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Beamline Design Issues

This presentation by D. R. Welch and D. V. Rose discusses beamline design requirements for ion beams in fusion experiments, emphasizing the importance of electron neutralization and protection from chamber plasmas. The study includes simulations of beam-plasma interactions and proposes solutions for suppressing electron motion using magnetic fields. Results show that a dipole magnetic field can effectively reduce emittance growth. The design aims to optimize ion beam neutralization while minimizing unwanted plasma interactions.

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Beamline Design Issues

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  1. Beamline Design Issues D. R. Welch and D. V. Rose Mission Research Corporation W. M. Sharp and S. S. Yu Lawrence Berkeley National Laboratory Presented at the ARIES Project meeting At UCSD, January 8, 2003

  2. Beamline design requirements (1) allow the ion beam to enter the chamber (2) provide electron neutralization of the beam space charge - provided by 2 plasma plugs of 1012 cm-3 density (Lsp sim) (3)inhibit rapid upstream motion of electrons (4) provide protection from chamber plasmas and gas expanding upstream after the fusion explosion - initial plasma plug increases in density to 1015 ionizing neutral gas (analytic calc), dipole magnet diverts plasma to wall (Lsp sim)

  3. High perveance HIF beams can be neutralized with a localized plasma Plasma just outside final focus magnet fringe fields Plasma just inside port • After final focus magnets, beams must drift roughly 6 meters and strike a < 3 mm target radiator • Localized plasma or “plasma plug” can provide significant neutralization Xe+ Target Target Radiation e- e- e- e- e- 3 m 3 m

  4. Problem - Emittance growth has been calculated in Lsp simulations of NBT with a plasma plug • Plasma plug electrons are attracted by the beam space charge, downstream and upstream • If electrons are not uniform in density, the resulting anharmonic electrostatic potential increases the beam emittance before reaching plasma • For large currents, unacceptably large spot sizes have been calculated • Here, we focus on the upstream interaction of the ion beam with plasma electrons • Solution is consistent with NBT beam port design

  5. Lsp model for beam-plasma plug upstream interaction • Simulations are two and three dimensional • Beam is injected through open boundary on left: .25-4 kA current, 4 GeV Bi+ • Electrons are Child-Langmuir emitted from grounded plane on right • CL electrons react like electrons from a dense plasma plug without the need to resolve plasma parameters (simulations are fast)

  6. Bi+ ionselectrons Plasma electron emission at z =50 cm Plasma electrons are pulled into unneutralized beam as beam approaches plasma plug • 4-kA Bi+ beam (initially cold) • plasma electrons are modeled at CL emission from z=50 metal wall • 5-kA electron current flows back • For k=1.5x10-4, growth is 1.5 pi-mm-mrad • electrons stream back non-uniformly and oscillate Electron current at z=45 cm

  7. 6-cm, 4-kA 6-cm, 1-kA 6-cm, .25-kA Plasma electrons are pulled into unneutralized beam as beam approached plasma plug 3-cm, 4-kA • Emittance growth (at z = 50 cm) scales with current, higher for smaller radius beam • electron current (at z=45 cm) grows faster than linear with beam current Beam emittance Beam emittance 6-cm, 4-kA 6-cm, 4-kA no plasma 6-cm, 4-kA 3-cm, 4-kA 6-cm, 1-kA 6-cm, 4-kA 6-cm, .25-kA Electron current (A) Electron current (A)

  8. We now attempt to suppress upstream motion of electrons with applied magnetic fields in two geometries • Solenoidal field (Bz) • Dipole Field (By)

  9. 2-kG solenoidal field removes most of the emittance growth B • Electrons stream in uniformly, nearly harmonic forces, little emittance growth • B field would have to extend the length of beam 50-150 cm, plasma flow into field complicated No Bz 2-kG Bz

  10. 1 kG 4 kG 2 kG Dipole field >2 kG, only 5-cm long, suppresses backstreaming in 3d Lsp sims • Electrons eventually ExB drift into beam, drift time slowed by increasing B • Some emittance growth is due to beam steering in dipole field B 1 kG 2 kG 4 kG

  11. Beam plasma plug interaction can increase emittance for high K • effect scales with current • electron EM fields are non-uniform creating anharmonic potential, emittance growth • solenoidal field of order a kGauss can force electrons to be uniform and greatly reduce emittance growth • dipole magnetic field can suppress upstream motion of the beam, no plasma-B field interaction

  12. Dipole magnet is consistent with current beamline design Plasma just outside final dipole magnet fringe fields – plasma density increase to 1015 before dibris arrives Final Focus magnet • Dipole magnet and upstream plasma plug both have dual roles • 2 kG field suppress both electron and chamber plasma upstream motion • Initial plasma plug density ramps from 1012 to 1015 to both neutralize ion beam and ionize neutrals Plasma just inside port Target Xe+ Target Radiation e- e- e- e- e- 3 m 3 m

  13. Conclusions • Unacceptable emittance growth calculated for high-current beams entering plasma plug • Electron motion can be suppressed with a dipole magnetic field just upstream of plasma • 1-2 kG dipole magnetic field is consistent with current beamline design, verified in 3D Lsp simulations

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