Search Results (47 total)

Results of numerical calculations and experimental investigations of an active Ka-band microwave pulse compressor are presented. The compressor is based on a running-wave, three-mirror, quasioptical resonator utilizing a diffraction grating whose channels embody plasma discharge tubes that constitute the active switch. The principle of compression is based on rapidly changing the resonator’s output coupling coefficient (Q switching) by initiating plasma discharges in the grating channels. Excitation of the resonator was achieved with up to 250 kW of 34.29 GHz microwaves in 700 nS pulses from the magnicon at the Yale Ka-band Test Facility. A power gain of at least 7∶1 in the compressed pulse with a pulse duration of 10–15 ns was achieved.

Results obtained in several experiments on active rf pulse compression at X band using a magnicon as the high-power rf source are presented. In these experiments, microwave energy is stored in high-Q TE₀₁ and TE₀₂ modes of two parallel-fed resonators, and then discharged using switches activated with rapidly fired plasma discharge tubes. Designs and high-power tests of several versions of the compressor are described. In these experiments, coherent pulse superposition was demonstrated at a 5–9 MW level of incident power. The compressed pulses observed had powers of 50–70 MW and durations of 40–70 ns. Peak power gains were measured to be in the range of 7∶1–11∶1 with efficiency in the range of 50%–63%.

A new scheme for a dielectric wakefield accelerator is proposed that employs a cylindrical multizone dielectric structure configured as two concentric dielectric tubes with outer and inner vacuum channels for drive and accelerated bunches. Analytical and numerical studies have been carried out for such coaxial dielectric-loaded structures (CDS) for high-gradient acceleration. An analytical theory of wakefield excitation by particle bunches in a multizone CDS has been formulated. Numerical calculations are presented for an example of a CDS using dielectric tubes with dielectric permittivity 5.7, having external diameters of 2.121 and 0.179 mm with inner diameters of 2.095 and 0.1 mm. An annular 5 GeV, 6 nC electron bunch with rms length of 0.035 mm energizes a wakefield on the structure axis having an accelerating gradient of ∼600  MeV/m with a transformer ratio ∼8∶1. The period of the accelerating field is ∼0.33  mm. If the width of the drive bunch channel is decreased, it is possible to obtain an accelerating gradient of >1  GeV/m while keeping the transformer ratio approximately the same. Full numerical simulations using a particle-in-cell code have confirmed results of the linear theory and furthermore have shown the important influence of the quenching wave that restricts the region of the wakefield to within several periods following the drive bunch. Numerical simulations for another example have shown nearly stable transport of drive and accelerated bunches through the CDS, using a short train of drive bunches.

A compact rf pulse compressor, fed by a phase-modulated pulse, is described for producing high-power flattop pulses at Ka-band. The compressor consists of a single axisymmetrical cavity operating in a combination of TE_0n modes (n=1, 2, 3), that have zero electric fields at the walls. This feature enhances the breakdown strength of the system and makes it possible to increase the output power, as compared with a traditional SLED-II pulse compressor. Advantages include use of one channel instead of two, and no requirement for a 3-dB hybrid coupler. This 34 GHz one-channel pulse compressor (OC SLED-II) is designed to multiply peak power produced by the Yale/Omega-P Ka-band magnicon amplifier (30–40 MW, 0.5–1  μs) by (3–4)∶1 and to compress in time by (5–6)∶1. The efficiency of the proposed compressor is similar to that of SLED-II. Results of low-power tests carried out on a 30 GHz prototype of the compressor are discussed.

A formal eigenmode method to solve for electromagnetic fields in a longitudinally translationally invariant multizone dielectric-lined waveguide is presented. The method is specialized to the development of wakefield theory for rectangular dielectric-lined structures which have an arbitrary number of dielectric layers. It is shown that the fields excited by a drive particle moving through the vacuum beam channel in such a structure can simultaneously include both propagating and decaying modes. The decaying modes characterize the short-range self-fields that move together with the particle, while the propagating modes characterize the long-range radiation fields or wakefields that carry energy away from the particle. It is also shown that the formal solution obtained in rectangular structures is applicable to all translationally invariant dielectric-lined waveguides, for example, cylindrical structures. Two important identities which the method relies upon are computationally confirmed for rectangular two-zone dielectric-lined structures. This theory may be employed for calculations of space charge effects, particularly for low or moderate-energy beams where self-field effects cannot be neglected.

