R. W. Thomae

Ion source antenna development for the Spallation Neutron Source

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 appr...

Simulation of the ion source extraction and low energy beam transport systems for the Spallation Neutron Source

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.

Ion-source and low-energy beam-transport issues with the front-end systems for the spallation neutron source

The front-end systems (FES) of the spallation neutron source project are being built by Berkeley Lab and will deliver a pulsed 40 mA H− ion beam at 2.5 MeV energy to the subsequent drift-tube linac. The FES accelerator components comprise a rf driven, volume-production, cesium-enhanced, multicusp ion source; an electrostatic low-energy beam transport (LEBT) that includes provisions for transverse focusing, steering, and beam chopping; a radio-frequency quadrupole accelerator; and a medium-energy beam transport line. The challenges for ion source and LEBT design are the generation of a plasma suitable for creating the required high H− ion density, lifetime of the rf antenna at 6% duty factor, removal of the parasitic electron population from the extracted negative ions, and emittance conservation. The article discusses these issues in detail and highlights key experimental results obtained so far.

Plasma ignition schemes for the Spallation Neutron Source radio-frequency driven H- source

The H− ion source for the Spallation Neutron Source (SNS) is a cesiated, radio-frequency driven multicusp volume source which operates at a duty cycle of 6%. In pulsed rf driven plasma sources, ignition of the plasma affects the stability of source operation and the antenna lifetime. We report on ignition schemes, based on secondary electron generation by UV light, a hot filament, a low power rf plasma (cw, 13.56 MHz), as well as source operation solely with the high power 2 MHz rf. We find that the dual frequency, single antenna scheme is most attractive for the operating conditions of the SNS H− source.

Improvement of the lifetime of radio frequency antenna for plasma generation

At Lawrence Berkeley National Laboratory different antenna protection schemes have been investigated for the radio frequency-driven multicusp ion source. It was found that the antenna lifetime can be greatly enhanced by an additional shielding, which consists of porcelain, quartz or boron nitride. Different antenna configurations and their influence on the plasma generation will be discussed. Antenna life time greater than 500 hours continuous wave operation has been demonstrated in hydrogen plasma using a novel quartz antenna design.

Emittance Characteristics of High‐Brightness H− Ion Sources

A survey of emittance characteristics from high‐brightness, H− ion sources has been undertaken. Representative examples of each important type of H− source for accelerator application are investigated: A magnetron surface plasma source (BNL); a multi‐cusp‐surface‐conversion source (LANL); a Penning source (RAL‐ISIS) and a multi‐cusp‐volume source (LBNL). Presently, comparisons between published emittance values from different ion sources are difficult largely because of different definitions used in reported emittances and the use of different data reduction techniques in analyzing data. Although seldom discussed in the literature, rms‐emittance values often depend strongly on the method employed to separate real beam from background. In this work, the problem of data reduction along with software developed for emittance analysis is discussed. Raw emittance data, obtained from the above laboratories, is analyzed using a single technique and normalized rms and 90% area‐emittance values are determined along...

Beam measurements on the H− source and low energy beam transport system for the Spallation Neutron Source

The ion source and Low Energy Beam Transport section of the front-end systems presently being built by Berkeley Lab are required to provide 50 mA of H - beam current at 6% duty factor (1 ms pulses at 60 Hz) with a normalized rms emittance of less than 0.20 p-mm-mrad. Experimental results, including emittance, chopping, and steering measurements, on the performance of the ion source and LEBT system operated at the demanded beam parameters will be discussed.

