kA Beam Research Articles

Experimental Demonstration of Dual-Mode Relativistic Backward Wave Oscillator With a Beam Filtering Ring Packaged With Permanent Magnet

It is reported on the design and experiment of a dual-mode (TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</sub> -TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">02</sub> ) relativistic backward wave oscillator (RBWO) with a beam filtering ring (BFR) packaged with permanent magnet. The BFR makes use of the large transverse oscillation of electrons at low magnetic field. By absorbing part of the electron beam periodically, the amount of the electrons that would absorb power from the microwave in the slow-wave structure (SWS) is greatly reduced, so the beam-wave conversion efficiency is improved. Experimental results demonstrate that a 3.2 GW mixed-modes microwave at 9.87 GHz with an efficiency of 40% is obtained with a 675 kV, 11.8 kA beam guided by a 0.68 T magnetic field. The BFR has a considerable lifetime and can effectively improve the efficiency of the device with stable operation.

Tapered helical undulator system for high efficiency energy extraction from a high brightness electron beam

In this paper we discuss the design choices and construction strategy of the tapered undulator system designed for a high energy extraction efficiency experiment in the ultraviolet region of the electromagnetic spectrum planned for installation at the Argonne National Laboratory Linac Extension Area (LEA) beamline. The undulator is comprised of 4 sections pure permanent magnet Halbach array separated by short break sections, each one of them housing a focusing quadrupole doublet and a phase shifter. The quadrupoles use a novel hybrid design which allows one to vary the gradient and match the beam transversely. The undulator tapering profile is optimized to maximize the energy conversion efficiency from a 343 MeV 1 kA beam into coherent 257.5 nm radiation taking into account the longitudinal current profile generated by the linac.

Characteristics of High-Current Electron Gun with Plasma Anode Based on Hybrid Discharge

Spatial structure of glow and time dynamics of a hybrid discharge that combines high-current reflective discharge and vacuum arcs induced by spark breakdown on dielectric surface are studied at a supply voltage of up to 9 kV and working gas (argon) pressures of 0.1–1 mTorr. A plasma anode with an increased peripheral concentration of ions can be formed, in particular, with the aid of an auxiliary shield. Distributions of energy density are measured over the cross section of a nonrelativistic (up to 30 keV) high-current (up to 25 kA) beam generated in an electron gun with an explosive emission cathode and a plasma anode based on this (hybrid) discharge. The results are comparable with the results obtained using concentrators of magnetic field. It is expedient to employ the proposed method for improvement of uniformity of the beam, since application of the concentrators may be impossible.

Experimental Research of X-Band Dual-Frequency Coaxial Relativistic Backward Wave Oscillator

The design and experimental demonstration of an X-band coaxial dual-frequency relativistic backward wave oscillator (CRBWO) with sectioned slow wave structures (SWSs) is presented. In this paper, we describe the experimental measurement methods of frequency, power and operation mode for the high-power microwave signal. In experimental tests, with a 500 kV, 11.5 kA beam guided by a 0.82 T magnetic field, a microwave with an average power of 507 MW and pulsewidth of 50 ns has been obtained. Also, the dominant frequencies of the X-band dual frequency are 9.845 GHz and 10.517 GHz, and the dual-frequency difference ${\Delta }{f}\approx {672}$ MHz, which was similar to the particle-in-cell simulation results. The operation mode of the device is determined to be TM01 mode in our experiment.

Over-Sized Mode-Selective Relativistic Backward Wave Oscillator

An oversized mode-selective relativistic backward wave oscillator is proposed. This device operates using the pure TM02 mode in the beam-wave interaction area and transmits a pure TM01 mode in the output waveguide. In the beam-wave interaction area, mode selection is well realized through oversized slow-wave structures with a coaxial inner conductor that provides perfect boundary conditions for the TM02 mode and simultaneously eliminates other modes. A large proportion of the TM02 mode field is eventually exported into the output waveguide in the form of a pure TM01 mode. With the large diameter and high mode purity, the device can be expected to have high power capacity. With a 770 kV, 11.0 kA beam guided by a 3.5 T magnetic field, a 3.2 GW microwave at 9.42 GHz has been obtained in our simulation. The device efficiency is 38%.

