Dynamics of subnanosecond electron beam formation in gas-filled and vacuum diodes

The dynamic characteristics of a subnanosecond pulsed electron beam formation in the accelerating gap of a gas-filled or evacuated diode have been studied at a time resolution ～10^?11 s. In the air-filled gap, the electron beam pulse with a current amplitude of several amperes is formed up to about one hundred picoseconds earlier than the analogous pulse under vacuum conditions, and the measured pulse duration (～10^?10 s) is close to the electron flight time across a diode gap in the continuous acceleration regime. It is shown that a nanosecond prepulse plays an important role by initiating the emission of electrons that are subsequently accelerated by the high-voltage pulse with a subnanosecond front.     

Influence of nonuniformity of the ion background on the oscillation frequency of a virtual cathode

The oscillation frequency of a virtual cathode in a diode gap is investigated as a function of the density of the anode ion layer. It is shown that when the densities of the ion layer are high, a stable electron bunch forms in the flux, which leads to an increase in the oscillation frequency. Pis’ma Zh. Tekh. Fiz. 24, 74–80 (November 12, 1998)     

Electron bunch compression and acceleration in the laser wakefield

The compression and acceleration of an external electron bunch into the laser wakefield is studied using 3D modeling with the LAPLAC code and compared to analytical predictions. It is shown, for a laser propagating in a plasma channel, that the nonlinear laser pulse dynamics together with the finite laser spot size influence the electron bunch compression and acceleration due to the reduction of the laser pulse group velocity. The transverse bunch dynamics and loading effect determine the final bunch charge and density and restrict the compressed sizes of the trapped and accelerated electron bunch. The dynamics of the electron bunch are illustrated with a set of parameters where the accelerated bunch acquires an energy of the order of 2 GeV, and 1% energy spread with sub-micron sizes.     

Ultrashort electron beams generated on the flat part of a voltage pulse in nitrogen and helium

In the case of voltage pulses with a small amplitude, an ultrashort avalanche electron beam (UAEB) in a diode filled with nitrogen or helium is generated on a flat part of the pulse. UAEBs obtained at a voltage of 25 kV have a full width at half maximum (FWHM) of about 200 ps and are delayed relative to the voltage pulse front by a time reaching tens of nanoseconds. Waveforms of the electron beam current pulse with several peaks of subnanosecond duration have been observed. At elevated pressures in a gas-filled diode, the voltage across the gap decreases by 10–20% during the electron beam generation.     

Annular structures formed in a beam of ions during their collective acceleration in a system with dielectric anode

We have studied the collective acceleration of protons and deuterons in an electron beam emitted from plasma formed at the surface of a dielectric anode insert. The experiments were performed with a pulsed electron accelerator operating at an accelerating voltage up to 1 MV, current amplitude up to 40 kA, and pulse duration of 50 ns. Reduction of the accelerating voltage pulse front width and optimization of the diode unit and drift region ensured the formation of several annular structures in the electron beam. As a result, up to 50% of the radioactivity induced in a copper target was concentrated in a ring with 4.5-cm diameter and 0.2-cm width. The formation of high energy density in these circular traces and the appearance of an axial component of the self-generated magnetic field of the electron beam are related with the increasing efficiency of acceleration of the most intense group of ions.     

Development of a 300-kV Marx generator and its application to drive a relativistic electron beam

We have indigenously developed a twenty-stage vertical structure type Marx generator. At a matched load of 90–100 Ω, for 25 kV DC charging, an output voltage pulse of 230 kV, and duration 150 ns is obtained. This voltage pulse is applied to a relativistic electron beam (REB) planar diode. For a cathodeanode gap of 7.5 mm, an REB having beam voltage 160kV and duration 150ns is obtained. Brass as well as aluminum explosive electron emission-type cathodes have been used     

Variation of the beam parameters of runaway electrons in a gas discharge under the conditions of nonuniform preliminary ionization

Within the theoretical model of a high-pressure hybrid nanosecond discharge with runaway electrons, a strong dependence of the electron beam amplitude, duration, and energy spectrum on the conditions of the preliminary ionization of a gas in the discharge gap is demonstrated. The conditions with uniform and nonuniform distributions of initial electrons in a coaxial diode filled with sulfur hexafluoride at atmospheric pressure are simulated. It is shown that the amplitude and current pulse profile of the electron beam substantially change upon the variation of the initial distribution of the electrons in the discharge gap.     

