Spallation Neutron Source Linac Research Articles

Accuracy improvement of longitudinal network in China Spallation Neutron Source using stretched wire

In order to improve the transverse alignment accuracy of the control network in Chinese Spallation Neutron Source (CSNS) Linac, a 200-m stretched wire-based system was established in the tunnel. A wire position sensor based on bi-telecentric technology (tWPS) was specially designed to obtain the offset with respect to the stretched wire. A control network adjustment algorithm with additional wire constraints has been developed. The laser tracker measured station by station in a free station way, and measured the control network and the fiducials of the tWPS at the same time. The offset observation equation was combined with the angle observation equation and distance observation equation of the laser tracker for joint solution. As a result, the transverse accuracy of the control network was improved from 0.061 mm without wire constraint to 0.028 mm with wire constraint, which indicates that the addition of wire constraints can indeed effectively improve the transverse accuracy of the longitudinal control network.

Design and performance of the LLRF control system for CSNS linac

The China Spallation Neutron Source (CSNS) linac is designed with beam energy of 80 MeV and a peak current of 15 mA in the first phase. It consists of RFQ, two bunchers of medium-energy beam transmission line, four DTL tanks and one debuncher of linac-to-ring beam transmission line. Correspondingly, eight online RF power sources are used to power these accelerators. In order to stabilize the amplitude, phase and resonant frequency of the RF accelerating field, and minimize beam loss, we have established digital low-level RF (LLRF) control system. The LLRF system includes RF reference line, analog module (AM), clock distribution module, digital control module (DCM), high-power protection module, timing and RF interlock module and so on. The DCM is mainly responsible for the stability of the RF field amplitude and phase, and RF interlock module can quickly cut off the RF drive in case of arc in the RF transmission system, VSWR over threshold or cavity vacuum fault and so on. During beam commissioning, all of eight online units of LLRF control system were operating stably and reliably. The amplitude and phase variations of the linac fields have been achieved about ± 0.4% and ± 0.15° with 10-mA beam loading, much better than the design requirements of ± 1% in amplitude and ± 1° in phase. With the help of this system, we achieved stable operation under different beam loads. Also, many important progresses have been achieved in the LLRF control system for a more convenient operation and a higher stability performance. This article describes the design and implementation of the LLRF for CSNS linac.

Laser-assisted high-energy proton pulse extraction for feasibility study of co-located muon source at the SNS

We have experimentally demonstrated the first non-intrusive 1-GeV proton beam extraction for the generation of muons with a temporal structure optimized for Muon Spin Relaxation/Rotation/Resonance (μSR) applications. The proton pulses are extracted based on the laser neutralization of 1 GeV hydrogen ion (H−) beam in the high energy beam transport of the Spallation Neutron Source (SNS) accelerator. The maximum flux of the extracted proton beam based on laser neutralization causes only 0.2% reduction of the total proton beam used for neutron production, a marked difference from the 20% reduction at other co-located muon and neutron facilities, and thus the proposed method will result in negligible impact on the SNS operation. This paper describes the development of a fiber/solid-state hybrid laser system that has high flexibility of pulse structure and output power, initial experiments on laser neutralization of H− beam and separation of H0 beam from the existing SNS accelerator beam line, conversion of H0 to proton at the SNS linac dump, and measurement results of 30 ns/50 kHz proton pulses. This system conclusively demonstrates the feasibility of laser-based proton beam extraction to power a world-leading μSR facility at the SNS.

Design study for a 500 MeV proton synchrotron with CSNS linac as an injector

Using the China Spallation Neutron Source (CSNS) linac as the injector, a 500 MeV proton synchrotron is proposed for multidisciplinary applications, such as biology, material science and proton therapy. The synchrotron will deliver proton beam with energy from 80 MeV to 500 MeV. A compact lattice design has been worked out, and all the important beam dynamics issues have been investigated. The 80 MeV H− beam is stripped and injected into the synchrotron by using multi-turn injection. In order to continuously extraction the proton with small beam loss, an achromatic structure is proposed and a slow extraction method with RF knock-out is adopted and optimized.