Quality control of the concrete and corrosion damage of cables of prestressing reinforcement of KA beams, bridge in Chomutov

The aim of this paper is to assess the condition of the diagnosed bridge structure, at which previous bridge inspections identified corrosion damage of cables of prestressing reinforcement of KA beams, to assess the extent of corrosion damage of prestressing reinforcement, the condition of chloride ions in concrete of the bearing structure, and to draw up a detailed corrosion assessment of the prestressing reinforcement, then to propose a possible method of the bridge renovation with partial use of new materials (UHPC).

Compact single-pass X-ray FEL with harmonic multiplication cascades

The generation of X-ray radiation in cascaded single-pass free electron laser (FEL), which amplifies high harmonics of a two-frequency undulator, is studied. Power dynamics of FEL harmonics is explored with the help of the phenomenological model of a single pass FEL. The model describes both linear and non-linear harmonic generation, starting from a coherent seed laser and initial shot noise with account for main loss factors for each harmonic in each cascade individually: the energy spread and beam divergence, the coupling losses between FEL cascades, the diffraction etc. The model was validated with the experiment and with relevant 3-D simulations. It is employed for modeling the cascaded FELs with harmonic multiplication and analyzing the evolution of FEL harmonic power with the aim to obtain the maximum high harmonic power in the X-ray band at the shortest possible FEL length with the lowest possible seed frequency. The advantages of two-frequency undulators in HGHG FELs are elucidated. The requirements for the electron beam are studied; the need for low energy spread is evidenced: our evaluations yield σe<2×10−4. Several cascaded HGHG FELs with two-frequency undulators are modeled. Generation of soft X-ray radiation at λ=2.71nm, reaching ∼50 MW power with I0∼100 A in a cascaded FEL at just 40 m with 13.51 nm seed, matching peak reflectivity of Mo/Si, is demonstrated. The generation of 40 MW radiation power at λ=2.27nm with the beam current I0∼100 A, energy E=950MeV and the energy spread σe=2×10−4 is studied, using second and third harmonics in three-stage 45 m long FEL. The multistage FEL is modeled for generating radiation in nanometer band: ∼40 MW power at λ∼2.6 nm with I0∼175 A current in just ∼40 m long FEL with commercially available F2 excimer UV laser seed at 157 nm. The peak radiation power rises to ∼0.5 GW for ∼1 kA beam current.

A vacuum-sealed, gigawatt-class, repetitively pulsed high-power microwave source

A compact L-band sealed-tube magnetically insulated transmission line oscillator (MILO) has been developed that does not require bulky external vacuum pump for repetitive operations. This device with a ceramic insulated vacuum interface, a carbon fiber array cathode, and non-evaporable getters has a base vacuum pressure in the low 10−6 Pa range. A dynamic 3-D Monte-Carlo model for the molecular flow movement and collision was setup for the MILO chamber. The pulse desorption, gas evolution, and pressure distribution were exactly simulated. In the 5 Hz repetition rate experiments, using a 600 kV diode voltage and 48 kA beam current, the average radiated microwave power for 25 shots is about 3.4 GW in 45 ns pulse duration. The maximum equilibrium pressure is below 4.0 × 10−2 Pa, and no pulse shortening limitations are observed during the repetitive test in the sealed-tube condition.

