Positron Storage Ring Research Articles

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.

Effects of nuclear fusion produced neutrons on silicon semiconductor plasma X-ray detectors

The effects of nuclear fusion produced neutrons on the X-ray energy responses of semiconductor detectors are characterized. The degradation of the response of position-sensitive X-ray tomography detectors in the Joint European Torus (JET) tokamak is found after neutron exposure produced by deuterium–deuterium and deuterium–tritium plasma fusion experiments. For the purpose of further detailed characterization of the neutron degradation effects, an azimuthally varying-field (AVF) cyclotron accelerator is employed using well-calibrated neutron fluence. These neutron effects on the detector responses are characterized using synchrotron radiation from a 2.5 GeV positron storage ring at the Photon Factory (KEK). The effects of neutrons on X-ray sensitive semiconductor depletion thicknesses are also investigated using an impedance analyser. Novel findings of (i) the dependence of the response degradation on X-ray energies as well as (ii) the recovery of the degraded detector response due to the detector bias application are found and interpreted by systematically combined methods using a synchrotron facility, a cyclotron facility, a fusion plasma device, material analysis methods along with our recent theory on semiconductor X-ray responses.

Dynamical diffraction imaging of voids in nearly perfect silicon

X-ray diffraction topographs of extremely pure and perfect silicon singlecrystals are made using low-energy undulator radiation from a positron storagering. Typical defect images observed are rather large round images having ablack-white contrast and a diameter of about 40 µm. Applying the dynamicaltheory of x-ray diffraction, the defect contrast is explained by tensile strainin the lattice around voids close to the exit surface. This discovery ofvoid-like microdefects explains, at least in part, the reduced density of thecrystal intended to be used for a redefinition of the unit of mass, thekilogram.

Effective dose and organ doses due to gas bremsstrahlung from electron storage rings

Bremsstrahlung on residual gas is an important source of beam losses in electron–positron storage rings. The bremsstrahlung photons are emitted in a narrow cone in the forward direction, which produces a “hot spot” of dose at the end of a straight section. Estimates of radiation hazard due to gas bremsstrahlung have so far been performed by calculating the maximum dose equivalent (MADE) or similar quantities. However, the use of quantities conceived for broad parallel beams in the case of very narrow beams significantly overestimates the organ doses and effective dose. In this paper a more sophisticated computational model was used to calculate the values of effective dose and absorbed doses in various organs due to gas bremsstrahlung X-rays generated by 0.1–10 GeV electrons. The bremsstrahlung photons generated by the interaction of a monoenergetic electron beam in a 1 m long air target were made to impinge on a selected organ of an hermaphrodite anthropomorphic mathematical model placed at 1 and 10 m distances from the end of the target. Organ dose and effective dose were calculated for five representative organs, namely right eye, ovaries, breast, testes and thyroid. Fits to the calculated values are given, as well as the dependence of photon fluence and dosimetric quantities on various parameters. The results are compared with previous estimates based on MADE and with values of ambient dose equivalent.

Investigation of x-ray-energy responses of semiconductor detectors under deuterium–tritium fusion-produced neutron irradiation

For the purpose of investigating fusion-produced neutron effects on semiconductor x-ray detectors, detection characteristics of x-ray tomography detectors used before and after deuterium–tritium (DT) and/or DD fusion-plasma experiments in the Joint European Torus (JET) tokamak are studied using synchrotron radiation from a 2.5 GeV positron storage ring at the Photon Factory. Degradations in the responses after neutron exposure into the detectors are found to have functional dependence on x-ray energy. Changes in the depletion thicknesses of the detectors are investigated by means of impedance analyses. The Fusion Neutronics Source (FNS) facility of the Japan Atomic Energy Research Institute is also employed for well calibrated DT fusion-produced neutron irradiation onto these semiconductor x-ray detectors. Recovery of the response degradation is found due to a method for supplying the operational bias to the degraded detector. Our theory is applied to interpret these detector characteristics under the irradiation of DT fusion-produced neutrons in the JET tokamak and the FNS facility.