We report results from an experiment that demonstrates the successful superposition of wakefields excited by 50 MeV bunches which travel ∼50  cm along the axis of a cylindrical waveguide which is lined with alumina. The bunches are prepared by splitting a single laser pulse prior to focusing it onto the cathode of an rf gun into two pulses and inserting an optical delay in the path of one of them. Wakefields from two short (5–6 psec) 0.15–0.35 nC bunches are superimposed and the energy loss of each bunch is measured as the separation between the bunches is varied so as to encompass approximately one wakefield period (∼21   cm). A spectrum of ∼40   TM_0m eigenmodes is excited by the bunch. A substantial retarding wakefield (2.65   MV/m·nC for just the first bunch) is developed because of the short bunch length and the narrow vacuum channel diameter (3 mm) through which they move. The energy loss of the second bunch exhibits a narrow peak when the bunch spacing is varied by only 4 mm (13.5 psec). This experiment is compared with a related experiment reported by a group at the Argonne National Laboratory where the bunch spacing was not varied and a much weaker retarding wakefield (∼0.1   MV/m·nC for the first bunch) comprising only about 10 eigenmodes was excited by a train of long (∼9   mm) bunches.

We report the development of a nondestructive technique to measure bunch rms length in the psec range and below, and eventually in the fsec range, by measuring the high-frequency spectrum of wakefield radiation which is caused by the passage of a relativistic electron bunch through a channel surrounded by a dielectric. We demonstrate numerically that the generated spectrum is determined by the rms bunch length, while the specific axial and longitudinal charge distribution is not important. Measurement of the millimeter-wave spectrum will determine the rms bunch length in the psec range. This has been done using a series of calibrated mesh filters and the charge bunches produced by the 50 MeV rf linac system at ATF (Accelerator Test Facility), Brookhaven. We have developed the analysis of the factors crucial for achieving good accuracy in this measurement, and find the experimental data are fully understood by the theory. We point out that this technique also may be used for measuring fsec bunch lengths, using a prepared planar wakefield microstructure.

A tall, dielectric-lined rectangular wakefield microstructure is analyzed as a possible element of an advanced linear wakefield accelerator. This accelerator would be driven by a train of fs electron microbunches that would be chopped out of a longer bunch using a powerful CO₂ laser and then formed into a train of rectangular-profile bunches using a quadrupole. The bunches set up a periodic wakefield in the microstructure that can be built up to 400–600  MV/m, for example, using a train of ten 3-fs 1-pC bunches. Two major issues are examined. First, interference is studied using the particle-in-cell code KARAT between transition radiation and Cerenkov wakefield radiation, both set up by the passage of a charge bunch through a dielectric structure of finite length. Of significance is the difference in propagation speeds of transition radiation and Cerenkov radiation (which travels almost at the vacuum light speed c) and the magnitude of the respective fields. Second, stability is examined for drive and accelerated bunches using computations of test particle orbits in the longitudinal and transverse wakefields excited by the drive bunches. It is found that nearly all test electrons in the drive bunches are confined within the structure for a travel distance of 30 cm or more, while test electrons located in an accelerated bunch can have stable motion over greater than 30 cm without passing through the structure walls.

