High Intensity Linear Accelerators Research Articles

Overview of fusion-like neutron sources based on high-intensity linear accelerators

Overview of fusion-like neutron sources based on high-intensity linear accelerators

Beam Transverse Phase Space Portrait Tomography at the High-Intensity Linear Accelerator of Hydrogen Ions

Beam Transverse Phase Space Portrait Tomography at the High-Intensity Linear Accelerator of Hydrogen Ions

Thermomechanical responses of the drift tube to beam loss at the China spallation neutron source.

The China Spallation Neutron Source drift tube linac (DTL) is a high current intensity linear accelerator operated in pulsed mode. Beam loss is the most important cause of DTL performance degradation. Therefore, control of beam loss is very crucial to the linac's commission and routine operation. The beam loss in beam pulse width (∼several hundreds of microseconds) can be regarded as an instantaneous process, which may cause the local temperature of the drift tubes (DTs) to exceed the melting point or the thermal stress to exceed the material yield strength, resulting in permanent damage to the DTs. RF heating and beam loss are two major heat sources of the DTs during operation. First, temperature rise due to RF power dissipation is evaluated in this paper. Then, the thermomechanical response due to beam loss at different energies is investigated. It is found that the 3-MeV beam loss has the greatest stopping power and causes the largest temperature rise to the DTs. The maximum beam current loss for different working conditions is analyzed by both analytical method and ANSYS simulations, and the details will also be discussed in this paper.

Effects of beam spinning on the fourth-order particle resonance of 3D bunched beams in high-intensity linear accelerators

The aim of this study is to make the parameter space of zero-current phase advance greater than 90\ifmmode^\circ\else\textdegree\fi{} available to the high-intensity linear accelerator (linac) design and operation, which has been excluded to avoid the envelope instabilities and particle resonances. The earlier study of Cheon et al. [Nucl. Instrum. Methods Phys. Res., Sect. A 1013, 165647 (2021)] reported that the spinning of ion beams can mitigate the fourth-order particle resonance and the associated envelope instability in high-intensity linacs. In the present work, we further investigate the effects of beam spinning on the fourth-order particle resonance in the case of 3D bunched beams with fast acceleration. We also explore the space-charge-driven resonance in the longitudinal plane and confirm that the fourth-order particle resonance can be manifested when the longitudinal zero-current phase advance ${\ensuremath{\sigma}}_{z0}$ is larger than 90\ifmmode^\circ\else\textdegree\fi{} and the depressed phase advance ${\ensuremath{\sigma}}_{z}$ is less than 90\ifmmode^\circ\else\textdegree\fi{}, similar to the transverse case. The beam spinning effects are examined in both transverse and longitudinal planes during beam acceleration through periodic solenoid and quadrupole-doublet focusing channels.

Mitigation of space-charge-driven resonance and instability in high-intensity linear accelerators via beam spinning

For modern high-intensity linear accelerators, the well-known envelope instability and recently reported fourth-order particle resonance impose a fundamental operational limit (i.e., zero-current phase advance should be less than 90°). Motivated by the stability of spinning flying objects, we propose a novel approach of using spinning beams to surpass this limit. We discovered that spinning beams have an intrinsic characteristic that can suppress the impact of the fourth-order resonance on emittance growth and the associated envelope instability.

RF design studies on a 325 MHz rebuncher cavity for high intensity linear accelerators producing neutrons

Two identical Coupled Cavity Linac type (CCL) rebuncher cavities to be integrated in the Medium Energy Beam Transport line (MEBT) of a high intensity linac Accelerator have been studied for matching a 40-mA proton beam accelerated by a 3 MeV Radio-Frequency Quadrupole (RFQ) to a 13 MeV Drift Tube Linac (DTL). The rebuncher operates at 325 MHz, and is designed for β = 0.080. The rebuncher aperture radius is 20 mm required by transverse beam dynamics. It is a single gap structure operating in the TM_010 mode for providing longitudinal beam focusing. The unloaded quality Q0 and the required input power to achieve an effective voltage of 150 kV are 27253 and 20.5 kW, respectively. In this work, we present the RF design studies for the high power rebuncher resonator, as well as the considerations of the tuning scheme and the coupler design.