Proposal for a one GeV plasma wakefield acceleration experiment at SLAC

A plasma-based wakefield acceleration experiment E-157 has been approved at SLAC to study acceleration of parts of an SLC bunch by up to 1 GeV/m over a length of 1 m. A single SLC bunch is used to both induce wakefields in the 1 m long plasma and to witness the resulting beam acceleration. The experiment will explore and further develop the techniques that are needed to apply high-gradient plasma wakefield acceleration to large-scale accelerators. The 1 m length of the experiment is about two orders of magnitude larger than for other high gradient plasma wakefield acceleration experiments and the 1 GeV/m accelerating gradient is roughly ten times larger than that achieved with conventional metallic structures. Using existing SLAC facilities, the experiment will study high gradient acceleration at the forefront of advanced accelerator research.     

Modeling the formation of a deep potential well in a vacuum diode

The formation of a deep nonstationary potential well in the gap between the cathode flare front and the anode has been modeled with allowance for a floating potential at the boundary of an expanding plasma. It is shown that a long-lived potential well with a lifetime on the order of nanoseconds can form in the vacuum diode. The obtained results provide an explanation of the phenomenon of collective ion acceleration at the spark stage of vacuum discharge.     

Protecting the anode foil of a high-current electron accelerator against arc-discharge damage

The mechanism of anode foil damage during the extraction of a high-power pulsed electron beam from a high-current diode has been experimentally studied on a TEU-500 electron accelerator [1]. It is established that the breakage of the anode foil is caused by the appearance of cathode spots on its surface, the intense electron emission from these spots during positive voltage pulses (postpulses following the main negative pulse of accelerating voltage), and the formation of arc discharge in the interelectrode gap. The improvement of diode matching to the pulse-forming line of the accelerator and the use of an auxiliary electrode (anode) forming additional vacuum discharge gap (crowbar) with the cathode practically excludes the anode foil breakage by arc discharge and significantly increases the working life of the foil (up to ∼10⁵ electron beam pulses).     

Simulation of ultrarelativistic beam dynamics in the plasma wake-field accelerator

With two-dimensional hybrid code LCODE, the long-term dynamics of two drivers in plasma is studied. The bunch train is shown to produce a well-controlled wake-field good for particle acceleration. In this wake-field, a narrow witness bunch can be accelerated with a small energy spread (several percent), a high energy gain (exceeding the initial driver energy), and the driver-witness efficiency about 20%. The shaped long bunch is unusable for particle acceleration, since it is rapidly destroyed by the transverse two-stream instability.     

Moment of injection of runaway electrons at the front of accelerating pulse in air-filled diode with inhomogeneous field: From instability to determinacy

A simple method is proposed that makes it possible to determine the moment of injection of a bunch of runaway electrons relative to a subnanosecond leading front of the accelerating high-voltage pulse applied to the cathode in air-filled diode with inhomogeneous field. The moment of injection is determined by finding a minimum in dispersion of the time delay between the variable point of oscilloscope sweep triggering on the instable front of the accelerating voltage pulse and the detected front of the electron current pulse.     

On laser wakefield acceleration in plasma channels

The structure of the laser wakefield is analyzed for wide and narrow (in comparison with plasma wavelength) plasma channels with parabolic in radial direction plasma density distributions. The results of analytical theory are confirmed by the self-consistent nonlinear numerical modeling of laser pulse propagation and wakefield generation. In narrow plasma channels the accelerating longitudinal component of the wakefield decreases rapidly with the distance from a laser pulse. This makes possible a short single electron bunch acceleration even if the injected electron beam is much longer than a plasma wavelength.     

Wakefield radiation generated in a dielectric-filled rectangular accelerating structure

The Vavilov-Cherenkov radiation generated by a relativistic electron bunch that propagates in the vacuum channel of a rectangular dielectric-filled wakefield accelerating structure is analytically described. The approach is based on the expansion of wake fields in terms of the eigenfunctions of operators of the transverse electric and magnetic field components. Expressions for the wake fields generated by a Gaussian electron bunch are obtained. Using the proposed formalism, parameters of a diamond-based accelerating structure admitting the generation of potential gradients above 100 MV/m have been calculated for a typical electron bunch of the Argonne Wakefield Accelerator.     

