Bunch Current Research Articles

A Code for Bunch Lengthening Simulation

Bunch lengthening phenomenon is resulted from one of the most severe single bunch instabilities in storage rings. We develop a new code to calculate the single bunch length and energy spread in storage rings using FORTRAN. In this code, wake field is calculated using an analytical formula, which is different from the previous ones. The bunch length and energy spread under different bunch currents are calculated for BEPCII by using this code, and the tracking results are in good agreement with those from other codes. The calculated energy spread clearly shows that the longitudinal microwave instability threshold is around 65mA for BEPCII storage ring.

Measurement of the real time fill-pattern at the Australian Synchrotron

This article describes the development, commissioning and operation of a Fill-Pattern Monitor (FPM) for the Australian Synchrotron that measures the real-time intensity distribution of the electron bunches in the storage ring. Using a combination of an ultra-fast photodiode and a high-speed digitiser, real-time measurement of the fill-pattern at bunch-by-bunch resolution was achieved. The results compare very well with current methods of measuring the fill-pattern, such as a pick-up style detector. In addition, the FPM is fully integrated into the EPICS control system. The data provided by the FPM gives accurate RF bucket position and relative bunch currents over a wide range of stored beam currents, from 0.01mA in a single bunch to 200mA total beam current. The FPM monitors the success of an injection attempt into the storage ring and is used in a feedback loop to determine where to target the next injection. Using the FPM a beam top-up mode was successfully tested, resulting in a near constant beam current by periodic targeted injections over an 8h shift. Results are presented for dynamically topped up real-time injection, where the beam pattern was squared using an intensity-dependent injection algorithm.

Laser Seeding of the Storage-Ring Microbunching Instability for High-Power Coherent Terahertz Radiation

We report the first observation of laser seeding of the storage-ring microbunching instability. Above a threshold bunch current, the interaction of the beam and its radiation results in a coherent instability, observed as a series of stochastic bursts of coherent synchrotron radiation (CSR) at terahertz frequencies initiated by fluctuations in the beam density. We have observed that this effect can be seeded by imprinting an initial density modulation on the beam by means of laser "slicing." In such a situation, most of the bursts of CSR become synchronous with the pulses of the modulating laser and their average intensity scales exponentially with the current per bunch. We present detailed experimental observations of the seeding effect and a model of the phenomenon. This seeding mechanism also creates potential applications as a high-power source of CSR at terahertz frequencies.

Very low emittance light source storage ring

A very low emittance storage ring with an energy of 6 GeV and a circumference of 1999 m has been designed for use as a synchrotron radiation source. The ring consists of 32 ten-bend achromat cells and has a natural emittance of 34×10 −12 m rad. Simulation showed that the horizontal dynamic apertures at the center of a straight section are −4.2 and +2.9 mm and that they can be increased to −5.4 and +7.9 mm, respectively by changing the strength of sextupole magnets used for chromaticity correction. Emittance growth due to intrabeam scattering and Touschek lifetime were estimated. With 0.1% coupling, the horizontal emittance was 109×10 −12 m rad, and the lifetime was 0.17 h for a 0.1-mA bunch current. With 100% coupling, it was 21×10 −12 m rad, and the lifetime was 2.0 h. These results indicate that a storage ring with picometer-order emittance is possible with realistic parameters and is thus promising as a next-generation light source.

Modeling colliding beams with an element by element representation of the storage ring guide field

A detailed model of the Cornell Electron Storage Ring (CESR) guide field, including beam-beam interaction computed in the weak-strong regime, is the basis for a multiturn simulation of luminosity. The simulation reproduces the dependence of luminosity on bunch current that is measured in the storage ring, at both high-energy ($5.3\text{ }\mathrm{GeV}/\mathrm{\text{beam}}$) and in the wiggler-dominated low energy ($\mathrm{CESR}\mathrm{\text{\ensuremath{-}}}c$) configuration ($1.9\text{ }\mathrm{GeV}/\mathrm{\text{beam}}$). Dynamics are determined entirely by the physics of propagation through the individual guide field elements with no free parameters. Energy dependence of the compensation of the transverse coupling introduced by the experimental solenoid is found to significantly degrade specific luminosity. The simulation also indicates a strong dependence of limiting beam-beam tune shift parameter on the geometric mean of synchrotron tune and bunch length.

