On the Focussing and Transmission Properties of Electron Beam in High Power Microwave Sources Filled with Plasma

Based on the electron movement principles the beam paraxial trajectory and space trajectory equation in drifting space filled with plasma are given in this paper. The space is divided into two areas( and , contains ₁ and ₂), the behavior of electrons in these areas is studied. The equations are theoretically or numerically solved, the focussing and transmission properties are studied and some parameters affecting these properties are discussed in detail. The relativistic effect is also taken into account. The study shows that with plasma filled the beam can be focused efficiently and transmitted with high quality by optimally choosing the plasma filling fraction and the voltage.Plasma microwave electronics stemmed from 1949, however, it developed rapidly in recent years. In the process of high power microwave study, plasma is more and more introduced into microwave source. Because of the neutralization effect of plasma, the beam can transmit with high quality, even under weak or without magnetic field, it can propagate through drifting tube or slow wave structure by its self magnetic focusing force^[1–2]. By making full use of this property, microwave devices without magnetic field system can be made, this leads to a decrease in volume and weight of the device. Some experiments show that this kind of device can promote the output power and efficiency obviously. However, because of the presence of plasma, the beam's behavior changes greatly compared to those under vacuum condition. It is important to study the properties of the beam in drifting space or slow wave structure filled with plasma for the development of the device.     

Possible gravitational radiation detection using the geometric phase of a light beam

An effect of geometrical phase shift is predicted for a light beam propagating in the field of a gravitational wave. For the beam travelling orthogonally to the direction of propagation of the gravitational wave from an observer and returning back after being reflected, this phase is shown to grow proportionally toL/ whereL is the distance between the observer and reflecting system, and the characteristic wavelength of the gravitational wave packet (for light propagating parallel or antiparallel to the gravitational wave, the geometric phase shift is absent). Gravitational radiation detection experiments are proposed using this new effect, the corresponding estimates being given.On leave of absence from: Peoples' Friendship University, Moscow, RussiaOn leave of absence from: Krasnoyarsk State University, Krasnoyarsk, Russia     

Nonlinear interaction of a modulated beam with plasma

Nonlinear evolution of h. f. instabilities excited by a premodulated electron beam in a magnetized plasma is investigated both theoretically and experimentally. Only the erenkov type of excitation (k_¦|v_b) of the upper branch of the electron plasma oscillations has been observed. The dynamics of excitation of the quasistationary fundamental wave (having the frequency of modulation) has been determined numerically. The wave absorption has been included into the theoretical model through an effective collision frequency. Numerical results agree well with the measured evolution of the amplitude and of the phase velocity of the fundamental wave along the system, as well as with the beam distribution functions. The observed bunching of beam electrons and characteristic features of plasma electrons heating are also reported. Broadening of the frequency spectrum and the occurrence of satellite waves with lower phase velocities have been observed downstream. Physical mechanism of spantaneous excitation of these satellites is suggested and their role in the process of the beam relaxation is discussed. More detailed studies of the nonlinearly excited waves have been performed by modulating the beam at two frequencies.It is a pleasure for us to thank Dr .Körbel who has kindly performed the numerical calculations.     

The role of the initial suppression of beam-plasma interaction for short experimental devices

Conditions for excitation of the h.f. beam-instability in a relatively short (L _b/) beam-plasma system have been studied. The beginning of the effective energy transfer from the beam electrons to the excited wave measured from the instant of their entrance into the system is delayed by a certain timet _crit that reaches its maximum valuet _crit3/2 for a monoenergetic beam. If waves with the group velocity comparable to that of the beam grow up to a measurable value only provided that the waves are effectively reflected on both ends of the system, the efficiency of the interaction is further lowered. The systematical change of the phase velocity of the excited wave leads to the interruption of the instability after a few such reflections. The theoretical results have been confirmed by the experimental measurements on the ELMAN II machine.It is a pleasure to acknowledge the permission to publish experimental results obtained by Dr. V. Piffl and Dr. J. Ullschmied.     