Simulation study of a high-intensity linear accelerator operated at longitudinal phase advances above 90°

High intensity beams may suffer from serious beam quality degradation if the focusing scheme allows for occurrence of resonances or instabilities. For transverse focusing a commonly accepted and respected lattice design rule is to choose the phase advance per structure period below the 90\ifmmode^\circ\else\textdegree\fi{} resonant stop band, which was implicitly applied to the longitudinal phase advance the same way. A recent study pointed out that for lattice structures with more than one rf gap per period the 90\ifmmode^\circ\else\textdegree\fi{} restriction needs not to be applied the same way to the longitudinal focusing as to the transverse one [I. Hofmann and Oliver Boine-Frankenheim, Phys. Rev. Lett. 118, 114803 (2017)], thus offering more design flexibility. The present paper is motivated by an interest to accelerate intense proton beams above longitudinal 90\ifmmode^\circ\else\textdegree\fi{} in the new poststripper drift tube linac of the GSI universal linear accelerator by using the rf power supplies optimized for the much stiffer heavy ions. We confirm that a strictly periodic prolongation of the first cavity could allow acceleration above longitudinal 90\ifmmode^\circ\else\textdegree\fi{} with only minor beam quality degradation. However, the combination of a high longitudinal phase advance with intercavity sections breaking the periodicity shows that matching challenges---rather than resonances---determine emittance growth. In the present case it is found tolerable for up to three such sections. This confirms the validity of the principle and at the same time its limits under practical conditions.

Choice of Linac parameters to minimize the space-charge effects

In high-intensity linear accelerators, the tune spreads induced by the space-charge forces in the radial and longitudinal planes are key parameters for halo formation and beam losses. For matched beams, they are the parameters governing the number of resonances (including coupling resonances), which affect the beam and determine the respective sizes of the stable and halo areas in phase space. The number and strength of the resonances excited in mismatched beams leading to even higher amplitude halos are also directly linked to the tune spreads. In this paper, the equations making the link between the basic linac parameters (rf frequency, zero-current phase advances, beam intensity and emittances) and the tune spreads are given. The analysis of the way these linac parameters can be chosen to minimize the tune spreads is presented. The ESS linac parameters are used as an example for this study.

Analysis on the stop band of fourth-order resonance in high-intensity linear accelerators

The earlier study of [Jeon et al., Phys. Rev. Spec. Top.--Accel. Beams 12, 054204 (2009)] discovered 4σ=360° fourth-order particle resonance in linear accelerators (linacs) for any non-KV distributions (such as Gaussian and waterbag) and found that this particle resonance was predominantly manifested over the envelope instability. Several experiments have since been performed and demonstrated that fourth-order resonance indeed occurred for σ<90°. Nonetheless, the lower bound of the particle resonance stop band and detailed parametric studies (e.g., scanning with various beam currents and zero-current phase advances σ0) on the emittance growth from the fourth-order resonance have been somewhat limited. This paper presents analytical and numerical investigations to observe the fourth-order resonance under several focusing field types (e.g., solenoid and quadrupole) with a wide range of parameters. From these studies, the general stop band for the 4σ=360° fourth-order particle resonance is established to be σ0>90° and σ<90°. Self-consistent multi-particle simulations using TraceWin and PARMILA codes are performed with initially well-matched Gaussian beams to observe the emittance growth originating from the fourth-order particle resonance. It is found that the emittance growth rate varies depending on σ0 and tune depression (or equivalently beam current). It transpires that the region in which the envelope instability develops is consistent with the envelope instability stop band. The fourth-order particle resonance stop band is wider than the envelope instability stop band, and the envelope instability is induced following fourth-order particle resonance only within the envelope instability stop band in the tune-depression space.

P268] Monte Carlo study of photoneutrons from flattened filter free medical linear accelerator for BNCT applications

Purpose Studies in Italy have indicated the possibility of using photo-neutrons from high energy linac as a promising alternative neutron source to obtain intense neutron fluence rate for Boron Neutron Capture Therapy (BNCT) applications [1] . In continuation to these efforts, this study was carried out with aims: 1) to select high intensity linear accelerator that can be used for photo-neutron production; 2) to select the best materials for photo-neutron production. Methods Initially, photon planner fluence and photons distributions have been simulated for a range of medical linear accelerators to study photon intensity using BEAMnrc Monte Carlo code. Due to its high photon intensity, The FFF Linac was selected as a nominal source to study photo-neutrons. Monte Carlo Simulations were then performed for photo-neutrons production from flattening filter free (FFF) medical Linac using lead and tungsten convertors. Results Our results showed that photo-neutrons flux and yield increase with both photon energy and convertor thickness, saturating at 7 cm (lead) and 6 cm (tungsten) convertors. The average photoneutrons energy produced ranged from 0.480 to 0.863 MeV for the tungsten convertor and from 0.586 to 1.214 MeV for the lead convertor. A photoneutrons flux of 3.971 × 10 8 n/cm2s and 2.585 × 10 8 n/cm2s have been calculated for a 25 MeV medical linac equipped with 7 cm lead convertor and 6 cm tungsten convertor respectively. Conclusions In conclusion the FFF linac presented a considerable feasibility toward having it as an alternative neutron source for BNCT with lead and tungsten as a suitable photoconverters.

