A. Zholents - Academia.edu (original) (raw)

Papers by A. Zholents

Research paper thumbnail of The New Undulator Based FS-Slicing Beamline at the ALS

Proceedings of the 2005 Particle Accelerator Conference

The Femtoslicing beamline at the ALS employs a fs laser beam interacting resonantly with the elec... more The Femtoslicing beamline at the ALS employs a fs laser beam interacting resonantly with the electron beam in a wiggler. The induced energy spread over the fs duration is converted to a transverse displacement by exploiting the storage ring dispersion. The displaced fs pulse radiates and produces fs synchrotron radiation. Up to now a regular bending magnet was used as radiator. To improve the flux, a significant upgrade was implemented, replacing the modulator, installing an in-vacuum undulator as new radiator, and installing a higher repeptition rate laser system. The new beamline will provide 100-200 fs long pulses of soft and hard x-rays with a repetition rate of 10-40 kHz for experiments concerning ultrafast dynamics in solid state physics, chemistry and biology.

Research paper thumbnail of Design and simulation challenges for FERMI@elettra

Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2009

FERMI@elettra is a fourth-generation light source user facility under construction at the Elettra... more FERMI@elettra is a fourth-generation light source user facility under construction at the Elettra Laboratory in Trieste, Italy. The high-quality 1.2 GeV electron beam drives two-seeded Free Electron Lasers (FELs) in the wavelength range 100À10 nm. Wavelength tunability, variable polarization and higher electron beam energies to reach even shorter output wavelengths are also in the machine delivery plan. This paper describes the physics processes that have been modelled to simulate FERMI@elettra and the computer codes used to optimize the machine design. The paper focuses on several design challenges and how these translate into modelling and simulation challenges.

Research paper thumbnail of Pulsed Wire Measurements of a High Field Gradient Quadrupole Wiggler

Alignment of the quadrupoles in a quadrupole wiggler to sub micrometer precision is required for ... more Alignment of the quadrupoles in a quadrupole wiggler to sub micrometer precision is required for the collinear wakefield accelerator that is under consideration at Argonne National Laboratory for a compact Free-Electron Laser [1]. The pulsed wire measurement method is the only technique that we are aware of that allows for sub micrometer precision and the ability to distinguish between the various quadrupoles within the wiggler. A one period prototype wiggler was manufactured and subsequently measured using the pulsed wire technique. The goal of the measurements was to verify that the magnetic centers of each quadrupole could be located and aligned to each other within the required precision. The method and results are described.

Research paper thumbnail of Investigation of Various Fabrication Methods to Produce a 180GHz Corrugated Waveguide Structure in 2mm Diameter ­0.5m ­Long Copper Tube for the Compact Wakefield Accelerator for FEL Facility

Argonne National Laboratory is developing a 180 GHz wakefield structure that will house in a co-l... more Argonne National Laboratory is developing a 180 GHz wakefield structure that will house in a co-linear array of accelerators to produce free-electron laser-based X-rays. The proposed corrugated waveguide structure will be fabricated on the internal wall of 0.5m long and 2mm nominal diameter copper tube. The estimated dimensions of these parallel corrugations are 200 µm in pitch with 100 µm side length (height and width). The length scale of the structure and requirements of the magnetic field-driven dimensional tolerances have made the structure challenging to produce. We have employed several method such as optical lithography, electroforming, electron discharge machining, laser ablation, and stamping to produce the initial structure from a sheet form. The successive fabrication steps, such as bending, brazing, and welding, were performed to achieve the long tubular-structure. This paper discusses various fabrication techniques, characterization, and associated technical challenges...

Research paper thumbnail of A Summary of the Advanced Photon Source (Aps) Short Pulse X-Ray (SPX) R&D Accomplishments*

The Advanced Photon Source at Argonne conducted R&D on design, fabrication, prototyping, and test... more The Advanced Photon Source at Argonne conducted R&D on design, fabrication, prototyping, and testing of superconducting deflecting cavities aimed at the production of short x-ray pulses at the Advanced Photon Source. In collaboration with Jefferson National Laboratory, we focused on prototyping and testing a number of single-cell deflecting cavities. At ANL, we designed, prototyped, and tested silicon carbide as damping material for higher-order-mode (HOM) dampers, which are broadband, to handle the HOM power across the frequency spectrum produced by the APS beam. In collaboration with Lawrence Berkeley National Laboratory, we have been developing state-of-the-art timing and synchronization system for distributing stable rf signals over optical fiber capable of achieving tens of femtoseconds phase drift and jitter. Collaboration with the Advanced Computations Department at Stanford Linear Accelerator Center looked into simulations of complex, multi-cavity geometries. This contributi...