Status of the SNS H- ion source and low-energy beam transport system

Status of the SNS H - Ion Source and Low-Energy Beam Transport System* R. Keller, @ R. W. Thomae, @ M. P. Stockli, # and R. F. Welton # E. O. Lawrence Berkeley National Laboratory, # Oak Ridge National Laboratory 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 will satisfy the operational needs through the commissioning and early operating phases of SNS. The ion source was derived from the SSC ion source, and many of its original features have been improved to achieve reliable operation at 6% duty factor, producing beam currents up to the 50-mA range. The LEBT utilizes purely electrostatic focusing and includes static beam-steering elements and a pre-chopper. This paper discusses the latest design features of the ion source and LEBT as well as some future improvements, gives performance data for the integrated system, and reports on commissioning re- sults obtained with the SNS RFQ accelerator. INTRODUCTION Berkeley Lab has just completed building the linac injector (Front End, FE) for the Spallation Neutron Source project (SNS) and is commissioning the entire system. The main subsystems are the H - ion-source, the low-energy beam-transport system (LEBT), the 2.5-MeV radio-frequency quadrupole (RFQ) acceler- ator, 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 cho- sen approach, utilizing an rf driven discharge inside a multicusp plasma generator with magnetic filter, ce- sium enhancement, and electron suppression at low energy [2]. For the LEBT, a purely electrostatic focusing sys- tem [3] was chosen, thereby avoiding time-dependent space charge compensation usually encountered with magnetic LEBTs. The production version of the ion source and LEBT [4] aims at generating and transpor- ting a beam with 50-mA current, thought to be suffi- cient to satisfy the latest SNS design goal of 38 mA to be injected into the Linac while assuming a 20% beam loss in the RFQ. Actual RFQ transmission results [5], quoted below, indicate that a LEBT output current of about 44 mA should satisfy the SNS current goal. A schematic view of ion source and LEBT is shown in Figure 1, below. By now, four plasma gen- erators (including one “startup source”) and one LEBT have been built and tested, and the specific design features and performance data of this production sys- tem are discussed in the following sections. FIGURE 1. SNS Ion Source and LEBT schematic. ION SOURCE The production-version ion sources aim at gener- ating H - beams of up to 50-mA current. The goal for the normalized, transverse rms emittance is determined * This work is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC03-76SF-00098. ** The SNS project is being carried out as a collaboration of six US Laboratories: Argonne National Laboratory (ANL), Brook- haven National Laboratory (BNL), Thomas Jefferson National Accelerator Facility (TJNAF), Los Alamos National Laboratory (LANL), E. O. Lawrence Berkeley National Laboratory (LBNL), and Oak Ridge National Laboratory (ORNL). SNS is man- aged by UT-Battelle, LLC, under contract DE-AC05-00OR22725 for the U.S. Department of Energy.

Measurements on H− sources for spallation neutron source application

Lawrence Berkeley National Laboratory is engaged in the development of H− ion sources for the upgrade of the Los Alamos Neutron Science Center (LANSCE) facility and the spallation neutron source (SNS) to be built in the U.S. For the upgrade of the LANSCE facility, the H− ion generator has to deliver an output current of 40 mA. The repetition rate must be 120 Hz at a pulse length of 1 ms (12% duty factor). Furthermore, the normalized emittance must be less than 0.1π mm mrad. During the last years, the Ion Beam Technology Group of the LBNL improved the so-called surface conversion source for the generation of higher H− currents. In the first part of this article, we discuss the operation conditions of the source at the required 40 mA output current. The ion source for the 1 MW spallation neutron source is required to provide 35 mA of H− beam current at 6% duty factor (1 ms pulses at 60 Hz) with a normalized rms emittance of less than 0.2π mm mrad. The H− beam will be accelerated to 65 keV and matched into a...

Radio-frequency antenna studies for high-current, high-duty-cycle, H− volume sources (abstract)

The Spallation Neutron Source requires an ion source capable of delivering a high-current (∼50 mA) H− beam with a 6% duty cycle continuously for the three weeks between the scheduled maintenance periods. The cesium-enhanced, multicusp volume ion source under development on the integrated test facility at LBNL delivers H− ion currents up to 50 mA, increasing approximately linearly with the rf power. Initial experience using porcelain-coated copper antennas, however, indicates lifetimes will fall below the desired three-week period, mostly limited by antenna failures. In an effort to improve our understanding of the antenna limitations, we are in the process of developing an antenna test dome, which will allow us to visually observe and study the rf-initiated discharge at low-power levels. We hope to be able to test antennas for invisible defects by observing and measuring the onset of the discharge. In addition, we are planning to test different antennas. Results will be presented at the meeting.