Induction Linac Research Articles

Powerful oversized W-band free-electron maser with advanced Bragg resonator based on coupling of propagating and cutoff waves

We study, both theoretically and experimentally, a free-electron maser (FEM) with a two-mirror resonator that includes the advanced Bragg structure based on coupling of propagating and cutoff waves as an upstream reflector. Similar to gyrotrons, the presence of a cutoff wave in the feedback loop improves significantly the selectivity of the resonator at large values of the oversize factor and facilitates a stable single-mode oscillation regime in short wavelength bands. Proof-of-principle experiments conducted at the induction linac LIU-3000 (Dubna) demonstrate the operability of the FEM with the proposed modification of the Bragg resonator. At the transverse oversize factor of about 5, stable narrow-band generation was achieved with an output power of up to 7 MW and the oscillation frequency of about 80 GHz corresponding to the frequency of the cutoff mode excited in the advanced Bragg reflector. These investigations encourage the application of the advanced Bragg resonators for the development of high-power long-pulse FELs driven by induction linacs and operating in the subterahertz to terahertz frequency range.

Optimizing beam transport in rapidly compressing beams on the neutralized drift compression experiment – II

The Neutralized Drift Compression Experiment-II (NDCX-II) is an induction linac that generates intense pulses of 1.2 MeV helium ions for heating matter to extreme conditions. Here, we present recent results on optimizing beam transport. The NDCX-II beamline includes a 1-m-long drift section downstream of the last transport solenoid, which is filled with charge-neutralizing plasma that enables rapid longitudinal compression of an intense ion beam against space-charge forces. The transport section on NDCX-II consists of 28 solenoids. Finding optimal field settings for a group of solenoids requires knowledge of the envelope parameters of the beam. Imaging the beam on the scintillator gives the radius of the beam, but the envelope angle is not measured directly. We demonstrate how the parameters of the beam envelope (radius, envelop angle, and emittance) can be reconstructed from a series of images taken by varying the B-field strengths of a solenoid upstream of the scintillator. We use this technique to evaluate emittance at several points in the NDCX-II beamline and for optimizing the trajectory of the beam at the entry of the plasma-filled drift section.

First demonstration of simultaneous measurement of beam current, beam position, and beam tilt on induction linac using combined B-dot monitor.

The authors previously reported that the axial B-dots can be used to directly measure the beam tilt and demonstrated that the axial B-dots are applicable to a coaxial calibration stand. In this study, a combined B-dot monitor composed of four axial B-dot loops and four azimuthal ones is tested for the simultaneous measurement of the time-varying beam current, beam offset, and beam tilt at the output of the injector of the DRAGON-I induction linac. In the experiments, the beam offset and beam tilt at the position of the monitor are proportionally adjusted using a pair of steering coils. Eight waveforms acquired from the B-dot monitor are analyzed to reconstruct the time-varying beam current, beam offset, and beam tilt. The original signals of both the azimuthal B-dot and the axial B-dot ports change significantly with respect to the current applied to the steering coils. The measured beam tilt is linearly dependent on the current applied to the steering coils and agrees well with the measured beam offset.

High-power free-electron maser operated in a two-mode frequency-multiplying regime

The frequency multiplication effects in high-power free-electron masers (FEM) with Bragg cavities were studied to provide the advance of the oscillators into short-wavelength bands. Theoretical analysis of frequency-multiplying FEMs was carried out within the framework of the averaged coupled-wave approach. Proof-of-principle experiments were performed based on a moderately relativistic induction linac LIU-3000 (JINR). As a result, an FEM multiplier operated with a megawatt power level in the 6-mm and 4-mm wavelength bands at the second and third harmonics, respectively, was realized. The possibility of using two-mode bichromatic FEMs for powering a double-frequency accelerating structure was discussed.