Observation of Anomalously Large Spectral Bandwidth in a High-Gain Self-Amplified Spontaneous Emission Free-Electron Laser

Observation of ultrawide bandwidth, up to 15% full-width, high-gain operation of a self-amplified spontaneous emission free-election laser (SASE FEL) is reported. This type of lasing is obtained with a strongly chirped beam (deltaE/E approximately 1.7%) emitted from the accelerator. Because of nonlinear pulse compression during transport, a short, high current bunch with strong mismatch errors is injected into the undulator, giving high FEL gain. Start-to-end simulations reproduce key features of the measurements and provide insight into mechanisms, such as angular spread in emitted photon and electron trajectory distributions, which yield novel features in the radiation spectrum.

Development of wing-type HOM damper for KEKB

The KEK B-factory (KEKB) is a high-current electron–positron collider aiming at a luminosity goal of 1×10 34 cm −2 s −1 . The vacuum components are likely to be annoyed by the intense higher order modes (HOM) excited by the high bunch current. Some bellows near special vacuum components, for example, were heated up due to the HOM and their RF-shield fingers were damaged during the beam operation. To solve these HOM-related problems, and also to prepare for a future higher current operation, a wing-type HOM damper equipped with SiC HOM absorbers were developed to absorb the unnecessary HOM, especially TE-mode like HOM with a power of several kW. They were installed in the KEKB positron ring and relieved the heating of bellows and pump elements effectively. The maximum absorbed HOM power was about 5.4 kW for one HOM damper at the stored beam current of 1.6 A (bunch number=1284 and bunch length=7 mm). The structure and the properties of the wing-type HOM damper are reported, as are the results of beam tests in the KEKB.

Development of wing-type HOM damper for KEKB

The KEK B-factory (KEKB) is a high-current electron–positron collider aiming at a luminosity goal of 1×1034cm−2s−1. The vacuum components are likely to be annoyed by the intense higher order modes (HOM) excited by the high bunch current. Some bellows near special vacuum components, for example, were heated up due to the HOM and their RF-shield fingers were damaged during the beam operation. To solve these HOM-related problems, and also to prepare for a future higher current operation, a wing-type HOM damper equipped with SiC HOM absorbers were developed to absorb the unnecessary HOM, especially TE-mode like HOM with a power of several kW. They were installed in the KEKB positron ring and relieved the heating of bellows and pump elements effectively. The maximum absorbed HOM power was about 5.4kW for one HOM damper at the stored beam current of 1.6A (bunch number=1284 and bunch length=7mm). The structure and the properties of the wing-type HOM damper are reported, as are the results of beam tests in the KEKB.

The SPARC/X SASE-FEL Projects

SPARC and SPARX are two different initiatives toward an ItalianFreeElectronLaser (FEL) source operating in theSelfAmplifiedSpontaneousEmission (SASE) mode, in which several national research institutions are involved. SPARC is a high gain FEL project devoted to provide a source of visible and VUV radiation while exploiting the SASE mechanism. An advanced Photo-Injector system, emittance compensating RF-gun plus a 150 MeV Linac, will inject a high quality e-beam into the undulator to generate high brilliance FEL radiation in the visible region at the fundamental wavelength, (∼500 nm). The production of flat top drive laser beams, high peak current bunches, and emittance compensation scheme will be investigated together with the generation of higher harmonic radiation in the VUV region. SPARX is the direct evolution of such a high gain SASE FEL toward the 13.5 and 1.5 nm operating wavelengths, at 2.5 GeV. To get the required value for the bunch peak current, Ipeak≈ 2.5 kA, the “hybrid” scheme, RF-compression stage plus magnetic chicane, is analyzed and compared with the more standard double stage of magnetic compression. The two options are reviewed considering the tolerance to the drive laser pulse phase jitter.

Femto-second X-ray generation by the tera-watt laser slice method in the SPring-8 storage ring

Feasibility study of the generation of ultra-short pulse X-rays in the SPring-8 storage ring with a laser slice method is discussed. The electron beam energy is modulated by applying a tera-watt laser with a length of 100 fs in parallel to the electron beam running through a wiggler. Pulsed X-rays, emitted from a bending magnet, are separated from the X-rays due to the beam bunch with a slit. Parameters of the system are optimized based on a formalism with respect to the laser beam interaction in a wiggler, beam dynamics and laser beam optics. Numerical calculations of the interaction are carried out taking into account a Gaussian distribution of electron and laser beams. The expected flux is 8.1×10 4 photons/s/mrad/0.1% bandwidth at a bunch beam current of 10 mA with a pulse length of 240 fs in full width. If the lattice functions are modified such that an undulator is used as a radiator in a negative dispersion section, we can expect the pulsed X-rays with a flux of 2.9×10 7 photons/s/0.1% bw and a pulse length 180 fs. Because of a high beam energy of 8 GeV, a very high laser field is required, which induces limitations in the laser beam optics. Discussions are extended to the New SUBARU storage ring of 1.5 GeV.