High average power Q-switched green beam generation by intracavity frequency doubling of diode-side-pumped Nd:YAG/HGTR-KTP laser

To obtain high power and high beam quality second harmonic generation, a Q-switched system has been demonstrated by intracavity frequency doubling of two diode-side-pumped Nd:YAG modules with double AO-modulators in an astigmatism compensated cavity geometry. A maximum average frequency doubled power of 185 W is obtained when the pumping power is 600 W for each module. The corresponding optical-to-optical conversion efficiency is 15.4% and the pulse width is 180 ns at a repetition rate of 10 kHz. An instability of 2.5% was measured over a period of 2 h and the beam quality factors were measured to be , the maximum output power.     

X radiation sources based on accelerators

Light sources based on accelerators aim at producing very high brilliance coherent radiation, tuneable from the infrared to X-ray range, with picosecond or femtosecond light pulses.The first synchrotron light sources were built around storage rings in which a large number of relativistic electrons produce “synchrotron radiation” when their trajectory is subjected to a magnetic field, either in bending magnets or in specific insertion devices (undulators), made of an alternating series of magnets, allowing the number of curvatures to be increased and the radiation to be reinforced.These “synchrotron radiation” storage rings are now used worldwide (there are more than thirty), and they simultaneously distribute their radiation to several tens of users around the storage ring.The most effective installations in term of brilliance are the so-called 3rd generation synchrotron radiation light sources. The radiation produced presents pulse durations of the order of a few tens of ps, at a high rate (of the order of MHz); it is tuneable over a large range, depending on the magnetic field and the electron beam energy and its polarisation is adjustable (in the VUV-soft-X range). Generally, a very precise spectral selection is made by the users with a monochromator.The single pass linear accelerators can produce very short electron bunches (). The beam of very high electronic density is sent into successive undulator modules, reinforcing the radiation's longitudinal coherence, produced according to a Free Electron Laser (FEL) scheme by the interaction between the electron bunch and a light wave. The very high peak brilliance justifies their designation as 4th generation sources. The number of users is smaller because an electron pulse produces a radiation burst towards only one beamline. Energy Recovery Linacs (ERL) let the beam pass several times in the accelerator structures either to recover the energy or to accelerate the electrons during several turns, and thus provide subpicosecond beams for a greater number of users.A state-of-the-art of X sources using conventional (and not laser plasma based) accelerators is given here, underlying the performance already reached or forecast and the essential challenges. To cite this article: M.-E. Couprie, J.-M. Filhol, C. R. Physique 9 (2008).     

Expansion and Destruction of a Relativistic Electron Beam in a Uniform Magnetostatic Field under the Influence of a Symmetric TM Wave

We solve the problem of the motion of a flow of relativistic electrons guided by a uniform magnetostatic field in the channel of interaction with a symmetric TM wave. The problem is related to the analysis of operation of powerful oscillators of Cherenkov microwave radiation and can also have other applications. This problem is of special interest for a possible reduction of the magnetostatic field within the permissible limits of the beam expansion. For synchronous interaction of electrons with a TM wave, we obtain a simple estimate of the possible expansion of the beam through the energy argument and consider the characteristic features of motion in some cases on the basis of more accurate calculations including those for the case of violation of the radial finiteness of motion (microwave defocusing of the beam). Attention is drawn to the inconsistency of the known models of expansion of the beam with a fixed maximum velocity of the radial drift of electrons.     

Parametric upconversion of TM and Trivelpiece-Gould (TG) modes to high frequency free electron laser (FEL)

A backward wave oscillator (BWO) filled with a strongly magnetized plasma supports TM and Trivelpiece-Gould (TG) modes. At large amplitudes these modes may act as wigglers for generating millimeter waves via free electron laser instability. The nonlinear coupling between the wiggler, the beam space charge mode, and the high frequency free electron laser wave is dominated by parallel motions. In the Raman regime the growth rate of instability goes as _pb ^1/2 / ₀ ^9/4 , where _pb is the beam plasma frequency and ₀ is the relativistic gamma factor.Department of Physics, Indian Institute of Technology, India. Published in Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 37, No. 6, pp. 748–753, June, 1994.     