Research paper thumbnail of Development and operating experience of a 1.1-m-long superconducting undulator at the Advanced Photon Source

Physical Review Accelerators and Beams, 2017

Research paper thumbnail of Observation of High Transformer Ratio of Shaped Bunch Generated by an Emittance-Exchange Beam Line

Physical Review Letters, 2018

Collinear wakefield acceleration has been long-established as a method capable of generating ultr... more Collinear wakefield acceleration has been long-established as a method capable of generating ultrahigh acceleration gradients. Due to the success on this front, recently, more efforts have shifted towards developing methods to raise the transformer ratio (TR). This figure of merit is defined as the ratio of the peak acceleration field behind the drive bunch to the peak deceleration field inside the drive bunch. TR is always less than two for temporally symmetric drive bunch distributions and therefore recent efforts have focused on generating asymmetric distributions to overcome this limitation. In this paper, we report on using the emittance-exchange method to generate a shaped drive bunch to experimentally demonstrate a T R ≈ 5 in a dielectric wakefield accelerator. 65 demonstrate that TR higher than 4.5 is achieved. 66 The experimental setup is shown in Fig. 1. A 20-nC, 67 8 MeV electron bunch is generated by an L-band 1.5 cell 68 photoinjector and accelerated by 6 standing-wave acceler-69 ating structures (not shown) to 48 MeV. The linac phase 70 is negative 15 degree with respect to crest (0 •) in order to 71 suppress the energy chirp. This bunch is then sent into 72 the experimental area containing the EEX beamline con-73 sisting of the transverse-deflecting cavity (TDC1) flanked 74 by two identical dogleg sections.

Research paper thumbnail of Upper limit for the accelerating gradient in the collinear wakefield accelerator as a function of the transformer ratio

Physical Review Accelerators and Beams, 2017

Research paper thumbnail of Precision Control of the Electron Longitudinal Bunch Shape Using an Emittance-Exchange Beam Line

Physical review letters, Jan 10, 2017

We report on the experimental generation of relativistic electron bunches with a tunable longitud... more We report on the experimental generation of relativistic electron bunches with a tunable longitudinal bunch shape. A longitudinal bunch-shaping (LBS) beam line, consisting of a transverse mask followed by a transverse-to-longitudinal emittance exchange (EEX) beam line, is used to tailor the longitudinal bunch shape (or current profile) of the electron bunch. The mask shapes the bunch's horizontal profile, and the EEX beam line converts it to a corresponding longitudinal profile. The Argonne wakefield accelerator rf photoinjector delivers electron bunches into a LBS beam line to generate a variety of longitudinal bunch shapes. The quality of the longitudinal bunch shape is limited by various perturbations in the exchange process. We develop a simple method, based on the incident slope of the bunch, to significantly suppress the perturbations.

Research paper thumbnail of Monochromatic design for the Apiary B-factory

AIP Conference Proceedings, 1990

ABSTRACT

Research paper thumbnail of Design of a Quasi-waveguide Multicell Deflecting Cavity for the Advanced Photon Source

Physics Procedia, 2015

This paper reports the electromagnetic design of a 2815 MHz Quasi-waveguide Multicell Resonator (... more This paper reports the electromagnetic design of a 2815 MHz Quasi-waveguide Multicell Resonator (called QMiR) being considered as a transverse RF deflecting cavity for the Advanced Photon Source's (APS) Short Pulse X-ray project. QMiR forms a trapped dipole mode inside a beam vacuum chamber while High Order Modes (HOM) are heavily loaded. It results a sparse HOM spectrum, makes HOM couplers unnecessary and allows simplifying the cavity mechanical design. The form of electrodes is optimized for producing 2 MV of deflecting voltage and keeping low peak surface electric and magnetic fields of 54 MV/m and 75 mT respectively. Results of detailed EM analysis, including HOM damping at the actual geometry of beam vacuum chamber, will be presented.