The development of magnetic field measurement system for drift-tube linac quadrupole

In the China Spallation Neutron Source (CSNS) linac, a conventional 324MHz drift-tube linac (DTL) accelerating an H− ion beam from 3MeV to 80MeV has been designed and manufactured. The electromagnetic quadrupoles (EMQs) are widely used in a DTL accelerator. The main challenge of DTLQ׳s structure is to house a strong gradient EMQ in the much reduced space of the drift-tube (DT). To verify the DTLQ׳s design specifications and fabrication quality, a precision harmonic coil measurement system has been developed, which is based on the high precision movement platform, the harmonic coil with ceramic frame and the special method to make the harmonic coil and the quadrupoles coaxial. After more than one year׳s continuous running, the magnetic field measurement system still performs accurately and stably. The field measurement of more than one hundred DTLQ has been finished. The components and function of the measurement system, the key point of the technology and the repeatability of the measurement results are described in this paper.

Publisher’s Note: Evidence of a halo formation mechanism in the Spallation Neutron Source linac [Phys. Rev. ST Accel. Beams16, 040103 (2013)

Publisher’s Note: Evidence of a halo formation mechanism in the Spallation Neutron Source linac [Phys. Rev. ST Accel. Beams<b>16</b>, 040103 (2013)

Evidence of a halo formation mechanism in the Spallation Neutron Source linac

A new halo formation mechanism and its mitigation scheme [D. Jeon, J. Stovall, A. Aleksandrov, J. Wei, J. Staples, R. Keller, L. Young, H. Takeda, and S. Nath, Phys. Rev. ST Accel. Beams 5, 094201 (2002)] are verified experimentally through a series of emittance measurements performed during the drift tube linac tank 1 commissioning of the Spallation Neutron Source. This is a rare experiment evidence of a halo formation mechanism. As the simulation predicts, the emittance measurements clearly show a visible halo reduction as well as a significant rms emittance reduction when the proposed round beam optics is employed. The emittance measurement results are consistent with multiparticle simulations and also consistent with wire scanner results. These measurements serve as a valuable code benchmarking for a beam under an intense space charge effect.

Effect of magnetic fringe field and interference on beam matching in a Medium Energy Beam Transport line of the Spallation Neutron Source linac

A Medium Energy Beam Transport (MEBT) line is employed in the Spallation Neutron Source linear accelerator (linac) to match the beam from a radio frequency quadrupole to a drift tube linac and to perform other functions. The MEBT lattice consists of 14 electromagnetic quadrupoles and other devices. The quads have very small aspect ratios (steel length over aperture diameter), and they are densely packed in the lattice. Significant fringe fields and magnetic interference cause difficulties in beam matching. We have performed 3D modeling of the magnets, expanded simulation data to 3D multipoles, computed their linear optical properties, and compared their performance with that predicted by simple hard-edge models. This paper reports our findings and a proposed general solution to the problem.

Emittance coupling driven by space charge in the CSNS linac

In the conventional design of RF linacs, the bunched beams are not in thermal equilibrium. The space charge forces couple the particle motions between the transverse and the longitudinal directions. Furthermore it will cause the equipartitioning process which leads to emittance growth and halo formation. In the design of the China Spallation Neutron Source (CSNS) linac, three cases are investigated using the Hofmann stability charts. In this paper, we present the equipartitioning beam study of the CSNS Alvarez DTL linac.

Numerical simulations of stripping effects in high-intensity hydrogen ion linacs

Numerical simulations of ${\mathrm{H}}^{\ensuremath{-}}$ stripping losses from blackbody radiation, electromagnetic fields, and residual gas have been implemented into the beam dynamics code TRACK. Estimates of the stripping losses along two high-intensity ${\mathrm{H}}^{\ensuremath{-}}$ linacs are presented: the Spallation Neutron Source linac currently being operated at Oak Ridge National Laboratory and an 8 GeV superconducting linac currently being designed at Fermi National Accelerator Laboratory.