An accelerator facility for WDM, HEDP, and HIF investigations in Nazarbayev University

Nazarbayev University (NU) in Astana, Kazakhstan, is planning to build a new multi-MV, ∼10 to several hundred GW/cm2 ion accelerator facility which will be used in studies of material properties at extreme conditions relevant to ion-beam-driven inertial fusion energy, and other applications. Two design options have been considered. The first option is a 1.2 MV induction linac similar to the NDCX-II at LBNL, but with modifications, capable of heating a 1 mm spot size thin targets to a few eV temperature. The second option is a 2 - 3 MV, ∼200 kA, single-gap-diode proton accelerator powered by an inductive voltage adder. The high current proton beam can be focused to ∼1 cm spot size to obtain power densities of several hundred GW/cm2, capable of heating thick targets to temperatures of tens of eV. In both cases, a common requirement to achieving high beam intensity on target and pulse length compression is to utilize beam neutralization at the final stage of beam focusing. Initial experiments on pulsed ion beam neutralization have been carried out on a 0.3 MV, 1.5 GW single-gap ion accelerator at Tomsk Polytechnic University with the goal of creating a plasma region in front of a target at densities exceeding ∼1012 cm-3.

Beam dynamics for induction accelerators

An induction linac uses pulsed power that is applied directly, without any intervening resonant cavities, to accelerate a charged particle pulse. This approach can accommodate a large multiple-beam focusing lattice capable of transporting a large total beam current with a long pulse duration, which may be compressed while accelerating as well as afterward. The mean accelerating gradient is relatively low (less than about 1.5MV/m), but the potential efficiency of energy transfer can be large up to about 50%. A multiple-beam induction linac is therefore a natural candidate accelerator for a heavy ion fusion (HIF) driver. However, the accelerated beams must meet stringent requirements on occupied phase space volume in order to be focused accurately and with small radius onto the fusion target. Dynamical considerations in the beam injector and linac, as well as in the final compression, final focus, and the fusion chamber, determine the quality of the driver beams as they approach the target. Requirements and tolerances derived from beam dynamics strongly influence the linac configuration and component design.After a summary of dynamical considerations, two major topics are addressed here: transportable current limits, which determine the choice of focal system for the linac, and longitudinal control of the beams, which are potentially destabilized by their interaction with the pulsed power system.

Accelerators for Inertial Fusion Energy Production

Since the 1970s, high energy heavy ion accelerators have been one of the leading options for imploding and igniting targets for inertial fusion energy production. Following the energy crisis of the early 1970s, a number of people in the international accelerator community enthusiastically began working on accelerators for this application. In the last decade, there has also been significant interest in using accelerators to study high energy density physics (HEDP). Nevertheless, research on heavy ion accelerators for fusion has proceeded slowly pending demonstration of target ignition using the National Ignition Facility (NIF), a laser-based facility at Lawrence Livermore National Laboratory. A recent report of the National Research Council recommends expansion of accelerator research in the US if and when the NIF achieves ignition. Fusion target physics and the economics of commercial energy production place constraints on the design of accelerators for fusion applications. From a scientific standpoint, phase space and space charge considerations lead to the most stringent constraints. Meeting these constraints almost certainly requires the use of multiple beams of heavy ions with kinetic energies >1 GeV. These constraints also favor the use of singly charged ions. This article discusses the constraints for both fusion and HEDP, and explains how they lead to the requirements on beam parameters. RF and induction linacs are currently the leading contenders for fusion applications. We discuss the advantages and disadvantages of both options. We also discuss the principal issues that must yet be resolved.

Enhancing trappable antiproton populations through deceleration and frictional cooling

CERN currently delivers antiprotons for trapping experiments with the antiproton decelerator (AD), which slows the antiprotons down to about 5 MeV. This energy is currently too high for direct trapping, and thick foils are used to slow down the beam to energies which can be trapped. To allow further deceleration to $\ensuremath{\sim}100\text{ }\text{ }\mathrm{keV}$, CERN is initiating the construction of ELENA, consisting of a ring which will combine rf deceleration and electron cooling capabilities. We describe a simple frictional cooling scheme that can serve to provide significantly improved trapping efficiency, either directly from the AD or first using a standard deceleration mechanism (induction linac or rf quadrupole). This scheme could be implemented in a short time. The device itself is short in length, uses accessible voltages, and at reasonable cost could serve in the interim before ELENA becomes operational, or possibly in lieu of ELENA for some experiments. Simple theory and simulations provide a preliminary assessment of the concept and its strengths and limitations, and highlight important areas for experimental studies, in particular to pin down the level of multiple scattering for low-energy antiprotons. We show that the frictional cooling scheme can provide a similar energy spectrum to that of ELENA, but with higher transverse emittances.

