Longitudinal Beam Instability Research Articles

Взаємодія хвиль і частинок при інжекції модульованого електронного пучка в іоносферну плазму. Теорія та експеримент

We present the results of the active space experiment with charged particle beam's injection (electrons and xenon ions) carried out onboard Intercosmos-25 station and daughter Magion-3 subsatellite. The ones are obtained under conditions when the particle beams were injected in opposite directions relative to the magnetic field B0 in such a way that the electron injection was directed towards the Earth. Mechanisms of beam-plasma instabilities relative to the excitation of electrostatic and electromagnetic waves are considered during the electron beam injection (~10 keV, 0.1 A) from the Intercosmos-25 station. Development of transverse instability on the first cyclotron resonance leads to the excitation of whistler mode waves backward-propagating relative to the injected electrons (from the Earth). The investigation object was the beam-excited differential fluxes of ionospheric electrons in a wide energetic range of 27 eV — 412 keV registered by the charged particle spectrometers onboard the Magion-3 subsatellite. Thereby, the interaction of whistler waves with ionospheric electron fluxes is stimulated by the energy transfer mechanisms such as 'particle-wave-particle'. Numerical results of beam-plasma instabilities are compared also with thermal plasma parameters registered at different space points on the station and subsatellite. Excitation of longitudinal and transverse beam-plasma instabilities will inevitably lead to their competition, which will affect the results of the experiment. The data of stimulated fluxes of ionospheric electrons allow us to investigate the various effects of the wave-particle interaction, taking into account the influence of the growth rate of longitudinal instability on the excitation angle of whistlers and their structure. This approach is based on the results of laboratory experiments to determine the pattern of excited whistlers for an electric dipole antenna and the analogy of the beam-plasma channel with the radiating system. The results of the active space experiment confirm the dependence of the growth rate of whistler mode waves on the development of longitudinal beam instability.

Electron Beam Transport in Plasma-Accelerator-Driven Free-Electron Lasers in the Presence of Coherent Synchrotron Radiation and Microbunching Instability

Laser- and beam-driven plasma accelerators promise electron beam brightness at the exit of plasma cells suitable for X-ray free-electron lasers. Beam transport from the accelerator to the undulator may include a multi-bend, energy-dispersive switchyard, in which energy collimators can be installed to protect the undulator or to serve multiple photon beamlines. Coherent synchrotron radiation and microbunching instability in the switchyard can seriously degrade the brightness of the accelerated beam, reducing the lasing efficiency. We present a semi-analytical analysis of those collective effects for beam parameters expected at the exit of state-of-the-art plasma accelerators. Prescriptions for the linear optics design used to minimize transverse and longitudinal beam instability are discussed.

Beam loading effects and microwave instability in the booster ring of a high intensity heavy-ion accelerator facility

Transient beam loading effects and microwave instability are very important in ion synchrotrons. A simulation code longitudinal particle tracking code (LPTC) based on multiparticle tracking is developed to simulate the beam loading effects and the longitudinal beam instabilities in the booster ring (BRing) of the High Intensity heavy-ion Accelerator Facility (HIAF). A process of beam capture, acceleration, debunching, and the second capture is simulated with a reference ion beam of ${^{78}\mathrm{Kr}}^{19+}$. The simulation results show that the beam loading effects have major contribution to the beam behavior during the debunching stage and the second capture stage. A feed-forward system is suggested to compensate the beam loading effects, and the main parameters of the feed-forward system are given. The microwave instability of coasting ${^{78}\mathrm{Kr}}^{19+}$ beams is observed from the simulation results. The threshold of the microwave instability is also given. The development of self-bunching and methods to suppress the microwave instability in the BRing are discussed.

Effect of the transverse parasitic mode on beam performance for the ADS driver linac in China

The ADS (Accelerator Driven subcritical System) driver linac in China is designed to run in CW (Continuous Wave) mode with 10 mA designed beam current. In this scenario, the beam-induced parasitic modes in the ADS driver linac may make the beam unstable or deteriorate the beam performance. To evaluate the parasitic mode effect on the beam dynamics systematically, simulation studies using the ROOT-based numerical code SMD have been conducted. The longitudinal beam instability induced by the HOMs (High Order Modes) and SOMs (Same Order Modes) has little effect on the longitudinal beam performance for the current ADS driver linac design based on the 10 MeV/325 MHz injector I from previous studies. Here the transverse parasitic mode (i.e., dipole HOM) effect on the transverse beam performance at the ADS driver linac exit is investigated. To more reasonably quantify the dipole mode effect, the multi-bunch effective emittance is introduced in this paper.