Head-tail instability caused by electron clouds in positron storage rings

In positron or proton storage rings with many closely spaced bunches, an electron cloud can build up in the vacuum chamber due to photoemission or secondary emission. We discuss the possibility of a single-bunch two-stream instability driven by this electron cloud. Depending on the strength of the beam-electron interaction, the chromaticity and the synchrotron oscillation frequency, this instability either resembles a linac beam breakup or a head-tail instability. We present computer simulations of the instabilities, and compare the simulation results with analytical estimates.

The pressure and gas composition evolution during the operation of the LEP accelerator at 100 GeV

The operation of the electron–positron storage ring LEP at an energy of 100 GeV leads to the deposition of a high-power flux on the vacuum chamber walls. The high critical energy of the photons emitted as synchrotron light (740 keV), places the photon-stimulated desorption in this machine in a completely new energy range compared to the existing electron storage rings. At this high photon energy, the energy dissipation of the photons in aluminium is done predominantly via the Compton process. This absorption mechanism leads to a larger enhancement of the gas desorption with the photon energy compared to the photoelectric effect. Measurements made at 100 GeV to study the variation of the pressure increase, of the gas composition and of the cleaning rates as a function of the beam dose and energy are presented and discussed in the context of the absorption of high-energy photons in aluminium.

CLEO B physics results

I summarize some recent results on B meson physics from the CLEO collaboration obtained with the Cornell electron–positron storage ring operating at the ϒ(4S) resonance: (a) determinations of the elements of the CKM matrix from semileptonic decays and (b) rare nonleptonic decays to charmless final states.

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.

Design, performance and status of the CLEO III silicon detector

Abstract The CLEO III silicon detector is part of a general upgrade of the CLEO detector to allow for operation at a luminosity of 2×10 33 cm −2 s −1 , which will be provided by the Cornell Electron–Positron Storage Ring (CESR) beginning in 1999. The silicon detector is a four-layer barrel design covering radii from 2.5 to 10.2 cm with 93% solid angle coverage. The silicon sensors are DC-coupled and double-sided with double-metal readout on the p-side. The n-type strips measure φ , with 50 μ m pitch while the p-type strips measure z , the coordinate along the beam axis, with 100 μ m pitch. The readout electronics are mounted on BeO hybrids attached to the conical support structure and connected to the silicon sensors via a thin kapton flex cable. The electronics consist of an R / C chip with bias resistors and decoupling capacitors, a low-noise preamp/shaper chip and a digitizer/sparsifier chip. Readout is done using VME-based sequencer boards. Production of all detector components is nearing completion and installation of the detector will take place in early 1999.

Particle dynamics in a storage ring with a longitudinal magnetic field

Abstract The use of an electron cooling system with a circulating electron beam for the Fermilab Recycler permits to avoid the problems of a traditional electron cooling system and to cool down antiprotons at an energy of 8 GeV. In this system the Recycler ring is equipped with an additional electron ring, which is periodically filled with new cold electrons. The electron beam circulates in a longitudinal (quasitoroidal) magnetic field and long-term stability of the beam is provided by additional spiral coils, which form a quadrupole electrostatic field. Such a ring (called a modified betatron) has been designed in JINR and now is under construction. This ring also was proposed to be used as low-energy positron storage ring (5–20 keV) for positronium generation. The electron cooling of positrons is an essential property of this system. The positronium is generated by recombination of positrons and electrons in the electron cooling section of the ring. The problems of storage of particles with a low momentum spread (Δ p / p ⩽10 −3 ) in a ring with a longitudinal magnetic field having a racetrack configuration is discussed.