Pulse line ion accelerator concept

April 3, 2006 LBNL-59492 The pulse line ion accelerator concept Richard J. Briggs Science Applications International Corporation, Alamo, California, 94507 The Pulse Line Ion Accelerator concept was motivated by the desire for an inexpensive way to accelerate intense short pulse heavy ion beams to regimes of interest for studies of High Energy Density Physics and Warm Dense Matter. A pulse power driver applied at one end of a helical pulse line creates a traveling wave pulse that accelerates and axially confines the heavy ion beam pulse. Acceleration scenarios with constant parameter helical lines are described which result in output energies of a single stage much larger than the several hundred kilovolt peak voltages on the line, with a goal of 3-5 MeV/meter acceleration gradients. The concept might be described crudely as an “air core” induction linac where the PFN is integrated into the beam line so the accelerating voltage pulse can move along with the ions to get voltage multiplication. 1. Introduction In the “Pulse Line Ion Accelerator” (PLIA) concept described in this paper a ramped high voltage pulse is applied at the input end of a helical pulse line structure. The resulting traveling wave pulse on the line can accelerate an ion bunch to energies much greater than the peak voltage applied to the line. It should also be possible to achieve an axial acceleration gradient of several MeV per meter with realistic helix parameters. The development of this concept was originally motivated by a proposal to use moderate energy intense ion beams to heat matter to regimes of interest for studies of High Energy Density Physics (HEDP) and Warm Dense Matter (WDM) [ 1 ] . In this approach, very short pulses (~ a nanosecond or less) of “medium mass” ions with energies slightly above the Bragg peak are focused to mm scale spot sizes onto thin target “foils”, which may in fact be foams with mean densities of order 10% solid density or less. The high degree of uniformity and efficiency with volumetric and shock-less heating of the target to temperatures of order 1 to 10 eV are attractive features in this approach. The concept presented in this paper is an excellent fit to the accelerator requirements for this HEDP/WDM application. A helical pulse line inserted into a large bore 5 - 10T superconducting solenoid can transport and accelerate a singly charged Ne or Na ion bunch with a total charge of order 0.1 to 1 micro-coulombs, an axial bunch length of 10’s of cm, and a radius of a few cm. The required axial compression to bunch lengths of order one cm in a neutralized drift compression region following the accelerator is then only a factor of 10-30, which greatly eases the requirements on longitudinal emittance, velocity tilt, and drift distance.

Optimizing the adiabatic buncher and phase-energy rotator for neutrino factories

In the US scenario for a Neutrino Factory presented in “A feasibility study of a neutrino source based on a muon storage ring”, N. Holtkamp (Ed.), D. Finley (Ed.), Fermilab, April 15th, 2000), a large percentage of the cost is related to an induction linac for phase-energy rotation and bunching of the muon beam collected after the production target and decay channel. A more cost-effective adiabatic buncher and phase-energy rotator has been proposed to replace the induction linac system (D. Neuffer, A. Van Ginneken, High-frequency bunching and ( φ - δ E ) rotation for a muon source, Proceedings of the 2001 Particle Accelerators Conference, Chicago, 2001, p. 2029). The new method uses consecutive RF cavities with differing frequencies. The frequencies are changed to enable bunching and phase-energy rotation. In this paper, the theoretical concept is developed and demonstrated with simulation results obtained with the map code COSY Infinity ( http://cosy.pa.msu.edu). An optimization strategy is also explored.

High current ion sources and injectors for induction linacs in heavy ion fusion

Heavy ion beam driven inertial fusion requires short ion beam pulses with high current and high brightness. Depending on the beam current and the number of beams in the driver system, the injector can use a large diameter surface ionization source or merge an array of small beamlets from a plasma source. In this paper, we review the scaling laws that govern the injector design and the various ion source options including the contact ionizer, the aluminosilicate source, the multicusp plasma source, and the metal vapor vacuum arc (MEVVA) source.