Analytic modeling of instabilities driven by higher-order modes in the HLS II RF system with a higher-harmonic cavity

The utility of a passive fourth-harmonic cavity plays a key role in suppressing longitudinal beam instabilities in the electron storage ring and lengthens the bunch by a factor of 2.6 for the phase II project of the Hefei Light Source (HLS II). Meanwhile, instabilities driven by higher-order modes (HOM) may limit the performance of the higher-harmonic cavity. In this paper, the parasitic coupled-bunch instability, which is driven by narrow band parasitic modes, and the microwave instability, which is driven by broadband HOM, are both modeled analytically. The analytic modeling results are in good agreement with those of our previous simulation study and indicate that the passive fourth-harmonic cavity suppresses parasitic coupled-bunch instabilities and microwave instability. The modeling suggests that a fourth-harmonic cavity may be successfully used at the HLS II.

Measurement of the time structure of a coherent synchrotron radiation burst in NewSUBARU

The time structure of a burst of coherent synchrotron radiation (CSR) emitted from a high-peak-current beam in the electron storage ring NewSUBARU has been investigated. The source of this CSR burst is fine time structure in a bunch produced by longitudinal beam instability. This burst is unstable but very easy to obtain, so it can be used for some experimental applications with appropriate averaging of data. With an averaging period of 10ms, the fluctuation of the integrated power was about 10%.

Compact Storage Ring for Far-Infrared Coherent Synchrotron Radiation Source

We present the results of design studies for a compact storage ring that may produce coherent synchrotron radiation in the far-infrared frequency range. The momentum compaction factor in the ring is easily varied by controlling dispersion function and thus a short bunch length of a few picoseconds can be achieved. We have performed a detailed analysis of beam dynamics for a short bunch length in the ring. Within the controllable range of the second-order momentum compaction factor in the ring, it was shown that the alpha bucket and the rf bucket were created. It is shown that the compact ring with conservative machine errors has a sufficient dynamic aperture after the correction of closed orbit distortion. Multi-particle tracking simulations are performed to investigate longitudinal beam instability due to the broadband impedance in the ring. We investigated the effects of coherent synchrotron radiation wake in the ring on longitudinal beam distribution. From these studies, it is shown that the designed compact ring may provide stable electron beams of approximately around 2 ps rms.

Electron optics of a future SC ERL at KAERI

A project of a high-power FEL at Korea Atomic Energy Research Institute is described. The FEL is driven by a superconducting energy recovery linac. The future ERL will be connected to the existing machine without any modification. It consists of two 180° bents and two straight sections: one is for the FEL, another for a Compton X-ray source. One can choose the regime controlling the lenses. The total ERL is isochronous to avoid any problem with longitudinal beam instability. The total relative emittance degradation through the whole machine is ≈1.5. The FEL will be based on a 2 m helical in-vacuum undulator made of permanent magnets. One mirror of the optical cavity is blind and made of copper, the other one, the outcoupler, is semi-transparent and made of CVD diamond. The expected average power is a few kW, the tuning range 35–70 μm.

Quasi-Isochronous Optics for Sub-Picosecond Bunch Length at Pohang Light Source

A lattice design was developed for a quasi-isochronous storage ring, which may produce a sub-picosecond bunch length at the 2.5 GeV Pohang Light Source (PLS) and the analysis of transverse and longitudinal particle dynamics in the designed ring was performed. Small momentum compaction factors in the PLS storage ring were obtained by varying the dispersion function to be negative in bending magnets. The shortest bunch length in the designed ring was estimated to be less than 1 ps, which is three-tenths that of a normal operation. The effects of the second-order momentum compaction factor on the transverse and longitudinal particle dynamics in the quasi-isochronous ring were investigated. The relationship between the momentum compaction factor and the bunch length, which can be obtained in the quasi-isochronous PLS storage ring was also investigated. Multiparticle tracking simulations were performed to investigate the longitudinal beam instability in the quasi-isochronous ring using a broadband impedance model.

System for Suppressing Quadrupole Beam Oscillations in the U-70 Synchrotron

Slow self-excited oscillations of the bunch length in an acceleration cycle are usually observed in the U-70 proton synchrotron. This longitudinal beam instability is probably due to the cross interaction of the feedback circuits employed in the accelerating system. The initial conditions for the instability are the input errors and the beam perturbations occurring when the beam crosses from the injection plateau into the acceleration regime, crosses the critical energy, and moves from the acceleration regime onto the ejection plateau. The instability results in unjustified and poorly controlled increase of the momentum spread in the beam. In the fall (2000) session of U-70, a model variant of an amplitude feedback circuit on the beam was put into operation. This system successfully suppresses in-phase quadrupole oscillations of the bunches at any point in the acceleration cycle. A brief technical description of the suppression system is given. Experimental results obtained when the system was started up and adjusted are presented.