A mechanism for acceleration of protons is described, in which energy gain occurs near cyclotron resonance as protons drift through a sequence of rotating-mode TE₁₁₁ cylindrical cavities in a strong nearly uniform axial magnetic field. Cavity resonance frequencies decrease in sequence from one another with a fixed frequency interval Δf between cavities, so that synchronism can be maintained between the rf fields and proton bunches injected at intervals of 1/Δf. An example is presented in which a 122 mA, 1 MeV proton beam is accelerated to 961 MeV using a cascade of eight cavities in an 8.1 T magnetic field, with the first cavity resonant at 120 MHz and with Δf  =  8 MHz. Average acceleration gradient exceeds 40 MV/m, average effective shunt impedance is 223 MΩ/m, but maximum surface field in the cavities does not exceed 7.2 MV/m. These features occur because protons make many orbital turns in each cavity and thus experience acceleration from each cavity field many times. Longitudinal and transverse stability appear to be intrinsic properties of the acceleration mechanism, and an example to illustrate this is presented. This acceleration concept could be developed into a proton accelerator for a high-power neutron spallation source, such as that required for transmutation of nuclear waste or driving a subcritical fission burner, provided a number of significant practical issues can be addressed.

A new accelerator, LACARA (laser-driven cyclotron autoresonance accelerator), under construction at the Accelerator Test Facility at Brookhaven National Laboratory, is to be powered by a 1 TW CO₂ laser beam and a 50 MeV injected electron pulse. LACARA will produce inside a 2 m, 6 T solenoid a 100 MeV gyrating electron bunch, with ∼3% energy spread, approximately 1 psec in length with particles advancing in phase at the laser frequency, executing one cycle each 35 fsec. A beamstop with a small off axis channel will transmit a short beam pulse every optical cycle, thereby producing a train of about 30, 3.5 fsec long, 1–3 pC microbunches for each laser pulse. We describe here a novel accelerator, a micron-scale dielectric wake field accelerator driven by a 500 MeV LACARA-type injector that takes the output train of microbunches and transforms them into a near-rectangular cross section having a narrow dimension of ∼10 μm and height of ∼150 μm using a magnetic quadrupole; these bunches may be injected into a planar dielectric-lined waveguide (slightly larger than the bunch) where cumulative buildup of wake fields can lead to an accelerating gradient >1 GV/m. This proposed vacuum-based wake field structure is physically rigid and capable of microfabrication accuracy, factors important in staging a large number of accelerator modules. Furthermore, the accelerating gradients it promises are comparable with those for plasma accelerators. A LACARA unit for preparing suitable bunches at 500 MeV is described. Physics issues are discussed, including bunch spreading and transport, bunch shaping, coherent diffraction radiation from the aperture, dielectric breakdown, and bunch stability in the rectangular wake field structure.

First experimental observations are reported on stimulated coherent synchrotron radiation from highly relativistic electrons in a strong magnetic field. The experiment employed a quasioptical millimeter-wave resonator and a 6-MeV electron beam gyrating in a field of up to 25 kG. Coherent radiation at 54 GHz, corresponding to the 11th gyroharmonic, was observed and characterized. These observations demonstrate the possibility of a synchrotron resonance maser.

Experiments are reported on inverse free-electron-laser acceleration, including for the first time observations of the energy change as a function of relative injection phase of the electron bunches. The microwave accelerating structure consists of a uniform circular waveguide with a helical wiggler and an axial magnetic field. Acceleration of the entire beam by 6% is seen for 6 MeV electron bunches at optimum relative phase. Experimental results compare favorably, for accelerating phases, with predictions of a three-dimensional simulation that includes large-orbit effects.