Research paper thumbnail of Towards a Practical Multi-Meter Long Dielectric Wakefield Accelerator: Problems and Solutions

IEEE Transactions on Nuclear Science, 2016

Research paper thumbnail of Development and operating experience of a short-period superconducting undulator at the Advanced Photon Source

Physical Review Special Topics - Accelerators and Beams, 2015

Research paper thumbnail of Low Energy Ring lattice of the PEP-II asymmetric B-factory

Proceedings Particle Accelerator Conference

Developing a lattice that contains a very low beta value at the interaction point (IP) and has ad... more Developing a lattice that contains a very low beta value at the interaction point (IP) and has adequate dynamic aperture is one of the major challenges in designing the PEP-II asymmetric Bfactory. For the Low Energy Ring (LER) we have studied several different chromatic correction schemes since the conceptual design report (CDR) [1]. Based on these studies, a hybrid solution with local and semi-local chromatic sextupoles has been selected as the new baseline lattice to replace the local scheme in the CDR [2]. The new design simplifies the interaction region (IR) and reduces the number of sextupoles in the arcs. Arc sextupoles are paired at phase difference and are not interleaved. In this paper we describe the baseline lattice with the emphasis on the lattice changes made since the CDR.

Research paper thumbnail of Study on a test of optical stochastic cooling scheme in a single pass beam line

Research paper thumbnail of Methods of Attosecond X-Ray Pulse Generation

Proceedings of the 2005 Particle Accelerator Conference

We review several proposals for generation of solitary attosecond pulses using two types of free ... more We review several proposals for generation of solitary attosecond pulses using two types of free electron lasers which are envisioned as future light sources for studies of ultra-fast dynamics using soft and hard x-rays.

Research paper thumbnail of Electron beam-based sources of ultrashort x-ray pulses

A review of various methods for generation of ultrashort x-ray pulses using relativistic electron... more A review of various methods for generation of ultrashort x-ray pulses using relativistic electron beam from conventional accelerators is presented. Both spontaneous and coherent emission of electrons is considered.

Research paper thumbnail of Generation of short X-ray pulses using harmonics and beam conditioning

ABSTRACT If an optical or UV laser seed is used to modulate a longitudinal slice of an electron b... more ABSTRACT If an optical or UV laser seed is used to modulate a longitudinal slice of an electron beam, the modulated electrons can then produce radiation at a higher harmonic after entering a differently tuned undulator. Repeated stages form a ``harmonic cascade'' which yield very energetic photons. The timing and duration of the resulting X-ray pulse will be determined by that of the initial laser seed. Furthermore, the coherence of the output radiation will reflect that of the initial laser seed, allowing for the possibility of pulses whose bandwidth is limited only by the total number of wavelengths in the pulse rather than by the number of undulator periods (``bandwidth limited''). We study ways to generate large power output in a relatively short sytem, either by avoiding the need to amplify over many gain lengths or by reducing the gain length. In addition to varying the undulator design, we also consider the use of beam conditioning. Beam conditioning is a correlation between electron energy and transverse amplitude that is introduced in order to reduce some of the deleterious effects of beam emittance, which is a major limitation to FEL performance.

Research paper thumbnail of Trajectory Jitter and Single Bunch Beam Break Up Instability

ABSTRACT This paper addresses stability issues related to control of the single bunch beam break ... more ABSTRACT This paper addresses stability issues related to control of the single bunch beam break up (SBBU) instability in the FERMI@Elettra linac using local trajectory bumps [1]. Analytical study and simulations using the code elegant are presented. Three different parameters have been used to characterize the SBBU, i.e. the projected emittance, the bunch head-to-tail deviation, and the Courant-Snyder amplitude for the slice centroid. It is shown that shot-to-shot trajectory jitter in the injector affects the efficiency of the control of the BBU.

Research paper thumbnail of Non-gaussian beam tails at the Advanced Light Source

The coordinate and energy distributions of the electron beam density at the Advanced Light Source... more The coordinate and energy distributions of the electron beam density at the Advanced Light Source have been obtained from measurements of the beam lifetime at various storage-ring apertures in the horizontal and vertical planes. It is shown that these distributions have a Gaussian core extended approximately up to five rms beam sizes. Beyond this point the electron density is defined by the electron scattering on the residue gas and electron intra-beam scattering and decreases as the cubic of the distance from the beam center. This behavior continues approximately up to ¿¼ rms beam sizes in the horizontal plane and ¼ rms beam sizes in the vertical plane.

