm-thick copper wall chamber and a 1-cm-thick aluminum wall chamber with a copper absorber 6 m downstream of every dipole to absorb the synchrotron radiation in a local spot. The HER has a 0.5-cm-thick copper vacuum chamber. Shielding from the 5.4-m-long, C-shaped iron bending magnet for the HER was considered for the beam radiation, while no other structural shielding was assumed for the LER. Furthermore, the dose from all radiations scattering back from the tunnel concrete wall were also estimated using the albedo method or the MORSE code. The results of the dose estimations are presented.

Linac ring collider

The collision of a linear accelerator electron beam with a particle beam stored in a ring allows center of mass energies and luminosities beyond those achievable in pure storage ring colliders. The constraints of such a collider scheme and the unusual beam-beam interaction scenario with a highly disrupted electron beam are discussed. Machine parameters for a high luminosity B factory are given. The possibilities to use the LEP type superconducting acceleration cavities for the linac and a low emittance and high current positron ring analog to advanced synchrotron light sources are outlined. The option for a parallel useability of the electron beam for linac-ring collisions and fixed target operation is mentioned.

Operation of a five-pole superconducting wiggler in the DCI positron ring and design of the beamline (invited)

A five-pole superconducting wiggler has been installed in the DCI positron ring and operated without disturbing the machine characteristics at full energy (1.85 GeV) and maximum current (300 mA). Three beamlines have been built which feed six beam ports. The first two-crystal monochromator to be used for Compton scattering has been commissioned although the sagittal focusing has not yet been tested.

A five-pole superconducting wiggler for the DCI positron ring at LURE

A 5 T superconducting wiggler has been installed in the DCI storage ring and operated successfully without any noticeable disturbance of the 100 mA positron beam at 1.72 GeV. This device is primarily intended to increase the photon flux at short X-ray wavelengths by shifting the characteristic wavelength. However, because it is a five-pole magnet it delivers an X-ray beam far more intense than the one from the bending magnets, even at wavelengths longer than 1 Å. The magnet was designed and constructed at the DPhPE of CEA Saclay to specifications from LURE.

Lep

The Large Electron Positron ring LEP is presently being built at CERN. It will provide colliding beams of energies between 15 and 100 GeV each, to connect to the physics done at PETRA, PEP and TRISTAN and to cover Z 0 and W production. It has a circumference of about 27 km and uses existing CERN machines as injectors. The difficulties given by the large size of this machine concerns mainly the civil engineering, the fabrication of large quantities of components and the planing and installation, while the basic accelerator physics is not much different from other existing storage rings. In this report however we concentrate on this last point and show how the underlying accelerator physics let to the choice of cerntain parameters and hardware solutions like concrete magnets and storage cavities and on a check of the importance of some special effects for LEP.

Magnet developments for the new Orsay synchrotron radiation source Super ACO

A description is given of the vacuum ultraviolet (VUV) Super ACO ring accelerator. Three wiggler/undulator insertion devices under construction are described: a 3.2-m-long undulator for a free-electron laser, a 2.8-m-long classical permanent-magnet pseudo-wiggler, and a 3.0-m-long asymmetric hybrid wiggler. The dedicated Super ACO ring is a part of the LURE (Laboratoire pour l'Utilisation du Rayonnement Electromagnetique) storage ring complex which includes the 540-MeV electron ring ACO and the 1.85-GeV DCI positron ring. The authors also discuss start up, special optics for operation with insertions, scraper experiments, and their results, magnetic tolerances, and some of the automated measuring equipment installed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Capture of medium-energy positrons in deuterium.

A formalism for investigating the positron-capture reaction ${\mathrm{e}}^{+}$+d\ensuremath{\rightarrow}\ensuremath{\nu}${\ifmmode\bar\else\textasciimacron\fi{}}_{\mathrm{e}+\mathrm{p}+\mathrm{p}}$ at medium positron energy (i.e., 0.5--2.0 GeV) and small p-p excitation energies (i.e., ${E}_{\mathrm{rel}\mathrm{\ensuremath{\le}}10}$ MeV) is described. In view of the small p-p excitation energy ${E}_{\mathrm{rel}}$ being considered, we take into account only the $^{1}\mathrm{S}_{0}$ and $^{3}\mathrm{P}_{\mathrm{J}}$ p-p final states (J=0, 1, 2). For the polar vector current, an impulse approximation, modified to incorporate current conservation, is employed and additional meson-exchange currents are included in the dominant channel. For the axial vector current, the nucleon-only impulse approximation is used. Numerical results are presented to highlight the prospect of measuring the ${q}^{2}$ dependence of the nucleon axial form factor ${f}_{A}$(${q}^{2}$). The proposed precision measurement of the nucleon axial form factor may be feasible at the upcoming Continuous Electron Beam Accelerator Facility if the proposed positron ring can actually be constructed.

Status of the SLC Damping Rings

Electron beams of full design energy 1.21 GeV and nearly full design intensity 4×1010 particles/pulse (design 5×1010) have been extracted from the Stanford Linac and successfully stored in the electron damping ring. Beams of less intensity have been extracted from the ring and reinjected into the Linac. The present intensity limits are not thought to be fundamental. The operating experience with the electron ring and the status of the construction of the positron ring will be discussed.

Radiofrequency system for CHEER

The Institute of Particle Physics is preparing a proposal to design and build a 10 GeV electron–positron ring to study electron–proton collisions at the Fermi National Accelerator Laboratory using the 1000 GeV protons from the Tevatron. The design and operation of a high-power electron linear accelerator is very similar to the accelerating system in an electron–positron storage ring. This report describes the design of an rf system for the main ring which has many characteristics similar to that found in the Electron Test Accelerator at Chalk River. Discussion of quantum lifetime, the amplitude of synchrotron oscillations, and the consequent choice of the size of the beam aperture have led to the design of a new structure which operates at a higher frequency than that used in the large storage rings presently in operation.

An exact subdivision of the Radon transform and scanning with a positron ring camera

It has been pointed out (Cormack 1978) that if a beam of finite width, defined in a certain way, is used in the two-dimensional reconstruction of an object, the beam defines a finite region of the Radon transform of the object. A novel method of scanning was suggested in which the regions of the transform defined by different positions of the beam fitted together almost exactly in such a way that most of the transform was sampled only once, and only a small fraction of the transform was either not sampled or was sampled twice. A natural question to ask is whether there are other methods of scanning which sample the transform in such a way that the regions of the transform defined by different positions of the beam fit together exactly. The purpose of this Note is to point out that this is accomplished in an ideal positron ring scanner, and that this is closely related to the treatment of such a scanner by Marr (1974).

Single Beam Instabilities in Doris

In the electron-and positron rings of DORIS strong transverse and longitudinal single beam instabilities were observed, which caused current limitations. The severest transverse and longitudinal instabilities were induced by excitation of higher modes in the rf-cavities. When these instabilities were cured by damping the higher cavity modes, the thresholds for this type of instabilities were greatly increased. The origin of the remaining transverse and longitudinal instabilities is not yet understood. The thresholds of these instabilities were increased by Landau-damping and changing the betatron- and synchrotron frequencies from bunch to bunch. The head-tail effect was observed at high peak currents and was cured by compensating the chromaticity. In the electron ring a transverse instability is present which is induced by the ions. Ion clearing in combination with some vacuum improvements is sufficient to avoid this instability.