Excitation of wakefields from a short charge bunch moving parallel to the axis of a dielectric-lined cylindrical waveguide is analyzed. This situation amounts to generation of Cerenkov radiation in a transversely bounded system. Wakefields are expanded into an orthonormal set of hybrid electric-magnetic eigenfunctions for this waveguide geometry. The orthonormalization relations for this system are obtained, evidently for the first time, both for a stationary source and for a localized moving source such as a charge bunch; it is shown that these orthonormalization relations differ. Forces arising from wakefields are found, valid within and behind a distributed bunch. Deviation of bunch distribution from axisymmetry leads to generation of dipole modes of significant amplitude that may lead to instability. Poynting’s theorem is examined for this system, and it is shown that convected Coulomb field energy must be subtracted from the Poynting flux to obtain the radiation power. This power, which balances drag on the bunch as calculated directly from the fields, is shown to flow in a direction opposite to that of the charge bunch. The results are easily generalized to bunches of arbitrary length and charge distribution, and to a train of such bunches. Numerical examples are presented for monopole, dipole, and quadrupole wakefield forces, and sample electric field patterns are shown to assist in understanding the unusual nature of this type of Cerenkov radiation. For a 2-nC rectangular drive bunch of length 0.20 mm, moving along the axis of an alumina-lined waveguide (ɛ=9.50) with inner and outer radii of 0.50 and 5.0 mm, a peak accelerating gradient behind the bunch of 155 MeV/m is predicted. This relatively high magnitude of accelerating gradient suggests that a simple uniform dielectric pipe could be the basis for the structure of a future high-gradient electron/positron linear accelerator, once low-emittance, kiloampere, subpicosecond electron bunches are available in the laboratory.

Analysis is presented of the gyroresonant acceleration of electrons in a vacuum using a focused laser. Continuous and equal acceleration is shown for electrons injected at all optical phases over an interaction length of tens of centimeters. Beam stalling is avoided as beam energy increases. Acceleration from 50 to 178 MeV is predicted for a 4 TW, 10.6-μm laser focused to a waist radius of 1.0 mm; these parameters correspond to a planned experiment. A beam stop with an off-axis hole after acceleration is shown to create a train of optically chopped bunches with 3-fs bunch lengths and a 35-fs period.

It has been shown previously that a cyclotron autoresonance accelerator (CARA) has a practical upper energy limit that arises from axial velocity stalling in the rising axial magnetic field. This paper shows that this upper energy limit can be overcome by operating a series of CARA stages, with each succeeding stage at a higher gyroharmonic. The accelerator would be driven from a single external rf source, and gyroharmonic operation in stages beyond the first would be accomplished by stagewise reduction in the guide magnetic field. Analytical and simulation results are presented to show, under ideal conditions, that stalling can thereby be avoided and acceleration to high energy can be achieved. For a three-stage CARA with a 15-A, 250-kV injected electron beam driven by 75 MW of rf power at 11.424 GHz, the results show that an energy of over 5 MeV can be reached at the end of the third stage, whereas a practical single-stage CARA with a 250-kV injected beam has an upper energy limit of about 1.7 MeV. It is also shown that tandem operation of a large number of CARA stages could, in principle, allow acceleration of electrons up to GeV energies.

A wake-field accelerator is described based on the use of a waveguide structure in which many modes can participate in wake-field formation, and in which the wake-field period equals the period of a train of drive bunches. A dielectric-lined waveguide is analyzed that is shown to support multimode propagation with all modes having nearly equal phase velocities, equal to the initial velocity of injected charge bunches that drive the wake fields. For this waveguide, the ratio of wake field to drag field for a bare drive bunch is 4.7, as compared to 2.0 for a single-mode waveguide. The composite TM_0n wake field of such a structure is shown to include highly peaked axial electric fields localized on each driving bunch in a periodic sequence of bunches. This allows stimulated emission of wake-field energy to occur at a rate that is larger than the coherent spontaneous emission from a single driving bunch of equal charge and energy. This mechanism can make possible the design of a stimulated dielectric wake-field accelerator that has the potential of providing an acceleration gradient for electrons or positrons in the range of 50–100 MV/m, taking a driving bunch charge of a few nC. We present calculations for such wake fields from a bunched sheet beam in a two-dimensional dielectric waveguide. Numerical examples are given, including the acceleration of a 30 MeV test bunch to 155 MeV in a structure 200 cm in length, using ten identical 2 nC/mm drive bunches.