Measurement of beam loading at the SNS superconducting linac for a beam with β

When a beam passes through superconducting cavities, it excites beam induced fields in the cavities. A systematic study is performed to study the beam loading effects with β<1 beam on the β=0.81 superconducting cavities of the Spallation Neutron Source (SNS) linac. The analysis indicates that the shunt impedance equation is still valid even for a beam with β well below 1. It is also shown that the induced field level is consistent with the estimation and its effect on the beam dynamics is consistent with the model.

Benchmarking of multiparticle phase scan and acceptance scan techniques for the Spallation Neutron Source linac

The importance of accurately setting the RF field amplitude and phase, called the RF set-point is well recognized for a high intensity proton linac such as the Spallation Neutron Source (SNS) linac. Techniques such as phase scan and acceptance scan have been used to determine RF set-points. However, benchmarking study of phase scan and acceptance scan techniques based on experiment was never undertaken before. The benchmarking study demonstrated that the multiparticle phase scan technique based on realistic tracking produces RF set-points consistent with those obtained from the acceptance scan technique. Previous work of phase scan type was based on single-particle tracking and the RF set-point from the single-particle phase scan sometimes did not agree well with either that from the multiparticle phase scan or that from the acceptance scan. The analysis based on multiparticle phase scan showed an accurate agreement between the measurement and simulation even when the beam bunch is rather long (the full-width at half-maximum (FWHM) beam bunch length entering the SNS Drift Tube linac tank 1 is about 27°).

The commissioning of the SNS superconducting linac

The Spallation Neutron Source (SNS) superconducting linac has been commissioned in 2005 with an output beam energy tuned up to 950MeV and a peak beam current of approximately 40mA. Some of the most important contributions to the successful beam commissioning of the world’s first pulsed proton SC linac are discussed in this paper, including cryomodule commissioning, numerical modeling for beam acceleration and transport in the accelerator, as well as the development of superconducting linac tune-up algorithms. Beam commissioning results and some beam dynamics studies of the SNS linac are also briefly described.

Low-level rf control of Spallation Neutron Source: System and characterization

The low-level rf control system currently commissioned throughout the Spallation Neutron Source (SNS) LINAC evolved from three design iterations over 1 yr intensive research and development. Its digital hardware implementation is efficient, and has succeeded in achieving a minimum latency of less than 150 ns which is the key for accomplishing an all-digital feedback control for the full bandwidth. The control bandwidth is analyzed in frequency domain and characterized by testing its transient response. The hardware implementation also includes the provision of a time-shared input channel for a superior phase differential measurement between the cavity field and the reference. A companion cosimulation system for the digital hardware was developed to ensure a reliable long-term supportability. A large effort has also been made in the operation software development for the practical issues such as the process automations, cavity filling, beam loading compensation, and the cavity mechanical resonance suppression.

Overview of the Spallation Neutron Source Vacuum Systems

The Spallation Neutron Source (SNS) is an accelerator-based neutron source being built in Oak Ridge, Tennessee, by the U.S. Department of Energy. The SNS will provide the most intense pulsed neutron beams in the world for scientific research and industrial development. At a total cost of $1.4 billion, construction began in 1999 and will be completed in 2006 the culmination of the collaborative efforts of six National Laboratories. An overview of the contribution of these laboratories is provided specifically addressing the design and construction of the numerous vacuum systems and their integration into an operational facility (N. Holtkamp, The SNS Linac and Storage Ring: Challenges and Progress Towards meeting them, EPAC 2002).

The status of the spallation neutron source ion source

The ion source for the spallation neutron source (SNS) is a radio-frequency, multicusp source designed to deliver 45 mA of H− to the SNS accelerator with a pulse length of 1 ms and repetition rate of 60 Hz. A total of three ion sources have been fabricated and commissioned at Lawrence Berkeley National Laboratory and subsequently delivered to the SNS at the Oak Ridge National Laboratory. The ion sources are currently being rotated between operation on the SNS accelerator, where they are involved in ongoing efforts to commission the SNS LINAC, and the hot spare stand (HSS), where high-current tests are in progress. Commissioning work involves operating the source in a low duty-factor mode (pulse width ∼200 μs and repetition rate ∼5 Hz) for extended periods of time while the high-current tests involve source operation at full duty-factor of 6% (1 ms/60 Hz). This report discusses routine performance of the source employed in the commissioning role as well as the initial results of high-current tests performed on the HSS.