Studies of bunch lengthening at the BEPC

The Beijing Electron–Positron Collider (BEPC) has been operating for more than ten years. In addition to providing colliding beams for high energy physics and a single beam for synchrotron radiation study, we carried out many machine related beam experiments for understanding better the beam physics and for improving performance. Among them, measurements of bunch length and its lengthening, and the impedance implied thereby, play crucial roles in the future plans for luminosity upgrades. This paper reviews the measurement of the bunch length in the BEPC storage ring, concluding with the scaling law for the bunch lengthening above the microwave instability threshold. The low frequency impedance of the storage ring is also obtained from the bunch lengthening due to the potential well distortion in low bunch current.

A heavy-ion DTL design for HIDIF

In the framework of Heavy-Ion Driven Ignition Facility (HIDIF), the beams from several identical Bi + ion sources must be efficiently captured, accelerated and merged together in successive funnelling steps to produce an intense beam. Such a beam must be further accelerated, in our case from 10 to 50 MeV/u, in a main linac and then injected into some storage rings. As a main linac, a conventional 200 MHz Alvarez DTL, has been proposed. Results from particle dynamics calculations, performed with 400 mA bunch current, indicate that good beam quality – low emittance growth and high transmission – can be achieved. The output distribution of the linac has been used to test the beam behaviour in a preliminary transfer line to the rings, showing that space charge effects cannot be neglected.

The “zero” current and self-sustaining electron bunches in M-type devices

When the processes occurring in the entire interaction space of a magnetron device are included simultaneously in a numerical simulation it is possible to observe the onset and behavior of stable solitary electron bunches. It is shown that these self-sustaining electron bunches may be responsible for the existence of an anode current in the absence of rf interaction.

Initial dual-sweep streak camera measurements on the Duke storage ring OK-4 UV/visible FEL

Initial measurements at the Duke storage ring free electron laser (FEL) of both spontaneous emission radiation (SER) from the OK-4 and FEL radiation outcoupled from the oscillator cavity have been done with a dual-sweep streak camera. In the SER case, ANL's Hamamatsu C5680 streak camera was used to characterize stored electron-beam bunch lengths versus rf gap voltage and single-bunch current. Electron-beam bunch lengths were observed in a range from 50 to 300 ps (FWHM). Particle beam energies of 270 and 500 MeV in the storage ring (SR) were used with the OK-4 to generate SER from 500 nm to less than 200 nm. Sensitivity to single-bunch, single-turn SER was shown down to ∼4 μA beam current at λ=450 nm. By operating in the synchroscan mode and at bunch currents less than 1 mA, beam orbit length vs. the resonator round-trip time was investigated by detecting the relative arrival time of the second pass outcoupled from the resonator at λ≈200 nm. These measurements were used in support of first lasing in the UV-visible regime in November 1996. The dual-sweep streak mode was then used to monitor lasing intensity variations on the 1 and 50 ms time scales, to measure a lasing pulse length as short as ∼3 ps ( σ) at λ=388 nm at low beam current, and to track the orbit length detuning curve.

Micropulsed ion source via self-sputtering avalanche

In the proposed pulsed ion source, a steady-state sputtering avalanche sustained by secondary ions locked in phase with an ac voltage applied between plates creates high current bunches with energy monochromaticity and ion purity. In a two-surface cascade, the average current exhibits a Child–Langmuir-like scaling, due to the dynamical constraints on the normalized ac voltage. A single-surface cascade, using a bending magnetic field, operates with unconstrained voltage and exhibits much lower heat load for given current, as well as the ability for average current in excess of the Child–Langmuir limit. The pulsed source is particularly attractive for ion beams of solid materials with high vaporization temperature.