It is shown that the lowest TE_s,l mode in a cylindrical waveguide at frequency sω, with group velocity nearly identical to group velocity for the TE₁₁ mode at frequency ω, is that with s  =  7, l  =  2. This allows coherent radiation to be generated at the seventh harmonic during electron cyclotron autoresonance acceleration. Conditions are found where such co-generation of 7th harmonic power at 20 GHz is possible with overall efficiency greater than 80%. This mechanism could make possible high efficiency cm-wavelength high power rf sources suitable for driving a future multi-TeV electron-position collider.

Analysis and experimental tests have been carried out on a dielectrically lined waveguide, which appears to be a suitable structure for accelerating electrons. From the dispersion relation for the TM₀₁ mode, inner and outer radii of a copper-clad alumina pipe (ɛ=9.40) have been determined such that the phase and group velocities are 0.9732c and 0.1096c, respectively. Analysis and particle simulation studies for the injection of 6-MeV microbunches from a 2.856-GHz rf gun, and subsequent acceleration by the TM₀₁ fields, predict that an acceleration gradient of 6.3 Mv/m can be achieved with a traveling-wave power of 15 MW applied to the structure. Synchronous injection into a narrow phase window is shown to allow trapping of all injected particles. The rf fields of the accelerating structure are shown to provide radial focusing, so that longitudinal and transverse emittance growth during acceleration is small and that no external magnetic fields are required for focusing. The acceleration mechanism is the inverse of that in which electrons radiate as they traverse a waveguide at speeds exceeding the phase velocity of the microwaves (Čerenkov radiation) and is thus referred to as a microwave inverse Čerenkov accelerator. For 0.16-nC, 5-psec microbunches, the normalized emittance of the accelerated beam is predicted to be less than 5π mm mrad. Experiments on sample alumina tubes have been conducted that verify the theoretical dispersion relation for the TM₀₁ mode over a two-to-one range in frequency. No excitation of axisymmetric or nonaxisymmetric competing waveguide modes was observed. High power tests showed that tangential electric fields at the inner surface of an uncoated sample of alumina pipe could be sustained up to 8.4 MV/m without breakdown. These considerations suggest that a microwave inverse Čerenkov test accelerator can be built to examine these predictions using an available rf power source, a 6-MeV rf gun, and an associated beam line. © 1996 The American Physical Society.

First experimental results are reported on the operation of a multimegawatt 2.856 GHz cyclotron autoresonance accelerator (CARA). A 90–100 kV, 2–3 MW linear electron beam has had up to 6.6 MW added to it in CARA, with an rf-to-beam power efficiency of up to 96%. This efficiency level is larger than that reported for any fast-wave interaction between radiation and electrons, and also larger than that in normal conducting rf linear accelerators. The results obtained are in good agreement with theoretical predictions.

A multimegawatt gyroharmonic converter depends critically on the parameters of a spatiotemporally modulated gyrating electron beam prepared using a cyclotron autoresonance accelerator (CARA). This paper extends a prior analysis of CARA [B. Hafizi, P. Sprangle, and J. L. Hirshfield, Phys. Rev. E 50, 3077 (1994)] to identify an approximate constant of the motion and, therefore, to give limits to the beam energy from CARA that can be utilized in a harmonic converter. It is also shown that particles are strongly phase trapped during acceleration in CARA and thus are insensitive to deviations from exact autoresonance. This fact could simplify construction of the up-tapered guide magnetic field in the device and augurs well for production of high-quality multimegawatt beams using CARA.

Analysis and numerical evaluations are presented for the spatial growth of small-signal modified electron Bernstein waves that stand radially and propagate axially along a beam-filled coaxial waveguide. It is shown that these waves, when coupled strongly to fields of an electromagnetic TE₁₁ mode in the interior cylindrical waveguide of this configuration, can make possible an amplifier of cyclotron harmonic waves that would possess several unique properties. In particular, millimeter wavelength amplifiers based on this interaction would require neither high magnetic fields nor high electron beam voltages, and thus would not require the bulky superconducting magnets and highly insulated high voltage power supplies usually associated with fast-wave gyro devices. The analysis extends prior work on interactions of this type, including in the calculations simultaneous contributions from more than one term in the infinite series dispersion relation. Effects of finite axial beam velocity spread on the gain and bandwidth characteristics of the interactions are also calculated, thereby accounting for the irreducible velocity spreads attributable to space charge potential depression on the beam.