Effects of neutrons on semiconductor x-ray detectors including n-type Joint European Torus and p-type GAMMA 10 tomography detectors

Characterization experiments have been carried out so as to investigate the effects of fusion-produced neutrons on the x-ray-energy responses of semiconductor detectors for x-ray tomography in the Joint European Torus (JET) tokamak (n-type silicon) and the GAMMA 10 tandem mirror (p-type silicon). Neutron effects on the x-ray-energy responses of these detectors are studied using synchrotron radiation from a 2.5 GeV positron storage ring at the Photon Factory. Changes in the material properties of the detectors have been investigated using an impedance analyzer to estimate neutron effects on x-ray-sensitive depletion thicknesses. A cyclotron accelerator is employed for well-calibrated neutron irradiation onto these plasma x-ray detectors; a fluence of 2–5×1013 neutrons/cm2 is utilized for simulating the effects of fusion-produced neutrons in JET. Modifications of the x-ray responses after neutron exposure due to fusion plasma shots in JET as well as cyclotron-produced neutron irradiations are found to have a functional dependence on x-ray energy. Also, recovery of the detector energy response is found when detector bias is applied. Our theory consistently interprets such properties in terms of the neutron effects on the diffusion length and the resistivity of detectors.

The HERMES polarized 3He internal gas target

The HERMES experiment is investigating the spin structure of the proton and neutron via deep-inelastic scattering of polarized positrons from polarized nuclear targets. The polarized positrons are provided by the HERA positron storage ring at DESY, Hamburg, Germany. The targets are pure internal gas targets. Data acquisition began in 1995, utilizing a polarized 3He internal gas target to study the spin structure of the neutron. The target gas was polarized using the metastability-exchange optical-pumping technique and then injected into a cryogenically cooled target cell. The target was designed to operate with either longitudinal or transverse directions of polarization. Operating conditions included polarizations of up to 54% and target thicknesses of 1×10 15 nucleons/cm 2. In this paper the HERMES polarized 3He internal gas target is described in detail.

Proposal of a high-field superconducting wiggler for a slow positron source at SPring-8.

A low-energy positron beam is a unique probe of materials. In high-energy electron and positron storage rings it is possible to generate intense synchrotron radiation with a photon energy of 1-3 MeV by installing a high-field (8-10 T) superconducting wiggler. High-energy photons are converted to low-energy positrons by using a suitable target-moderator system. For an 8 GeV electron storage ring at a beam current of 100 mA, final yields are estimated to be about 10(8)-10(10) slow-e(+) s(-1) or larger depending on the moderation efficiency, with the size of the positron source 10(1)-10(2) cm(2). In the present work a wiggler magnetic system of 10 T is proposed. The main parameters of the superconducting wiggler are presented.

Operational evaluation of ultrahigh-vacuum protection systems for intense-photon-flux wiggler beamlines.

The ultrahigh-vacuum (UHV) protection system at the 2.5 GeV synchrotron radiation source (positron storage ring) at the Photon Factory allows the intense-photon-flux wiggler beamlines to operate safely during synchrotron radiation experiments for a long time. There are six high-power wiggler/undulator beamlines that provide intense photon-flux beams to the experimental hall. In the case of a possible instantaneous vacuum failure at the experimental hall, the intense photon-flux radiation from the wiggler could cause a meltdown of the titanium-alloy fast-closing valve. The authors have developed a UHV protection system. Upon a vacuum failure, the protection system can dump the positron beam by turning off the RF power in the four RF klystrons (150 kW maximum), and then initiate a blade closure of the fast-closing valve. In this paper, the operational performance of the vacuum protection system for the intense-photon-flux wiggler beamlines is evaluated and discussed regarding the actual vacuum deterioration downstream of a beamline at the experimental hall.

Time-resolved surface magnetometry in the nanosecond scale using synchrotron radiation

The dynamics of magnetization reversal at surfaces and interfaces is studied by a specific photoelectric effect experiment with pulsed soft x rays. This novel time-resolved surface magnetometry is based on the measure of the spin polarization of the total photoejected electron yield by means of a Mott scattering experiment and on the acquisition synchronism with the multibunch structure of the positron storage ring SuperAco at Orsay (500 ps pulses at 120 ns intervals). We present results on the fast magnetization reversal of Fe ultrathin films deposited on amorphous low-coercivity ferromagnetic ribbons.