Classification of Space-Charge Resonances and Instabilities in High-intensity Linear Accelerators

As the beam intensity increases in modern high-power accelerators, self-field effects of the beam become significant and it is the utmost goal to minimize the beam loss of halo particles by avoiding or minimizing contributions of various halo mechanisms. There are two distinct families of space-charge halo mechanisms in high-intensity linear accelerators, and yet they need to be differentiated: resonances (single particle resonances or incoherent resonances) and instabilities (parametric resonances or coherent resonances). What we call resonances are resonances of the beam particle excited through the space-charge nonlinear multipoles. What we call instabilities (or envelope instabilities) are instabilities of the beam envelope. Instabilities are also called parametric resonances because they are parametric resonances of the envelope equation. They would better be called envelope parametric resonances to distinguish them from particle parametric resonances. Resonances and instabilities may look alike in the phase space, and yet they have distinct differences. Instabilities (or envelope parametric resonances) do not have the resonant frequency component. Various orders of resonances and instabilities are presented along with the beam distributions with which the particular mechanism is observed.

Characteristics of the fourth order resonance in high intensity linear accelerators

For the 4σ = 360° space-charge resonance in high intensity linear accelerators, the emittance growth is surveyed for input Gaussian beams, as a function of the depressed phase advance per cell σ and the initial tune depression (σo – σ). For each data point, the linac lattice is designed such that the fourth order resonance dominates over the envelope instability. The data show that the maximum emittance growth takes place at σ ≈ 87° over a wide range of the tune depression (or beam current), which confirms that the relevant parameter for the emittance growth is σ and that for the bandwidth is σo – σ. An interesting four-fold phase space structure is observed that cannot be explained with the fourth order resonance terms alone. Analysis attributes this effect to a small negative sixth order detuning term as the beam is redistributed by the resonance. Analytical studies show that the tune increases monotonically for the Gaussian beam which prevents the resonance for σ > 90°. Frequency analysis indicates that the four-fold structure observed for input Kapchinskij-Vladmirskij beams when σ < 90°, is not the fourth order resonance but a fourth order envelope instability because the 1/4 = 90°/360° component is missing in the frequency spectrum.

Revisiting the Longitudinal 90° Limit in High Intensity Linear Accelerators

Parametric envelope and sum envelope resonances are analyzed to revisit the validity of an assumed stop band and design limit of high intensity linear accelerators at a longitudinal phase advance of 90° per focusing lattice period. While the 90° limit is unquestioned in the transverse plane, we show here that it can be dropped as longitudinal limit for lattices with two or more rf gaps per focusing period. A new limit arises, however, from a novel transverse-longitudinal parametric sum envelope instability. The resulting sum instability rule allows the phase advance to exceed 90° longitudinally provided that transversely it remains correspondingly under 90°. We suggest that the additional design freedom opens the possibility for larger accelerating gradients and stronger longitudinal focusing with potential length and cost saving in the design of advanced superconducting linear accelerator concepts-as long as technological cavity limits are not reached.

Interplay of space-charge fourth order resonance and envelope instability

Since the finding of the space charge fourth order 4σ=360° resonance [Jeon et al., 2009 [3]], studies have been carried out to better understand the fourth order resonance and the envelope instability in high intensity linear accelerators. The questions remained unanswered under what conditions the envelope instability is excited or suppressed following the development of the fourth order resonance. Simulations suggest that the variation of σo and σ along the linac, the halo particles and the resonance islands of the fourth order resonance play an important role in exciting or suppressing the envelope instability after the fourth order resonance is manifested for initially well-matched beams. Here σ(σo) is the depressed (zero-current) phase advance per cell. The envelope instability is excited from the mismatch generated by the fourth order resonance, 1) when σo is maintained approximately constant or increases along the linac with σo>90° and 2) when the extent of the fourth order resonance shrinks, as σ increases approaching 90°. On the other hand, when σ stays almost constant (maintaining the fourth order resonance) or when σo decreases and gets close to 90° (getting out of the envelope instability), the envelope instability is suppressed and the halo particles formed by the fourth order resonance preserves a four-fold structure in the phase space.