Improved foilless Ku-band transit-time oscillator for generating gigawatt level microwave with low guiding magnetic field

An improved foilless Ku-band transit-time oscillator with low guiding magnetic field is proposed and investigated in this paper. With a non-uniform buncher and a coaxial TM02 mode dual-resonant reflector, this improved device can output gigawatt level Ku-band microwave with relatively compact radial dimensions. Besides the above virtue, this novel reflector also has the merits of high TEM reflectance, being more suitable for pre-modulating the electron beam and enhancing the conversion efficiency. Moreover, in order to further increase the conversion efficiency and lower the power saturation time, a depth-tunable coaxial collector and a resonant cavity located before the extractor are employed in our device. Main structure parameters of the device are optimized by particle in cell simulations. The typical simulation result is that, with a 380 kV, 8.2 kA beam guided by a magnetic field of about 0.6 T, 1.15 GW microwave pulse at 14.25 GHz is generated, yielding a conversion efficiency of about 37%.

A Ku-band coaxial relativistic transit-time oscillator with low guiding magnetic field

AbstractA novel coaxial relativistic transit-time oscillator with low guiding magnetic field is proposed and investigated to generate high power microwave at Ku-band. With the coaxial structure and a quasi body wave adopted as the operating mode, the device has a larger space-charge limiting current, higher power handling capacity, and lower guiding magnetic field. Moreover, for further improving the output power, a coaxial TM02mode resonant reflector is well designed. Main structure parameters of the device are optimized by particle in cell simulations. A typical simulation result is that, with a 358 keV, 7.25 kA beam guided by a magnetic field of about 0.7 T, an 810 MW microwave pulse at 14.25 GHz is generated, yielding a conversion efficiency of about 31%. The primary experiments are also carried out. At a low guiding magnetic field of 0.7 T, a microwave pulse with power of 400 MW, pulse duration of 30 ns, frequency of 14.3 GHz close to the simulation one, and efficiency of 15.4% is generated.

An oversized X-band transit radiation oscillator

An oversized transit radiation oscillator is designed to generate high power microwave at X-band. By using a coaxial structure, the power capacity of the device is improved significantly. In order to realize long-pulse operation, the cathode-anode gap, the collector location, and the surface field strength have been emphatically considered in our design. With a 710 kV, 14.5 kA beam guided by a 0.8 T magnetic field, a 2.7 GW microwave at 9.38 GHz has been obtained in the simulation. The power conversion efficiency is 26.2%. The simulation also indicates that the highest axial electric field strength on the surface of electrodynamic structure is only 590 kV/cm, which could be further decreased by increasing the radial dimension of the X-band device.

Dual-band relativistic backward wave oscillators based on a single beam and dual beams

Two types of relativistic backward wave oscillators (RBWOs) used to produce dual-band microwaves are proposed and investigated by use of the particle-in-cell (PIC) simulation code KARAT [V. P. Tarakanov, User’s Manual for Code Karat (Berkeley Research Associates, Springfield, VA, 1992)]. The first type of RBWO, for generation of C-band and X-band microwaves, is designed based on a single beam and a sectioned structure. With an electron beam of 650 keV and 5.0 kA guided by a magnetic field of 2.0 T, an average power of 380 MW with a total power conversion efficiency of 11.7% is obtained and the frequencies are 5.48 and 9.60 GHz, respectively. By changing the distance between the two sections, single-band oscillations are realized with higher power conversion efficiency than that of the dual-band oscillation. The second type, based on a coaxial structure and dual parallel annular beams, is a dual-band RBWO designed with separated beam-wave interaction regions for generation of C-band and X-band microwaves. With a dual beam of 650 keV and 11.8 kA (the outer beam current is 6.4 kA and inner beam current is 5.4 kA) guided by a magnetic field of 2.0 T, an output power of 1400 MW with a total power conversion efficiency of 18.3% is generated and the frequencies are 4.60 and 8.40 GHz, respectively. PIC simulations demonstrate that the two beam-wave interaction regions operate independently. The two types of dual-band RBWO are also compared and analyzed.