An analysis of the electron beam dynamics in a cyclotron autoresonance accelerator (CARA) is presented. The beam is to be employed in harmonic convertor experiments to generate high-power centimeter-wavelength microwaves, with potential application as a driver for a next-generation electron-positron collider. The presentation will highlight the quality of the electron beam generated by this acceleration mechanism. For beam energies up to about 1 MeV and beam currents up to 50 A, the evolution of the axial velocity spread is determined by a self-consistent numerical solution of the governing nonlinear equations for an ensemble of electrons with finite initial emittance. It is shown that the requirement for an up-tapered guide magnetic field to maintain resonance leads to a diminution of the axial velocity (due to the mirror effect) that eventually causes the acceleration to cease. It is also shown that under some conditions a CARA can have an acceleration efficiency approaching 100%. An estimate is given, and examples are considered for the deterioration of a CARA beam’s spatiotemporal character arising from self-fields.

Linear and nonlinear theory is presented for the generation of gyroharmonic radiation from spatio-temporally modulated electron beams in cylindrical waveguides. Selection rules for axisymmetric beams are derived that show coupling at the mth temporal harmonic to be absent for all TM modes, and for TE modes other than those with an azimuthal mode index of m. Estimates from linear theory are given for the effects of spreads in axial momentum and guiding-center radius. A nonlinear multimode theory is developed in order to treat mode saturation and mode competition for TE modes in a down-tapered guide magnetic field. Numerical simulations of the multimode nonlinear generation of fifth-harmonic radiation at 94 GHz from a 150-kV, 6.667-A, α=2 beam show that nearly 90% of the initial transverse energy of the beam can be converted to fifth-harmonic radiation in the TE₅₁ mode, and that less than 2.5% of the beam energy is converted to other harmonics. Use of a linearly tapered guide magnetic field gives lower saturated fifth-harmonic conversion efficiency than does use of a nonlinear taper. But the linear-taper case shows greater resilience to axial momentum spread than does the nonlinear taper case. The spent beam is nearly monoenergetic and phase coherent at saturation, suggesting that overall harmonic-conversion efficiencies can approach 100% if a single-stage depressed collector is used to recover the spent beam energy.

A multimode nonlinear particle simulation code is used to find the saturated efficiency for power transfer into modes of a cylindrical waveguide carrying a spatiotemporally modulated gyrating electron beam. For A TE₅₁-mode fifth harmonic 94-GHz harmonic converter using a 150-kV, 6.7-A cold beam, this code predicts a conversion efficiency of 57% when a linearly tapered guide magnetic field is used, and 70% when a nonlinear taper is used. Efficiency in the linear taper case is shown to be insensitive to beam axial velocity spread, while both cases show negligible power flow into competing modes.

A previously published analysis [J. L. Hirshfield, Phys. Rev. A 44, 6845 (1991)] of the first-order transfer of power into fields of a TE_0m rectangular waveguide from a relativistic electron beam carrying spatiotemporal modulation is extended to TE_lm modes. Non-axis-encircling orbits, circularly polarized excitations, and competing modes are incorporated into the expanded analysis. Selection rules and phase-matching conditions are found that govern the power transfer; these are shown to allow wave power to increase quadratically with both the interaction length and the dc beam current. Examples of fifth-harmonic conversion are presented for both TE₃₂ and TE₀₃ modes, the latter in a square waveguide supporting a circularly polarized wave. Power levels of 100 kW or more at 94 GHz appear to be achievable in a conceptual fifth-harmonic device using a 200-kV, 1-A beam. The circularly polarized mode appears to be relatively free of mode competition. Devices with lower beam energies are also shown to be capable of fifth-harmonic operation at 94 GHz, albeit with lower output power than for devices with higher beam energies.