Measurement of the absolute energy and energy spread of the ESRF electron beam using undulator radiation

Two simple methods of characterizing the average energy and energy spread of the electron beam have been developed at the ESRF. Both are based on analysis of the x-ray spectrum from an undulator. The first allows the absolute energy of the electrons to be determined. It is based on the dependence between the harmonics wavelengths and the electron beam energy. The x-ray beam is monochromatized at 21 keV by a silicon crystal in backscattering geometry. By adjusting the magnetic gap, one makes the third harmonic of the radiation from an undulator coincide with the energy selected by the crystal. The main errors come from the uncertainties in the undulator’s magnetic field and period. By operating the undulator at low field (K=0.36), an absolute accuracy of 10−3 is reached for the electron energy. The energy spread measurement is performed by analyzing the broadening of the harmonics’ profile. It is deduced from the measured ratio between the height of the peak of the seventh harmonic at 29 keV and the height of a secondary maximum at lower energy. The measured low current energy spread is 1.1×10−3±20%. It increases with the single bunch current due to turbulent bunch widening.

Some aspects of the design study on a fourth-generation-like VUV/SXR light source

Some aspects of the design study on a fourth-generation-like [1] vacuum ultraviolet/soft X-ray (VUV/SXR) light source are summarized. A storage ring lattice adopting a seven-bend achromat (SBA) [2] structure is presented. High brightness and coherence of SR light are emphasized. The natural electron beam emittance reaches as low as 0.25 nm.rad at 2 GeV. The ring circumference is 473.68 m. Eight 7-m long and four 18-m long dispersion-free straights are available for installation of insertion devices (IDs) and other kinds of equiqment. Sources based on long undulators placed in long straights would be able to provide SR spectra with a photon beam brightness up to 10 21 photons/s/mrad 2/mm 2/0.1%BW. The Touschek lifetime, which is reputed as one of the most crucial issues for design of such low energy (2–4 GeV), low emittance (in the 10 −10 m.rad range) light sources, has been studied by taking into account the bunch lengthening and intra-beam scattering effects. We find that at lower energies (around 2 GeV) both the bunch length and momentum spread increase by a factor of 2.7 due to the longitudinal microwave instability, and the emittance blows up by factors of 2.7–4.4, corresponding to coupling ratios of 10%-1%, caused by the intra-beam scattering; however, at the higher energies (around 4 GeV), there are no remarkable changes for these parameters (an average bunch current of 1 mA is assumed throughout the calculation). Thus, the energy dependence of the Touschek lifetime is modified correspondingly, and the Touschek lifetime issue becomes less serious at the cost of the bunch lengthening and widening and the emittance blowup at the low energy range.

Beam-beam interaction with a horizontal crossing angle

We report measurements of the dependence of luminosity and beam-beam tune-shift parameter on horizontal crossing angle at a single interaction point in the Cornell Electron Storage Ring. The report is based on data collected between September 1991 and January 1992 at CESR. For head-on collisions (zero crossing angle) the achieved tune-shift parameter is ξ v = 0.03 ± 0.002 at 11 mA/bunch. For a crossing half-angle of θ c = ± 2.4 mrad, we achieve ξ v = 0.024 ± 0.002 at similar bunch currents. The data suggest some degradation of performance if the trajectory through the interaction region is distorted magnetically even while head-on collisions are preserved. Therefore at least some of the observed dependence of tune-shift parameter on crossing angle may be due to the associated large displacement of the beam trajectories in the interaction region optics. Furthermore, with the introduction of the crossing angle, the algorithm for optimizing luminosity is significantly complicated due to linear optical errors and the solenoid compensation. We interpret the measured tune-shift parameter at θ c = 2.4 mrad as a lower limit to what can ultimately be achieved.

Present development at MAX-lab

The development at the present MAX I ring concerning a new beam position monitor system, enhanced injection efficiency, harmonic generation of coherent radiation, the suppression of longitudinal instabilities and some development work on the 100 MeV race-track microtron to increase the bunch current is presented. The lattice and design philosophy of the 1.5 GeV MAX II third generation synchrotron light source currently being built at MAX-lab are also discussed.

A racetrack microtron for free electron laser use

Owing to a number of parameters the racetrack microtron is suitable as an electron laser. Here follows a short description of the racetrack microtron at MAX, and the subharmonic prebunching system used to increase the peak bunch current. A description of the beam parameters is also included, especially with FEL in mind. Finally, a possible FEL using the MAX racetrack microtron is discussed.