Towards a Modular Point Design for Heavy Ion Fusion

We report on an ongoing study on modular Heavy Ion Fusion (HIF) drivers. The modular driver is characterized by ~20 nearly identical induction linacs, each carrying a single high current beam. In this scheme, one of the full size induction linacs can be tested as an “integrated Research Experiment” (IRE). Hence this approach offers significant advantages in terms of driver development path. For beam transport, these modules use solenoids, which are capable of carrying high line charge densities, even at low energies. A new injector concept allows compression of the beam to high line densities right after the source. The final drift compression is performed in a plasma in which the large repulsive space charge effects are neutralized. Finally, the beam is transversely compressed onto the target, using either external solenoids or current-carrying channels (in the assisted pinch mode of beam propagation). We report on progress towards a self-consistent point design from injector to target. Considerations of driver architecture, chamber environment as well as the methodology for meeting target requirements of spot size, pulse shape and symmetry are also described. Finally, some near-term experiments to address the key scientific issues are discussed.

Magnetic field measurements of quadrupoles in the High-Current Experiment

The High-Current Experiment (HCX) at Lawrence Berkeley National Laboratory is part of the US program to explore heavy-ion beam transport at a scale representative of the low-energy end of an induction linac driver for fusion energy production. Four pulsed magnetic quadrupoles are being used to study gas and electron effects with a 0.2 A, 1-MeV K + beam. The magnets, originally designed and built for a prototype pulsed magnetic quadrupole array, have an elliptical beam tube (6×10 cm) and iron yoke. The magnet coil and field length are ≈31 cm, and operating gradients are 10–40 T/m. To establish that the field quality of the prototype quadrupoles is satisfactory for the experiments, a 1-cm pickup loop was used to measure the flux B r( θ) at the magnet mid-plane and also at the lead and return ends. A longer probe was used to measure the integrated flux of B θ ( θ) along the magnet. The field quality appears satisfactory for the short transport experiments through these quadrupoles.

High current transport experiment for heavy ion inertial fusion

The High Current Experiment (HCX) at Lawrence Berkeley National Laboratory is part of the US program to explore heavy-ion beam transport at a scale representative of the low-energy end of an induction linac driver for fusion energy production. The primary mission of this experiment is to investigate aperture fill factors acceptable for the transport of space-charge-dominated heavy-ion beams at high space-charge intensity (line-charge density /spl sim/ 0.2 /spl mu/C/m) over long pulse durations (>4 /spl mu/s) in alternating gradient electrostatic and magnetic quadrupoles. This experiment is testing - at driver-relevant scale - transport issues resulting from nonlinear space-charge effects and collective modes, beam centroid alignment and beam steering, matching, image charges, halo, electron cloud effects, and longitudinal bunch control. We present the results for a coasting 1 MeV K/sup +/ ion beam transported through the first ten electrostatic transport quadrupoles, measured with beam-imaging and phase-space diagnostics. The latest additions to the experiment include measurements of the secondary ion, electron and atom coefficients due to halo ions scraping the wall, and four magnetic quadrupoles to explore similar issues in magnetic channels.

Experimental study of the transport limits of intense heavy ion beams in the HCX

The High Current Experiment (HCX) at Lawrence Berkeley National Laboratory is part of the US program to explore heavy-ion beam transport at a scale representative of the low-energy end of an induction linac driver for fusion energy production. The primary mission of this experiment is to investigate aperture fill factors acceptable for the transport of space-charge-dominated heavy-ion beams at high space-charge intensity (line charge density up to ∼0.2 μC/m) over long pulse durations (4 μs) in alternating gradient focusing lattices of electrostatic or magnetic quadrupoles. The experiment also contributes to the practical baseline knowledge of intense beam manipulations necessary for the design, construction and operation of a heavy ion driver for inertial fusion. This experiment tests transport issues resulting from nonlinear space-charge effects and collective modes, beam centroid alignment and beam steering, matching, image charges, halo, electron cloud effects, and longitudinal bunch control. We first present the results for a coasting 1 MeV K + ion beam transported through the first ten electrostatic transport quadrupoles, measured with optical beam-imaging and double-slit phase-space diagnostics. This includes studies at two different radial fill factors (60% and 80%), for which the beam transverse distribution was characterized in detail. Additionally, beam energy measurements will be shown. We then discuss the first results of beam transport through four pulsed room-temperature magnetic quadrupoles (located downstream of the electrostatic quadrupoles), where the beam dynamics become more sensitive to the presence of secondary electrons.