Research paper thumbnail of The New Undulator Based FS-Slicing Beamline at the ALS

Proceedings of the 2005 Particle Accelerator Conference

The Femtoslicing beamline at the ALS employs a fs laser beam interacting resonantly with the elec... more The Femtoslicing beamline at the ALS employs a fs laser beam interacting resonantly with the electron beam in a wiggler. The induced energy spread over the fs duration is converted to a transverse displacement by exploiting the storage ring dispersion. The displaced fs pulse radiates and produces fs synchrotron radiation. Up to now a regular bending magnet was used as radiator. To improve the flux, a significant upgrade was implemented, replacing the modulator, installing an in-vacuum undulator as new radiator, and installing a higher repeptition rate laser system. The new beamline will provide 100-200 fs long pulses of soft and hard x-rays with a repetition rate of 10-40 kHz for experiments concerning ultrafast dynamics in solid state physics, chemistry and biology.

Research paper thumbnail of Design and simulation challenges for FERMI@elettra

Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2009

FERMI@elettra is a fourth-generation light source user facility under construction at the Elettra... more FERMI@elettra is a fourth-generation light source user facility under construction at the Elettra Laboratory in Trieste, Italy. The high-quality 1.2 GeV electron beam drives two-seeded Free Electron Lasers (FELs) in the wavelength range 100À10 nm. Wavelength tunability, variable polarization and higher electron beam energies to reach even shorter output wavelengths are also in the machine delivery plan. This paper describes the physics processes that have been modelled to simulate FERMI@elettra and the computer codes used to optimize the machine design. The paper focuses on several design challenges and how these translate into modelling and simulation challenges.

Research paper thumbnail of Pulsed Wire Measurements of a High Field Gradient Quadrupole Wiggler

Alignment of the quadrupoles in a quadrupole wiggler to sub micrometer precision is required for ... more Alignment of the quadrupoles in a quadrupole wiggler to sub micrometer precision is required for the collinear wakefield accelerator that is under consideration at Argonne National Laboratory for a compact Free-Electron Laser [1]. The pulsed wire measurement method is the only technique that we are aware of that allows for sub micrometer precision and the ability to distinguish between the various quadrupoles within the wiggler. A one period prototype wiggler was manufactured and subsequently measured using the pulsed wire technique. The goal of the measurements was to verify that the magnetic centers of each quadrupole could be located and aligned to each other within the required precision. The method and results are described.

Research paper thumbnail of Investigation of Various Fabrication Methods to Produce a 180GHz Corrugated Waveguide Structure in 2mm Diameter ­0.5m ­Long Copper Tube for the Compact Wakefield Accelerator for FEL Facility

Argonne National Laboratory is developing a 180 GHz wakefield structure that will house in a co-l... more Argonne National Laboratory is developing a 180 GHz wakefield structure that will house in a co-linear array of accelerators to produce free-electron laser-based X-rays. The proposed corrugated waveguide structure will be fabricated on the internal wall of 0.5m long and 2mm nominal diameter copper tube. The estimated dimensions of these parallel corrugations are 200 µm in pitch with 100 µm side length (height and width). The length scale of the structure and requirements of the magnetic field-driven dimensional tolerances have made the structure challenging to produce. We have employed several method such as optical lithography, electroforming, electron discharge machining, laser ablation, and stamping to produce the initial structure from a sheet form. The successive fabrication steps, such as bending, brazing, and welding, were performed to achieve the long tubular-structure. This paper discusses various fabrication techniques, characterization, and associated technical challenges...

Research paper thumbnail of A Summary of the Advanced Photon Source (Aps) Short Pulse X-Ray (SPX) R&D Accomplishments*

The Advanced Photon Source at Argonne conducted R&D on design, fabrication, prototyping, and test... more The Advanced Photon Source at Argonne conducted R&D on design, fabrication, prototyping, and testing of superconducting deflecting cavities aimed at the production of short x-ray pulses at the Advanced Photon Source. In collaboration with Jefferson National Laboratory, we focused on prototyping and testing a number of single-cell deflecting cavities. At ANL, we designed, prototyped, and tested silicon carbide as damping material for higher-order-mode (HOM) dampers, which are broadband, to handle the HOM power across the frequency spectrum produced by the APS beam. In collaboration with Lawrence Berkeley National Laboratory, we have been developing state-of-the-art timing and synchronization system for distributing stable rf signals over optical fiber capable of achieving tens of femtoseconds phase drift and jitter. Collaboration with the Advanced Computations Department at Stanford Linear Accelerator Center looked into simulations of complex, multi-cavity geometries. This contributi...