Experimental evidence of space charge driven resonances in high intensity linear accelerators

In the construction of high intensity accelerators, it is the utmost goal to minimize the beam loss by avoiding or minimizing contributions of various halo formation mechanisms. As a halo formation mechanism, space charge driven resonances are well known for circular accelerators. However, the recent finding showed that even in linear accelerators the space charge potential can excite the $4\ensuremath{\sigma}=360\ifmmode^\circ\else\textdegree\fi{}$ fourth order resonance [D. Jeon et al., Phys. Rev. ST Accel. Beams 12, 054204 (2009)]. This study increased the interests in space charge driven resonances of linear accelerators. Experimental studies of the space charge driven resonances of high intensity linear accelerators are rare as opposed to the multitude of simulation studies. This paper presents an experimental evidence of the space charge driven $4\ensuremath{\sigma}=360\ifmmode^\circ\else\textdegree\fi{}$ resonance and the $2{\ensuremath{\sigma}}_{x(y)}\ensuremath{-}2{\ensuremath{\sigma}}_{z}=0$ resonance of a high intensity linear accelerator through beam profile measurements from multiple wire-scanners. Measured beam profiles agree well with the characteristics of the space charge driven $4\ensuremath{\sigma}=360\ifmmode^\circ\else\textdegree\fi{}$ resonance and the $2{\ensuremath{\sigma}}_{x(y)}\ensuremath{-}2{\ensuremath{\sigma}}_{z}=0$ resonance that are predicted by the simulation.

Sixth-order resonance of high-intensity linear accelerators.

It is shown that the sixth-order 6σ=720° (or 6∶2) resonance is manifested for high-intensity beams of linear accelerators through the space charge potential when the depressed phase advance per cell σ is close to and below 120° but no resonance effect is observed for σ above 120°. Simulation studies show a clear emittance growth by this resonance and a characteristic sixfold resonance structure in phase space. To verify that this is a resonance, a frequency analysis was conducted and a study was performed of crossing the resonance from above and from below the resonance. Canonical perturbation is carried out to show that this resonance arises through perturbation of strong 2σ=360° (2∶1) and 4σ=360° (4∶1) space charge resonances. Simulations also show that the space charge 6σ=360° (or 6∶1) resonance is very weak.

Evaluation of integral quantities in an accelerator driven system using different nuclear models implemented in the MCNPX Monte Carlo transport code

The MCNPX code offers options based on physics packages; the Bertini, ISABEL, INCL4 intra-nuclear models, and Dresner, ABLA evaporation–fission models and CEM2k cascade-exciton model. This study analyzes the main quantities determining ADS performance, such as neutron yield, neutron leakage spectra, heating and neutron and proton spectra in the target and in the beam window calculated by the MCNPX-2.5.0 Monte Carlo transport code, which is a combination of LAHET and MCNP codes. The results obtained by simulating different models cited above and implemented in MCNPX are compared with each other. The investigated system is composed of a natural lead cylindrical target and stainless steel (HT9) beam window. The target has been optimized to produce maximum number of neutrons with a radius of 20cm and 70cm of height. The target is bombarded with a high intensity linear accelerator by a 1GeV, 1mA proton beam. The protons are assumed uniformly distributed across the beam of radius 3cm, and entering the target through a hole of 5.3cm radius. The proton beam has an outer radius of 5.3cm and an inner radius of 5.0cm. The maximum value of the neutron flux in the target is observed on the axis ∼10cm below the beam window, where the maximum difference between 7 different models is ∼15%. The total neutron leakage of the target calculated with the Bertini/ABLA is 1.83×1017n/s, and is about 14% higher than the value calculated by the INCL4/Dresner (1.60×1017n/s). Bertini/ABLA calculates top, bottom and side neutron leakage fractions as 20%, 2.3%, 77.6% of the total leakage, respectively, whereas, the calculated fractions are 18.6%, 2.3%, 79.4%, respectively, with INCL4/Dresner combination. The largest heat deposition density by considering all particles in the beam window calculated with CEM2k model is 104W/cm3/mA, which is 9.0% greater than the lowest value predicted with INCL4/Dresner model (95.4W/cm3/mA). The maximum average heat deposition density for all particles in the target is calculated as 6.87W/cm3/mA with INCL4/ABLA.