Scientific issues in future induction linac accelerators for heavy-ion fusion

The worldwide heavy-ion fusion effort has made a large amount of progress over almost three decades of research on the intense beam physics and concepts needed for an inertial fusion driver. In the 1970s, ‘80s, and ‘90s in the US, most experiments used driver parameters scaled to low energy and low line charge density in order to study, most often with a single beam, the physics of high-perveance beams. More recently, the US program has introduced higher line charge density experiments, where the beam space charge potential is significant. This allows investigation of the production, and effect on the beam, of stray electrons. Increasingly, the possibilities for neutralizing plasmas are also being investigated, both just upstream and in the target chamber. This talk will outline the scientific issues that remain to be examined. These include issues of longitudinal compression, dynamic aperture, electron/ion instabilities, effect of electrons and gas on the beam, longitudinal emittance evolution, halo production, inductive effects at high energy, multiple beams, innovative final focus approaches, and driver issues for solenoidal transport. An outline of future experiments to address these issues will be presented and discussed.

Overview of US heavy-ion fusion progress and plans

Significant experimental and theoretical progress has been made in the US heavy-ion fusion program on high-current sources, injectors, transport, final focusing, chambers and targets for high-energy density physics (HEDP) and inertial fusion energy (IFE) driven by induction linac accelerators. One focus of present research is the beam physics associated with quadrupole focusing of intense, space–charge dominated heavy-ion beams, including gas and electron cloud effects at high currents, and the study of long-distance-propagation effects such as emittance growth due to field errors in scaled experiments. A second area of emphasis in present research is the introduction of background plasma to neutralize the space charge of intense heavy-ion beams and assist in focusing the beams to a small spot size. In the near future, research will continue in the above areas, and a new area of emphasis will be to explore the physics of neutralized beam compression and focusing to high intensities required to heat targets to high-energy density conditions as well as for inertial fusion energy.

Solenoid transport for heavy ion fusion

Solenoid transport of high current, heavy ion beams is considered for several stages of a heavy ion fusion driver. In general, this option is more efficient than magnetic quadrupole transport at sufficiently low kinetic energy and/or large e/ m, and for this reason it has been employed in electron induction linacs. Ideally, an ion beam would be transported in a state of Brillouin flow, i.e. cold in the transverse plane and spinning at one half the cyclotron frequency. The design of appropriate solenoids and the equilibrium and stability of transported ion beams are discussed. An outline of application to a fusion driver is also presented.

Overview of US heavy ion fusion research

Significant experimental and theoretical progress has been made in the US heavy ion fusion programme on high-current sources, injectors, transport, final focusing, chambers and targets for high-energy density physics and inertial fusion energy (IFE) driven by induction linac accelerators. One focus of the present research is the beam physics associated with quadrupole focusing of intense, space-charge-dominated heavy ion beams, including gas and electron cloud effects at high currents, and the study of long-distance-propagation effects such as emittance growth due to field errors in scaled experiments. A second area of emphasis in the present research is the introduction of background plasma to neutralize the space-charge of intense heavy ion beams and assist in focusing the beams to a small spot size. In the near future, research will continue in the above areas, and a new area of emphasis will be to explore the physics of neutralized beam compression and focusing to high intensities required to heat targets to high-energy density conditions as well as for IFE.

Vircator with ballistic focusing of an electron beam

A high-power microwave oscillator (vircator) is built around an ironless induction linac. The feature of this device is ballistic focusing of an electron beam in a diode-type system with a concentric spherical cathode and anode. The possibility of the vircator to generate high-power microwave pulses is demonstrated.