Author: Benitez, J.Y.
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Extending the Performance of Venus Ion Source  
  • D. Xie, J.Y. Benitez, C.M. Lyneis, D.S. Todd
    LBNL, Berkeley, California, USA
  • W. Lu
    IMP/CAS, Lanzhou, People's Republic of China
  Continuing efforts to improve the VENUS performance, especially those started since 2014, have led to greatly enhanced source performance. Improved cooling of the plasma chamber and a slightly modified 28 GHz microwave coupling have resulted in reliable operations, increased injected power of up to 10 kW, and a substantial enhancement in VENUS overall performance. VENUS has produced the following ion beams: 600 eµA of 16O8+, 4 eµA of 40Ar18+, 17 eµA of 78Kr31+, 0.8 eµA of 124Xe45+, 0.6 eµA of 197Au58+ and 0.1 eµA of 209Bi61+. Compared to prior performance, ion beam intensity has increased up to a factor of 6 for the same high charge state and there has been an increase of five charge states upwards for Au and Bi beams where beam intensity is about 1 eµA. In addition a number of ultra-high charge state ion beams at lower intensities have been produced. With ion beams of 124Xe47+, 48+, 49+ injected from VENUS, the total beam energy has reached a new record of ~2.6 GeV for a K140 cyclotron. VENUS produced for the first time oxygen charge state distribution (CSD) peaking at O7+ which is a good step for VENUS and a milestone for ECRIS technology, indicating that there may still be room for improvements.  
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Simulation and Experimental Studies of Biased Disc Behavior in the Superconducting ECR Ion Source VENUS  
  • D.S. Todd, J.Y. Benitez, C.M. Lyneis, D. Xie
    LBNL, Berkeley, California, USA
  The use of a biased disc in place of a first stage injector in an ECR ion source was first reported at the 1990 ECRIS workshop. The improvements in both charge state distribution and beam current were significant: so much so that almost all ECR ion sources built since use biased discs instead of a first stage injector. Though now ubiquitous, exactly how a biased disc is affecting the plasma is not well-understood. The superconducting ECR ion source VENUS provides a good test bench for further studies of the biased disc as VENUS' flexibility allows for the production of moderate current, high current, and highly charged ion beams, and the biased disc operation can be vastly different for each of these operation modes. We present experimental results from varied biased disc operation with VENUS, and also for modifications to the biased disc such as decreasing its size and boring a hole in its center to inject additional electrons from a thermal cathode. Additionally, we have developed a code to investigate electron heating in VENUS. We will discuss results from these simulations which, when coupled with experimental results, will provide a better understanding of the biased disc.  
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Axial X-Ray Cutoff in Small, Dense Plasma in the VENUS ECR  
  • J.Y. Benitez, C.M. Lyneis, D.S. Todd, D. Xie
    LBNL, Berkeley, California, USA
  The coil structure of the superconducting ECR ion source VENUS allows for considerable control over the plasma-confining magnetic field while investigating axially-emitted x-rays from the source. Generally, it is found that axial x-ray intensity increases with input RF power. However, when the minimum field at the source center is relatively high this increase with power is only over a limited range. Past this range there is a sudden decrease in x-ray intensity followed by a complete disappearance of axial x-rays for any further input heating. As ion beams are still extracted, it is clear that plasma is still present. We will present measured data in the forms of x-ray spectra, extracted beam currents, and transmitted and reflected microwave power to explore the possibility that the plasma may have reach critical density on axis and effectively turned off RF heating in this region.  
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