Development of high intensity linear accelerator for heavy ion inertial fusion driver

In order to verify the direct plasma injection scheme (DPIS), an acceleration test was carried out in 2001 using a radio frequency quadrupole (RFQ) heavy ion linear accelerator (linac) and a CO2-laser ion source (LIS) (Okamura et al., 2002) [1]. The accelerated carbon beam was observed successfully and the obtained current was 9.22mA for C4+. To confirm the capability of the DPIS, we succeeded in accelerating 60mA carbon ions with the DPIS in 2004 (Okamura et al., 2004; Kashiwagi and Hattori, 2004) [2,3]. We have studied a multi-beam type RFQ with an interdigital-H (IH) cavity that has a power-efficient structure in the low energy region. We designed and manufactured a two-beam type RFQ linac as a prototype for the multi-beam type linac; the beam acceleration test of carbon beams showed that it successfully accelerated from 5keV/u up to 60keV/u with an output current of 108mA (2×54mA/channel) (Ishibashi et al., 2011) [4]. We believe that the acceleration techniques of DPIS and the multi-beam type IH-RFQ linac are technical breakthroughs for heavy-ion inertial confinement fusion (HIF). The conceptual design of the RF linac with these techniques for HIF is studied. New accelerator-systems using these techniques for the HIF basic experiment are being designed to accelerate 400mA carbon ions using four-beam type IH-RFQ linacs with DPIS. A model with a four-beam acceleration cavity was designed and manufactured to establish the proof of principle (PoP) of the accelerator.

Beam dynamics studies on the 100 MeV/100 kW electron linear accelerator for NSC KIPT neutron source

We designed a 100 MeV/100 kW electron linear accelerator for NSC KIPT, which will be used to drive a neutron source on the basis of subcritical assembly. Beam dynamics studies have been conducted to reach the design requirements (E = 100 MeV, P = 100 kW, dE/E < 1% for 99% particles). In this paper, we will present the progress of the design and the dynamic simulation results. For high intensity and long beam pulse linear accelerators, the BBU effect is one big issue; special care has been taken in the accelerating structure design. To satisfy the energy spread requirement at the linac exit, the particles with large energy difference from the synchronous particle should be eliminated at a low energy stage to ease the design of the collimation system and radiation shielding. A dispersion free chicane with 4 bending magnets is introduced downstream of the 1st accelerating section; the unwanted particles will be collimated there.

Comparisons of the Calculations Using Different Codes Implemented in MCNPX Monte Carlo Transport Code for Accelerator Driven System Target

The MCNPX code offers options based on physics packages; the Bertini, ISABEL, INCL4 intra-nuclear models, and Dresner, ABLA evaporation-fission models and CEM2k cascade-exciton model. The study analyzes the main quantities determining ADS performance, such as neutron yield, neutron leakage spectra, and neutron and proton spectrain the target andin the beam window calculated by the MCNPX-2.5.0 Monte Carlo transport code, which is a combination of LAHET and MCNP codes. The results obtained by simulating different models, cited above and implemented in MCNPX are compared with each other.The investigated system is composed of a natural lead cylindrical target and stainless steel (HT9) beam window. Target has been optimized to produce maximum number of neutrons with a radius of 20 cm and 70 cm of height. Target is bombarded with a high intensity linear accelerator by a 1 GeV, 1 mA proton beam. The protons are assumed uniformly distributed across the beam of radius 3 cm, and entering the target through a hole of 5.3 cm radius. The proton beam has an outer radius of 5.3 cm and an inner radius 5.0 cm. The maximum of the neutron flux in the target is observed on the axis ~ 10 cm below the beam window, where the maximum difference between 7 different models is ~ 15 %. The total neutron leakage out of the of the target calculated with the Bertini/ABLA is 1.83×1017 n/s, and is about 14 % higher than the value calculated by the INCL4/Dresner (1.60×1017 n/s). Bertini/ABLA calculates top, bottom and side neutron leakage fractions as 20 %, 2.3 %, 77.6 % of the total leakage, respectively, whereas, they become 18.6 %, 2.3 %, 79.4 % with INCL4/Dresner combination.