A time-dependent non-linear theory of ion channel electron cyclotron maser

A time-dependent non-linear theory is developed for ion channel electron cyclotron maser (ICECM), and the corresponding numerical calculation results are given for the first time. The theoretical analysis shows that the longitudinal plasma wave tends to enhance the energy exchange between the beam and wave. Using the numerical calculation of the self-consistent time evolution of scattering wave growth, the non-linear effect of ion density on the beam-wave interaction efficiency has been studied. The time norm, which must be satisfied in the operation of ICECM, is verified. Taking 6.8 × 1013 cm−3 ion density, 1 MV accelerated voltage and 1.5 KA beam current, the present numeric calculation results show that the pulse scattering wave power and the frequency can achieve about 200 MW and 284 G rad/s respectively.

Intense electron-beam ionization physics in air

In this paper we study, experimentally and theoretically, the interactions of an intense electron beam with an initially-neutral background gas. The Naval Research Laboratory’s Gamble II generator [J. D. Shipman, Jr., IEEE Trans. Nucl. Sci. NS-18, 243 (1971)] was used to drive an annular 900 kV, 800 kA beam, whose effects on background air in the pressure range ∼0.01 to 10 Torr were studied. Experimental diagnostics included a sophisticated two-color interferometer for time-resolved measurements of the background electron density, B-dot monitoring of the global net current, and x-ray pinhole images of the beam location. Data obtained were compared to extensive simulations using three numerical models that incorporated complex beam physics, atomic processes, and the capability for simulating strongly-disturbed gases. Good simulation agreement with net current and electron density as a function of pressure was obtained using a scaled pressure. Simulated and experimental net current fractions (at peak beam current) for the 1–10 Torr collision-dominated transport regime were on the order of 10%, while ionization fractions after the beam pulse were 20% for 10 Torr, rising to nearly 100% at the lower pressure of 0.5 Torr. More advanced model development is underway to better understand the important physics of beam–gas interactions.

Grad-B drift transport and focusing of high-current electron beams

This paper studies, experimentally and theoretically, the grad-B drift transport and focusing of an intense electron beam. The Naval Research Laboratory’s Gamble II generator [J. D. Shipman, Jr., IEEE Trans. Nucl. Sci. NS-18, 243 (1971)] was used to drive a 900 kV, 850 kA beam. This beam drifted in the radial magnetic field gradient produced by an on-axis guide wire carrying a current of 40 kA. An additional axial magnetic field gradient generated by focusing spokes was used to radially compress the beam. Experimental diagnostics included radially resolved calorimetry and radiachromic film to study wall losses. Extensive simulations were performed to better understand the experiments. Energy transport efficiencies were slightly below 70%, and the focusing spokes were observed to compress the beam radius by approximately 10%.

Electron trapping in high-current ion beam pipes

The space charge voltage depression in a drifting heavy ion beam during the final stages of current pulse compression can be hundreds of kV. For example, a 1 kA beam of ions at β= v/ c=0.4 would have a beam center-to-edge potential difference of 75 kV. With suitable clearance from beam edge to the beam pipe, this amount is typically increased by a factor of 2–3 by the (1+2 ln( b/ a)) term that accounts for the ratio of pipe radius, b, to beam radius, a. Such high voltages, and resulting high electric fields at the pipe wall, will result in electrons being pulled into the beam pipe. These electrons which are emitted from the grounded beam pipe, will pass through the ion beam at high velocity and then turn around without (usually) striking the wall and continue to pass through the beam on repeated oscillations. It is possible to control the longitudinal motion of these trapped electrons by suitably varying the pipe size while considering the beam diameter. A segment of the beam pipe that has a larger diameter will result in a potential well that traps the electrons longitudinally. In a constant current scenario in a uniform pipe, the electrons will drift in the direction of the beam. However, the head and especially the tail of the ion beam will have a dramatic effect on the electrons, causing them to be pulled into the ion beam. These complex processes will continue until the ion beam passes through an optical element such as a beam transport magnet that will effectively block the motion of the electron clouds following the ions. In this paper, we will show examples of how electrons can be trapped and controlled by varying the conditions determining their emission and confinement. Ray tracing simulations using the EGN2 (Herrmannsfeldt, SLAC Electron Trajectory Program, SLAC-331 (1988)) computer code will be used to model the electron trajectories in the presence of a high current heavy ion beam. The self-magnetic field of the ion beam, while not sufficient to affect the ions themselves significantly, has a strong influence on the electron orbits.