First results from the HERMES experiment

The Hermes experiment at the HERA proton positron collider has been designed to study the spin structure of the nucleon. Hermes measures the inclusive and semi-inclusive spin dependent deep inelastic scattering of polarized positrons off polarized nucleons with a large acceptance forward spectrometer. Two novel techniques are employed, the usage of an internal polarized gas target and the spin rotation of the transversely polarized positrons in the HERA positron storage ring. In 1995, its first year of data taking, Hermes measured the spin structure function g 1 n( x) of the neutron and its first moment Γ 1 n = −0.037 ± 0.013( stat) ± 0.005( sys) ± 0.006( extrapol) [1], in good agreement with existing data. As a first semi-inclusive result, the hadron asymmetries have been extracted from the 1995 data. They are slightly negative and have small statistical errors. Additionally, the angular distribution of π + mesons from the ϱ 0 → π +π − decay could be extracted with high accuracy. this yields the ratio of longitudinal to transversely polarized ϱ 0s, assuming the validity of s-channel helicity conservation. The comparison of ϱ 0 production on 3He and H targets can be used to investigate possible effects of nuclear transparency. The data on hydrogen measured in 1996 are a factor of 4 better in figure of merit. The conversion of the existing threshold Čerenkov counter into a RICH detector, which will take place during the 1997 winter shutdown, will significantly improve the particle identification capabilities of Hermes.

Use of lead-glass capillaries for micro-focusing of highly-energetic (0–60 keV) synchrotron radiation

The performance of ellipsoidally shaped lead-glass capillaries for focusing the polychromatic synchrotron beam produced by a bending magnet of the DORIS positron storage ring (Hasylab, Hamburg, Germany) is discussed. The size, intensity and energy distribution of the focused beam produced by such capillaries are compared with those of beams generated by means of straight borosilicate capillaries, indicating that beam sizes ofca. 4 µm at the sample surface can be obtained with a total flux density that is ca. ten times higher than when a collimated beam is employed. Synchrotron radiation with energies up to 60 keV is focused, leaving the original energy distribution of the white synchrotron beam virtually unchanged. The analytical characteristics of the µ-XRF set-up at Beamline L of Hasylab, when equipped with a lead-glass capillary, were investigated by means of NIST SRMs and indicate that interference-free absolute/relative detection limits in the 1–10 fg/0.8–2 ppm range are achievable from 100 µm silicate-type samples for the elements from Mn (Z=25) to Gd (Z=64) using their Kα lines within 1000 s counting time. Elemental yields are situated in the 10–100 counts s–1 per 100 mA per (µg cm–2) range. As illustrations of the type of investigations these highly energetic, micrometre-sized beams make possible, the two-dimensional mapping of the distribution of REEs (rare earth elements) and other heavy elements in geological igneous rock samples and the three-dimensional non-destructive analysis of heavy metals (such as V, Fe, Ni and Mo) in individual fly-ash particles by means of fluorescence microtomography are briefly described.

An estimate of gas desorption in the damping rings of the next linear collider

We express the gas desorption in a high-energy electron or positron storage ring in terms of the photoemission and the electron-induced gas-desorption yields of the vacuum-chamber material. The derived expression is used to estimate the gas desorption in the two main damping rings of the Next Linear Collider (NLC) and, in particular, its dependence on the height of the slots which connect the beam pipe proper to an antechamber.

Conceptual design of the low energy positron storage ring

A method for the generation of intense antihydrogen and positronium streams using a low energy positron storage ring is considered. A longitudinal magnetic field for the focusing system and electron cooling of the positrons are essential features of the ring. The general limitations of the storage ring parameters are determined by the relation between scattering and cooling processes. The positron beam energy is determined by physics processes which are to be studied; it lies in the range from 5 up to 30 keV. The positron lifetime is about 100 s at a vacuum pressure of 100 pTorr. The generation of slow antihydrogen atoms by using electrostatic deceleration of positrons and antiprotons is described. The lowest antihydrogen energy in such a scheme is about a few eV and the generation rate is about 1 atom per second. The positron storage ring design is described. The general parameters of the positron storage ring, which is under design in JINR, are presented. The main purpose of the proposed machine construction is the performance of experimental studies of intense positron beam storing, electron cooling and deceleration in a straight section.