Research paper thumbnail of Development and operating experience of a 1.1-m-long superconducting undulator at the Advanced Photon Source

Physical Review Accelerators and Beams, 2017

Research paper thumbnail of Observation of High Transformer Ratio of Shaped Bunch Generated by an Emittance-Exchange Beam Line

Physical Review Letters, 2018

Collinear wakefield acceleration has been long-established as a method capable of generating ultr... more Collinear wakefield acceleration has been long-established as a method capable of generating ultrahigh acceleration gradients. Due to the success on this front, recently, more efforts have shifted towards developing methods to raise the transformer ratio (TR). This figure of merit is defined as the ratio of the peak acceleration field behind the drive bunch to the peak deceleration field inside the drive bunch. TR is always less than two for temporally symmetric drive bunch distributions and therefore recent efforts have focused on generating asymmetric distributions to overcome this limitation. In this paper, we report on using the emittance-exchange method to generate a shaped drive bunch to experimentally demonstrate a T R ≈ 5 in a dielectric wakefield accelerator. 65 demonstrate that TR higher than 4.5 is achieved. 66 The experimental setup is shown in Fig. 1. A 20-nC, 67 8 MeV electron bunch is generated by an L-band 1.5 cell 68 photoinjector and accelerated by 6 standing-wave acceler-69 ating structures (not shown) to 48 MeV. The linac phase 70 is negative 15 degree with respect to crest (0 •) in order to 71 suppress the energy chirp. This bunch is then sent into 72 the experimental area containing the EEX beamline con-73 sisting of the transverse-deflecting cavity (TDC1) flanked 74 by two identical dogleg sections.

Research paper thumbnail of Upper limit for the accelerating gradient in the collinear wakefield accelerator as a function of the transformer ratio

Physical Review Accelerators and Beams, 2017

Research paper thumbnail of Precision Control of the Electron Longitudinal Bunch Shape Using an Emittance-Exchange Beam Line

Physical review letters, Jan 10, 2017

We report on the experimental generation of relativistic electron bunches with a tunable longitud... more We report on the experimental generation of relativistic electron bunches with a tunable longitudinal bunch shape. A longitudinal bunch-shaping (LBS) beam line, consisting of a transverse mask followed by a transverse-to-longitudinal emittance exchange (EEX) beam line, is used to tailor the longitudinal bunch shape (or current profile) of the electron bunch. The mask shapes the bunch's horizontal profile, and the EEX beam line converts it to a corresponding longitudinal profile. The Argonne wakefield accelerator rf photoinjector delivers electron bunches into a LBS beam line to generate a variety of longitudinal bunch shapes. The quality of the longitudinal bunch shape is limited by various perturbations in the exchange process. We develop a simple method, based on the incident slope of the bunch, to significantly suppress the perturbations.

Research paper thumbnail of Monochromatic design for the Apiary B-factory

AIP Conference Proceedings, 1990

ABSTRACT

Research paper thumbnail of Design of a Quasi-waveguide Multicell Deflecting Cavity for the Advanced Photon Source

Physics Procedia, 2015

This paper reports the electromagnetic design of a 2815 MHz Quasi-waveguide Multicell Resonator (... more This paper reports the electromagnetic design of a 2815 MHz Quasi-waveguide Multicell Resonator (called QMiR) being considered as a transverse RF deflecting cavity for the Advanced Photon Source's (APS) Short Pulse X-ray project. QMiR forms a trapped dipole mode inside a beam vacuum chamber while High Order Modes (HOM) are heavily loaded. It results a sparse HOM spectrum, makes HOM couplers unnecessary and allows simplifying the cavity mechanical design. The form of electrodes is optimized for producing 2 MV of deflecting voltage and keeping low peak surface electric and magnetic fields of 54 MV/m and 75 mT respectively. Results of detailed EM analysis, including HOM damping at the actual geometry of beam vacuum chamber, will be presented.