HIGH-POWER, INTENSE-CURRENT-DENSITY, LOW-EMITTANCE, HIGH-BRIGHTNESS PSEUDOSPARK ELECTRON BEAM DRIVEN BY IMPROVED PULSE LINE ACCELERATOR

High-power ( 200 kV), intense-current-density, low-emittance, high-brightness electron beam was generated in a 10 gap pseudospark chamber filled with 15 Pa N2 and driven by an improved pulse line accelerator. The ejected electron beam was characterized by 1mm diameter, 2. 2 kA beam current, and 400ns pulse length, and propagated 20cm in neutral N2 ionizing channel in the drift tube. At 5 cm downstream of the anode, it penetrated consecutively an acid-sensitive discoloring film and a 0.05 mm-thick copper foil, and left 0.6 mm and 0.3 mm holes on them, separately. According to the pinching equilibrium condition, the beam's normalized emittance and brightness are respectively calculated to be 23 πmm-mrad and 8×1010A/(m rad)2. That the 10 shots on the same acid-sensitive film produced a hole of 1. 6mm in diameter at 7cm showed a good repeatability of the electron beam. After 60 shots the pseudospark chamber was observed that there was almost no destructive damage traces left on the surfaces of the electrodes and insulators. It was found that the quality of electron beam produced in the pseudospark discharge chamber driven by the modified pulse line accelerator is much better than that generated by cold cathode discharges.

Beam generation, acceleration, and extraction on a stellatron accelerator

The stellatron accelerator of the University of California at Irvine (UCI) [Phys. Fluids B 2, 3149 (1990)] was modified to accommodate beam extraction. The magnetic field coils were reconstructed to adapt an extraction port. A fiberglass–graphite–epoxy chamber replaced the glass chamber that cracked frequently during the previous experiment. Improvement in the magnetic field configuration has significantly reduced the current loss during acceleration. A 1.1 kA, 12 MeV electron beam was generated by initially forming a 1.2 kA beam with plasma start-up. The beam radius was 5–7 mm. Beam extraction was performed with two auxiliary pulsed current coils and up to 15% of the beam was extracted from the chamber. The extraction process was analyzed by computational orbit simulations.

Interaction of intense MeV light-ion beams with low-pressure gases*

The interaction of intense proton beams with low-pressure (0.25 to 4 Torr) background gases is studied to evaluate beam-current neutralization during transport. Electrons to neutralize the beam are provided by beam-induced ionization of the gas. In experiments with 1 MeV, 1 kA/cm2 protons, net currents outside the beam envelope and electron densities within the beam envelope are measured for helium, neon, argon, and air. Net-current fractions are 2% to 8% and ionization fractions are 0.6% to 5% for 5 to 7 kA beams. Simulations of the experiments for helium and argon suggest that fast electrons play an important role in generating a significant fraction of the return current in a halo outside the beam. As a result, net currents inside the beam may be larger than inferred from magnetic-field measurements outside the beam. Ions at the head of the beam are observed to lose more energy than expected from collisional energy losses in the background gas.

Electron beam tracking in a preformed density channel

High-current charged particle beams can be guided by reduced density channels. Such guiding occurs when the distribution of plasma currents in the density channel causes a net attractive force to be exerted on the beam. In particular, a relativistic electron beam (REB) injected parallel to a spatially offset, reduced density channel is pulled toward the channel. The force exerted on the beam is predicted to increase as the beam current increases and as the offset between the beam and the channel increases out to offsets equal to the channel radius. An experiment with a 1 MV, ≊10 kA beam was performed that demonstrates this effect.