A highly efficient 5 kW peak power Nd:YAG laser with time-shared fiber optic beam delivery

Here, we report on the development of an efficient, high peak power lamp pumped Nd:YAG laser with time-shared fiber optic beam delivery. A maximum average output power of 270 W with 100 J maximum pulse energy and 5 kW peak power has been achieved with an electrical to laser conversion efficiency of 5.4%, which is on higher side for typical lamp pumped solid-state lasers. We have improved efficiency by spectral conversion and water flow optimization in the pump cavity, with a resulting beam quality comparable to commercial systems of similar power level. The resonator has been designed for stable operation from single-shot to 200 Hz repetition rate. A study of pulse-to-pulse laser energy stability for different resonator configurations has also been performed. The resonator was designed to achieve a good beam quality for the whole range of operation with a maximum beam parameter product of 15 mm mrad (M²45). A simple mechanism for time-shared fiber optic port selection has also been devised. Material processing applications such as cutting of stainless steel sheets up to 14 mm thickness and welding of metals such as carbon steel with weld depths up to 2 mm using the developed laser system has also been reported.     

Are quantum wave packets observable? Sokolov effect: puzzling generation of 2p states in the 2s hydrogen beam passing trough a wide metal slit

The observation of excitation of 2p states in a collimated 2s hydrogen beam passing through a wide metal slit with no direct contacts or electric field applied (Sokolov effect) up to now has had no reasonable explanation. A solution presented in this paper is formulated within the standard quantum-mechanical framework with a consecutive wave packet treatment of the atomic center-of-mass wave function. It is found that a very weak interaction of the beam diffraction halo with the slit, though negligible for center-of-mass motion, coherently affects the intrinsic state of an atom in the beam and efficiently induces transitions. High sensitivity of this interference phenomena may be used to measure transverse coherence length of the beam.Received: 25 March 2003, Published online: 15 July 2003PACS: 03.65.-w Quantum mechanics     

Assignments of fir laser emissions from CH3OH optically pumped by an intra-cavity etalon tuned TEA-CO2 laser

The author reports that TEA-CO₂ with an intra-cavity etalon is a useful pumping source which can deliver widely tunable beam (tunable range 1 GHz) with accurate oscillation frequencies and with high power compared to a tunable wave guide laser.The source is applied to the excitations of CH₃OH and the FIR emissions from it are well assigned for there absorption transitions and FIR emissions. This proves that the source display not only usefulness for development of new FIR emissions but also for molecular spectroscopy.     

The reflection of electrons from standing light waves

Experiments are described in which a slow electron beam is deflected by a standing light wave produced by a high intensity laser. This interaction is believed to occur as a Bragg reflection of the electrons in the standing wave intensity pattern set up in the light beam, as was predicted byKapitza andDirac in 1933. No energy interchange takes place but a momentum transfer which results in a Bragg relationship for the angle of reflection. The probability is proportional to the square of the light intensity.Kapitza-Dirac's original particle treatment of the effect is shortly reviewed and a wave mechanical treatment is added. Two types of experiments A and B are described. In type A an electron beam is scanned through a standing light wave and any deflection is detected with a differential amplifier. In type B the electron beam is locked in at maximum deflection and the laser output is simultaneously displayed with the deflected electrons on a fast dual beam oscilloscope. The electron beam deflections follow the fine structure of the laser spikes. Results from both types of experiments stay within the right magnitude of the theoretical predictions.     

Electrostatic deceleration of the LEAR antiprotons

A high voltage electrostatic column is proposed for an efficient deceleration of the 60 MeV/c LEAR antiprotons down to an energy where they will be trapped and cooled. The column, made of standard NEC tubes connects the LEAR beam line to an SF6 isolated terminal in which the energized (but disconnected) superconducting solenoid and associated components are placed. The charging chain ramps the terminal voltage to its final value (slightly less than 2 MV) during the few seconds preceding the LEAR beam injection and returns the voltage to a preselected low value within the following few seconds. The decelerated antiprotons have less than 10 keV energy spread and can be optimally trapped, cooled and extracted into a high quality low energy beam.     