Research paper thumbnail of Towards a Practical Multi-Meter Long Dielectric Wakefield Accelerator: Problems and Solutions

IEEE Transactions on Nuclear Science, 2016

Research paper thumbnail of Development and operating experience of a short-period superconducting undulator at the Advanced Photon Source

Physical Review Special Topics - Accelerators and Beams, 2015

Research paper thumbnail of Low Energy Ring lattice of the PEP-II asymmetric B-factory

Proceedings Particle Accelerator Conference

Developing a lattice that contains a very low beta value at the interaction point (IP) and has ad... more Developing a lattice that contains a very low beta value at the interaction point (IP) and has adequate dynamic aperture is one of the major challenges in designing the PEP-II asymmetric Bfactory. For the Low Energy Ring (LER) we have studied several different chromatic correction schemes since the conceptual design report (CDR) [1]. Based on these studies, a hybrid solution with local and semi-local chromatic sextupoles has been selected as the new baseline lattice to replace the local scheme in the CDR [2]. The new design simplifies the interaction region (IR) and reduces the number of sextupoles in the arcs. Arc sextupoles are paired at phase difference and are not interleaved. In this paper we describe the baseline lattice with the emphasis on the lattice changes made since the CDR.

Research paper thumbnail of Study on a test of optical stochastic cooling scheme in a single pass beam line

Research paper thumbnail of Methods of Attosecond X-Ray Pulse Generation

Proceedings of the 2005 Particle Accelerator Conference

We review several proposals for generation of solitary attosecond pulses using two types of free ... more We review several proposals for generation of solitary attosecond pulses using two types of free electron lasers which are envisioned as future light sources for studies of ultra-fast dynamics using soft and hard x-rays.

Research paper thumbnail of Electron beam-based sources of ultrashort x-ray pulses

A review of various methods for generation of ultrashort x-ray pulses using relativistic electron... more A review of various methods for generation of ultrashort x-ray pulses using relativistic electron beam from conventional accelerators is presented. Both spontaneous and coherent emission of electrons is considered.

Research paper thumbnail of Generation of short X-ray pulses using harmonics and beam conditioning

ABSTRACT If an optical or UV laser seed is used to modulate a longitudinal slice of an electron b... more ABSTRACT If an optical or UV laser seed is used to modulate a longitudinal slice of an electron beam, the modulated electrons can then produce radiation at a higher harmonic after entering a differently tuned undulator. Repeated stages form a ``harmonic cascade'' which yield very energetic photons. The timing and duration of the resulting X-ray pulse will be determined by that of the initial laser seed. Furthermore, the coherence of the output radiation will reflect that of the initial laser seed, allowing for the possibility of pulses whose bandwidth is limited only by the total number of wavelengths in the pulse rather than by the number of undulator periods (``bandwidth limited''). We study ways to generate large power output in a relatively short sytem, either by avoiding the need to amplify over many gain lengths or by reducing the gain length. In addition to varying the undulator design, we also consider the use of beam conditioning. Beam conditioning is a correlation between electron energy and transverse amplitude that is introduced in order to reduce some of the deleterious effects of beam emittance, which is a major limitation to FEL performance.

Research paper thumbnail of Trajectory Jitter and Single Bunch Beam Break Up Instability

ABSTRACT This paper addresses stability issues related to control of the single bunch beam break ... more ABSTRACT This paper addresses stability issues related to control of the single bunch beam break up (SBBU) instability in the FERMI@Elettra linac using local trajectory bumps [1]. Analytical study and simulations using the code elegant are presented. Three different parameters have been used to characterize the SBBU, i.e. the projected emittance, the bunch head-to-tail deviation, and the Courant-Snyder amplitude for the slice centroid. It is shown that shot-to-shot trajectory jitter in the injector affects the efficiency of the control of the BBU.

Research paper thumbnail of Non-gaussian beam tails at the Advanced Light Source

The coordinate and energy distributions of the electron beam density at the Advanced Light Source... more The coordinate and energy distributions of the electron beam density at the Advanced Light Source have been obtained from measurements of the beam lifetime at various storage-ring apertures in the horizontal and vertical planes. It is shown that these distributions have a Gaussian core extended approximately up to five rms beam sizes. Beyond this point the electron density is defined by the electron scattering on the residue gas and electron intra-beam scattering and decreases as the cubic of the distance from the beam center. This behavior continues approximately up to ¿¼ rms beam sizes in the horizontal plane and ¼ rms beam sizes in the vertical plane.