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Abstract:The ion source and Low-Energy Transport (LEBT) system that will provide H -ion beams to the Spallation Neutron Source (SNS)** Front End and the accelerator chain have been developed into a mature unit that fully satisfies the operational requirements through the commissioning and early operating phases of SNS. Compared to the early R&D version, many features of the ion source have been improved, and reliable operation at 6% duty factor has been achieved producing beam currents in the 35-mA range and above. LEBT operation proved that the purely electrostatic focusing principle is well suited to inject the ion beam into the RFQ accelerator, including the steering and pre-chopping functions. This paper will discuss the latest design features of the ion source and LEBT, give performance data for the integrated system, and report on commissioning results obtained with the SNS RFQ and Medium-Energy Beam Transport (MEBT) system. Prospects for further improvements will be outlined in concluding remarks. INTRODUCTIONBerkeley Lab has just completed building the linac injector (Front End, FE) for the Spallation Neutron Source project (SNS), and the commissioning of the entire system is proceeding. The main subsystems are the H -ion-source, the low-energy beam-transport system (LEBT), the 2.5-MeV radio-frequency quadrupole (RFQ) accelerator, and the medium-energy beam-transport system (MEBT). Ion source and LEBT are the subject of this paper; their task is to create a 65-keV, 38-mA ion beam, to match and steer it into the RFQ, and to pre-chop it into mini-pulses of about 600 ns duration. The nominal duty factor is 6%, with 1-ms macro-pulse length and 60-Hz repetition rate.Based upon the main design features of the SSC ion source, 1 an R&D version of the SNS ion source was built first to demonstrate the viability of the chosen approach, utilizing an rf driven discharge inside a multicusp plasma generator with magnetic filter, cesium enhancement, and electron suppression at low energy.2 This source version did not allow implementing cesium enhancement and electron suppression at the same time, but both features were proven to work satisfactorily in separate tests.

An analysis of propagation of surface polaritons–plasmons along the interface of gyrotropic plasma has been conducted. It has been shown that new wave characteristics appear due to the Rayleigh kind of the wave. The influence of both the plasma and the bounding media has been considered. The role of the interface has been demonstrated. The existence of many new phenomena is proved: the end point of the dispersion in strongly magnetized plasma, singular waves, the nonzero field and Poynting’s vector of the intersection of the dispersion curve and the complex zone, etc.

Spherical silver nanoparticles were prepared by means of ion beam synthesis in lithium niobate. The embedded nanoparticles were then irradiated with energetic (84)Kr and (197)Au ions, resulting in different electronic energy losses between 8.1 and 27.5 keV nm(-1) in the top layer of the samples. Due to the high electronic energy losses of the irradiating ions, molten ion tracks are formed inside the lithium niobate in which the elongated Ag nanoparticles are formed. This process is strongly dependent on the initial particle size and leads to a broad aspect ratio distribution. Extinction spectra of the samples feature the extinction maximum with shoulders on either side. While the maximum is caused by numerous remaining spherical nanoparticles, the shoulders can be attributed to elongated particles. The latter could be verified by COMSOL simulations. The extinction spectra are thus a superposition of the spectra of all individual particles.

The volumetric Kα emission rate of argon emitted from the electron cyclotron resonance (ECR) heated plasmas of the JYFL (University of Jyväskylä, Department of Physics) 14 GHz ECR ion source (ECRIS) and the 14.5 GHz Grenoble Test Source (GTS) at iThemba Laboratory for Accelerator Based Sciences have been measured to gain an understanding of the influence of the ion source tune parameters on the absolute inner shell ionization rate. It was observed that the behaviour of the ionization rate and the extracted ion beam currents react differently, depending the parametric sweep performed. The neutral gas pressure and incident microwave power was found to have the strongest influence on the ionization rate. At high neutral gas pressure, the absolute inner shell ionization rate was found to saturate. This observation is as a result of the plasma energy content becoming insufficient to sustain the growth in ionization rate. It was also observed that the incident microwave power should be increased much more to counter the decrease in high charge state production as a result of the gas increase and subsequent increase in charge exchange. At low incident microwave, the ionization rate per unit absorbed microwave power was found to be high, which suggests that the inner shell ionization process is driven by the ion dynamics as opposed to the electron dynamics. The influence of the biased disc voltage and magnetic field configuration on the ionization rate was found to be minimal. This led to the suggestion that these two tune parameters does not directly impact the warm electron population of the ECRIS plasma during Preprint submitted to Nuclear Instruments and Methods in Physics Research, AFebruary 2, 2018 *Manuscript Click here to view linked References the parametric sweeps. The Kα emission rate can be used as an additional tool for benchmarking the results of numerical simulation codes on ECRIS plasmas.

The operational lifetime of a radio-frequency (rf) ion source is generally governed by the length of time the insulating structure protecting the antenna survives during exposure to the plasma. Coating the antenna with a thin layer of insulating material is a common means of extending the life of such antennas. When low-power/low-duty factor rf excitation is employed, antenna lifetimes of several hundred hours are typical. When high-power, >30 kW, and high-duty cycles, ∼6%, are employed, as is the case of the Spallation Neutron Source (SNS) ion source, antenna lifetime becomes unacceptably short. This work addresses this problem by first showing the results of microanalysis of failed antennas from the SNS ion source, developing a model of the damage mechanism based on plasma-insulator interaction, using the model to determine the dimensional and material properties of an ideal coating, and describing several approaches currently under way to develop a long-lived antenna for the SNS accelerator. These approaches include thermal spray coatings, optimized porcelain enamel coatings, refractory enamel coatings, and novel antenna geometries designed to operate with low rf electric fields.

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The ion source for the Spallation Neutron Source is a radio-frequency (rf) multi-cusp, volume-type H− source that is coupled to a rf quadrupole accelerator through a low energy beam transport (LEBT) system consisting of five electrostatic elements. To gain a deeper understanding of the operation of this system and to continue to refine the design, we have performed ion extraction and transport simulations using the computer code PBGUNS. A comparison is presented between simulation and the measured phase space of the beam for various values of LEBT electrode potentials. Both the emittance magnitude and orientation in phase space were found to be in reasonable agreement with measurement. A design study is also presented where the angle of the source outlet electrode has been optimized with the aid of PBGUNS simulations, resulting in a substantial reduction of the emittance.