High-current-density subnanosecond electron beams formed in a gas-filled diode at low pressures

The formation of electron beams in a gas diode filled with various gases at low and medium pressures under the action of nanosecond voltage pulses has been studied. It is shown that subnanosecond pulses of the beam current in helium, hydrogen, neon, nitrogen, argon, methane, sulfur hexafluoride, krypton, and xenon can be obtained both at atmospheric pressure and at a pressure of several units or dozens of Torr. In particular, a beam current density above 2 kA/cm² behind the foil at a pulse duration (FWHM) of 250 ps has been obtained in helium-filled diode. On the passage from the regime of ultrashort avalanche electron beam formation to the vacuum diode regime, the beam current pulse amplitude decreases, while both the beam pulse duration (FWHM) and the pulse front width increase.     

Matching a double forming line to explosive-emission diode

The regime of operation of an explosive-emission diode is affected by a matching transformer between this diode and a double forming line of a high-current electron accelerator. Preliminary forced demagnetization of the transformer core makes the shape of the voltage pulse applied to the cathode close to the optimum, corresponding to a decrease in the resistance of the anode-cathode gap related to expansion of the explosive-emission plasma. In addition, matching of the double forming line to the explosive-emission diode significantly decreases the amplitude of parasitic prepulses and increases (to 90–92%) the fraction of energy supplied to the diode during the main current pulse.     

Effect of nitrogen pressure on the energy of runaway electrons generated in a gas diode

The energy spectra of runaway electrons generated in a gas diode under the action of voltage pulses with a front width of ∼300 ps and amplitude of ∼140 kV have been studied using a time-of-flight spectrometer at nitrogen pressures in a range of 0.1–760 Torr. The delay of runaway electron beam pulse relative to the driving voltage pulse has been determined. The electron energy depends in a complicated manner on the nitrogen pressure in the gas diode and on the cathode geometry. A minimum breakdown voltage for a gap between tubular cathode and flat anode has been observed at a nitrogen pressure of ∼100 Torr. A decrease in the nitrogen pressure below 100 Torr leads to an increase in the maximum of voltage drop on the gap and the energy of the main fraction of electrons.     

Laser wakefield acceleration: the injection issue. Overview and latest results

External injection of electron bunches into laser-driven plasma waves so far has not resulted in 'controlled' acceleration, i.e. production of bunches with well-defined energy spread. Recent simulations, however, predict that narrow distributions can be achieved, provided the conditions for properly trapping the injected electrons are met. Under these conditions, injected bunch lengths of one to several plasma wavelengths are acceptable. This paper first describes current efforts to demonstrate this experimentally, using state-of-the-art radio frequency technology. The expected charge accelerated, however, is still low for most applications. In the second part, the paper addresses a number of novel concepts for significant enhancement of photo-injector brightness. Simulations predict that, once these concepts are realized, external injection into a wakefield accelerator will lead to accelerated bunch specs comparable to those of recent 'laser-into-gasjet' experiments, without the present irreproducibility of charge and final energy of the latter.     

The effective anode-cathode gap in an ion diode operating in a bipolar-pulse regime

The effective anode-cathode gap (ACG) in a self-magnetically insulated ion diode operating in a double (bipolar) pulse regime has been studied. In this diode, the ACG is bounded by a plasma layer at the anode surface and by electrons drifting near the cathode surface. Analysis of the system operation showed that, during the first voltage pulse, the effective ACG decreases at a constant velocity of 1.5 ± 0.1 cm/μs from 9 to 1–2 mm (depending on the pulse duration) and is not completely bridged by plasma. After reversal of the voltage polarity, the effective gap width is restored for 10–20 ns on a nearly initial level. During the second pulse, electrons drift within a 1- to 1.5-mm-thick layer near the anode, while the thickness of a plasma layer on the anode surface does not exceed 0.5 mm.     

Dynamics of plasma and ion flux in a vacuum neutron tube

Results of the numerical simulation of the formation of the ion beam in the accelerating gap of a vacuum neutron tube are presented. Calculations are performed with the KARAT code in a two-dimensional nonstationary formulation for plasma formed in arc discharge and inflowing into an accelerating gap with the given time dependences of parameters (density, expansion velocity). The small duration of the vacuum arc leads to a considerable change of parameters of inflowing plasma during the accelerating pulse. Two geometries are considered: the conventional and sectioned diode, in which the total voltage is divided between the anode, intermediate electrode, and cathode.