High current operation of a storage-ring free-electron laser

The operation of storage-ring free-electron lasers (SRFEL) at high current still represents a challenge because of the growth of longitudinal beam instabilities. One of these, the quadrupolar coherent synchrotron oscillation, is very harmful for free-electron-laser (FEL) operation. On the Super-ACO storage ring, they either prevent the FEL start-up, or result in a very poor stability of the FEL source. A new feedback system to damp the quadrupolar coherent synchrotron oscillation has been installed on the ring and the stabilized beam parameters have been systematically measured. As a result, the FEL gain is higher and the FEL operates more easily and with a higher average power. Its stability, which is very critical for user applications, has been significantly improved as it has been observed via systematic measurements of FEL dynamics performed with a double sweep streak camera.

Longitudinal beam instability in plasmas with several species of heavy particles

The interaction between electron beams and plasmas containing several species of heavy particles (i.e., atomic particles possessing polarisable electronic envelopes) is analysed. It is established that the longitudinal beam instability at ω = ωpe ≈ kzu is possible not only in the resonant case (i.e., when the electron plasma frequency is very close to the frequency of radiation emitted in transitions between the two working energy levels), but also in the non-resonant situations. The characteristics of the instability in this latter case are discussed in detail. It is found that, generally, the trends of the amplitude increments are opposite to those in the equivalent resonant cases.

Measurement of bunch lengthening in TERAS

The bunch length was measured by using a highly sensitive streak camera with a time resolution of 2 ps. It was found that fine structures appeared in the electron bunch shape and that the shapes of electron bunches were described by a Gaussian distribution on the average. The dependence of bunch length on beam current was measured for an electron beam of 607 MeV. The bunch length was well represented by a power function of beam current with an exponent of 0.197 at currents lower than 35 mA or 0.30 at high currents. The experimental results suggest that the longitudinal coupled-bunch beam instability takes place at low beam currents and the turbulent instability dominates at high currents. It was also found from the three-dimensional bunch shape measurements that the bunch shape tended to blow up at high currents.

A storage ring FEL for the VUV

A free electron laser for the VUV operating in a storage ring requires an electron beam of high density and low energy spread and a short wavelength, narrow-gap undulator. These conditions tend to produce longitudinal and transverse beam instabilities, excessive beam growth through multiple intra-beam scattering and a short gas-scattering lifetime. Passing the beam only occasionally through the undulator in a by-pass straight section, as proposed by Murphy and Pellegrini [1], allows operation in a high-gain, single-pass mode and a long gas-scattering lifetime. Several storage ring designs have been considered to see how best to satisfy the several requirements. Each features a by-pass, a low-emittance lattice and built-in wigglers for enhanced damping to counteract the intra-beam scattering.

Superconducting Cavity Beam Test in CESR

The first use of a superconducting accelerating structure in a storage ring is described. Two 5-cell 1500 MHz ''muffin tin'' superconducting niobium cavities (1 m active length) were operated in the Cornell 8 GeV e/sup +/e/sup -/ storage ring, CESR. A 6.1 mA beam was captured and stored at 3.5 GeV using only the superconducting cavity. Up to 12.5 kW was transferred to the beam by the cavity. Single bunch currents up to 12 mA were reached in conjunction with the normal 500 MHz RF. Beam induced higher order mode (HOM) powers of 290 W were carried to room-temperature loads by four couplers and waveguides; no HOM-induced cryogenic dissipation was detectable. Q's of the 76 most prominent resonances were measured. Transverse and longitudinal beam instabilities were studied by mechanically sweeping the tune of the cavities; current thresholds are compared to theory. The cavity exhibited the ability to survive breakdown at high incident power without degradation.

Longitudinal Behavior of the Beam in KEK Booster

The measurements of the longitudinal beam properties and high intensity effects of the KEK booster are presented. The capture efficiency has been improved by using two kinds of cure for beam loading instabilities appeared just after injection. Operation of the second RF station in cooperation with the existing one reduces the beam loss during acceleration. Severe longitudinal beam instabilities observed at the end of acceleration have been cured by incorporating a 2fs feedback loop.

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.