Cumulative beam break-up study of the spallation neutron source superconducting linac

Abstract Beam instabilities due to High Order Modes (HOMs) are a concern to superconducting (SC) linacs such as the Spallation Neutron Source (SNS) linac. The effects of pulsed mode operation on transverse and longitudinal beam breakup instability are studied for H− beam in a consistent manner for the first time. Numerical simulation indicates that cumulative transverse beam breakup instabilities are not a concern in the SNS SC linac, primarily due to the heavy mass of H− beam and the HOM frequency spread resulting from manufacturing tolerances. As little as ±0.1 MHz HOM frequency spread stabilizes all the instabilities from both transverse HOMs, and also acts to stabilize the longitudinal HOMs. Such an assumed frequency spread of ±0.1 MHz HOM is small, and hence conservative compared with measured values of σ=0.00109(fHOM−f0)/f0 obtained from Cornell and the Jefferson Lab Free Electron Laser cavities. However, a few cavities may hit resonance lines and generate a high heat load. It is therefore prudent to have HOM dampers to avoid the danger of quenching a cavity.

Formation and mitigation of halo particles in the Spallation Neutron Source linac

A halo generation mechanism in the nonperiodic lattices such as the SNS (Spallation Neutron Source) linac MEBT [medium-energy beam transport between radio-frequency quadrupole and DTL (drift tube linac)] is reported. We find that the nonlinear space charge force resulting from large transverse beam eccentricity $\ensuremath{\sim}2:1$ in the $\ensuremath{\sim}1.6\mathrm{m}$-long MEBT chopper section is responsible for halo formation. As a result, the beam distribution, based on the front end emittance measurements and multiparticle simulation studies, develops halo that leads to beam loss and radioactivation of the SNS linac. Designing lattices with transverse beam eccentricity close to 1:1 suppresses this kind of halo generation. Modifying the MEBT optics and introducing adjustable collimators in the MEBT significantly reduced beam losses in the coupled cavity linac, which is a preferred scheme for mitigating halo. It turns out that the DTL collimation does not effectively remove halo and presents a risk of overheating drift tubes.

Systematic comparison of position and time dependent macroparticle simulations in beam dynamics studies

Macroparticle simulation plays an important role in modern accelerator design and operation. Most linear rf accelerators have been designed based on macroparticle simulations using longitudinal position as the independent variable. In this paper, we have done a systematic comparison between using longitudinal position as the independent variable and using time as the independent variable in macroparticle simulations. We have found that, for an rms-matched beam, the maximum relative moment difference for second, fourth moments and beam maximum amplitudes between these two types of simulations is $0.25%$ in a 10 m reference transport system with physical parameters similar to the Spallation Neutron Source linac design. The maximum $z$-to- $t$ transform error in the space-charge force calculation of the position dependent simulation is about $0.1%$ in such a system. This might cause a several percent error in a complete simulation of a linac with a length of hundreds of meters. Furthermore, the error may be several times larger in simulations of mismatched beams. However, if such errors are acceptable to the linac designer, then one is justified in using position dependent macroparticle simulations in this type of linac design application.

Anisotropic free-energy limit of halos in high-intensity accelerators.

We study halo emittance growth in anisotropic beams and show that the rms emittance growth resulting from mismatch is highly anisotropic, depending on the tune ratio. We find that the free-energy limit calculated by Reiser [J. Appl. Phys. 70, 1919 (1991)] for an axisymmetric 1D halo can be extended to 2D if understood as an upper bound to the rms emittance growth averaged per degree of freedom. The thus-obtained "free-energy limit" of an ideal transport system is compared with the halo rms emittance growth in simulations of the Spallation Neutron Source linac.