High spatial intensity 10 W-CuBr laser with hydrogen additives

High beam quality output has been demonstrated from a single medium-scale (2 cm dia. × 50 cm) CuBr laser with hydrogen additives. The addition of hydrogen leads to beam divergence reduction and laser power lift-up finally yielding 5-fold increase in laser power spatial intensity compared with the case of no hydrogen added. With a simple positive-branch unstable resonator practical (pulse-average) divergence of 80 rad is achieved. The easy-feasible sealed-off version of CuBr laser makes it an attractive source for many high-quality laser beam applications.     

Efficient Raman frequency conversion of high‐power fiber lasers in diamond

We report high‐power frequency conversion of a Yb‐doped fiber laser using a double‐pass pumped external‐cavity diamond Raman oscillator. Pumping with circular polarization is shown to be efficient while facilitating high‐power optical isolation between the pump and Raman laser. We achieved continuous‐wave average power of 154 W with a conversion efficiency of 50.5% limited by backward‐amplified light in the fiber laser. In order to prove further scalability, we achieved a maximum steady‐state Raman‐shifted output of 381 W with 61% conversion efficiency and excellent beam quality using 10 ms pump pulses, approximately a thousand times longer than the transient thermal time‐constant. No power saturation or degradation in beam quality is observed. The results challenge the present understanding of heat deposition in Raman crystals and foreshadow prospects for reduced thermal effects in diamond than originally anticipated. We also report the first experimental evidence for stimulated Brillouin scattering in diamond.

     

Imaging and focusing of an atomic beam with a large period standing light wave

A novel atomic lens scheme is reported. A cylindrical lens potential was created by a large period ( 45 m) standing light wave perpendicular to a beam of metastable He atoms. The lens aperture (25 m) was centered in one antinode of the standing wave; the laser frequency was nearly resonant with the atomic transition 2³ S ₁–2³ P ₂ (=1.083 m) and the interaction time was significantly shorter than the spontaneous lifetime (100 ns) of the excited state. The thickness of the lens was given by the laser beam waist (40 m) in the direction of the atomic beam. Preliminary results are presented, where an atomic beam is focused down to a spot size of 4 m. Also, a microfabricated grating with a period of 8 m was imaged. We discuss the principle limitations of the spatial resolution of the lens given by spherical and chromatic aberrations as well as by diffraction. The fact that this lens is very thin offers new perspectives for deep focusing into the nm range.     

On the transversal vibrations of a conveyor belt with a low and time-varying velocity. Part II: the beam-like case

In this paper, initial-boundary-value problems for a beam equation (with string effect) are considered. These problems can be used as simple models to describe the vertical vibrations of a conveyor belt, for which the velocity is small with respect to the wave speed. In this paper, the belt is assumed to move with a time-varying velocity . Formal asymptotic approximations of the solutions are constructed to show the complicated dynamical behaviour of the belt. Complicated dynamical behaviour of the belt system occurs when the frequency Ω is the sum or difference of any two natural frequencies of the system with zero belt velocity. For special values of the belt parameters these sum type and difference type of internal resonances coincide giving rise to even more complicated dynamical behaviour. Some examples (including detuning cases) have been studied in detail.     

Ground-state hyperfine splitting of antihydrogen

ASACUSA collaboration at CERN’s antiproton decelerator (CERN AD) plans to measure the ground-state hyperfine splitting (GS-HFS) of antihydrogen () to test the CPT symmetry to high precision. Our scheme is to produce an (anti-) atomic beam with a novel two-frequency superconducting Paul trap, and to use sextupole magnets and a 1.4-GHz cavity to analyze the HFS resonance frequency.      

An experimental investigation of high permittivity dielectric resonators for millimeter-wave microstrip components

This paper proposes a new type of high permittivity whispering gallery mode cylindrical dielectric resonators with a geometry which is very suitable for use in microstrip type integrated circuits. These oversized resonators, together with high Q factors and ease of fabrication, are potentially interesting for millimeter wave integrated filters and oscillators. In this paper, we report an experimental investigation of such resonators carried out in Ka band (26.5–40 GHz). Measured results including resonant frequencies and loaded quality factors will be presented. Application to millimeter wave filters will also be demonstrated.