Robert F' Welton

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Ion Source RD at the SNS

• Robert F. Welton

Accelerator Physics and Technology Forum October
13, 2005
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Outline

• Introduction Commissioning Performance,
  Facility Requirements and the H- Ion Source
• Early Endurance Runs at Full Duty-Factor
• Development of a New Cesiation Technique
• Later Endurance Runs at Full Duty-Factor
• RD Directions External antennas, New Cs
  collars, Improved extraction systems and Helicon
  Plasma Generators

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Source Performance During Commissioning
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SNS Operational Requirements
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2002 Survey of H- Ion Sources
In routine use at major facilities or extensively
commissioned
- R.F. Welton, Overview of High-brightness H-
Ion Sources, LINAC02, Gyeongju, Korea
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The SNS Ion Source
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Early High-Duty Factor Test Runs
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Data Analysis

• During each run an optical spectrometer monitored
  plasma density which was found to be constant.
  This suggests a problem with surface production ?
  Cs distribution.
• Each Cs dispenser contains a compressed powder
  mixture of Cs2CrO4, Al and Zr (St101) which
  release Cs though these reaction pathways

• Thus if elemental Zr and Al become depleted Cs
  will not be released!

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Data Analysis

• Computational thermodynamic analysis (HSC) shows
  that free Zr and Al will form stable (and
  useless) compounds with residual gases evolved
  from the source during source conditioning at
  temperatures greater than 250 C.

• The new operating procedure is simple Fully
  condition the source to full duty factor before
  raising the Cs collar temperature above 250 C!

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Later High-Duty Factor Test Runs
Each run employed the new source conditioning
technique
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Ongoing Source Development Efforts at SNS
A significant improvement but does yet meet the
SNS operational or PUP requirement

• Ongoing Source Development.
• External Antennas
• New Cs Collars
• Improved Extraction Systems
• Helicon Plasma Generators

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The External Antenna Module

• Motivation DESY has shown a several year source
  lifetime at 40 mA but with low duty factor 0.1.
• Status We have designed and built a similar
  system.

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External Antenna 100 ms Operation

• Status
• Highest beam current ever produced by an
  uncesiated SNS ion source in our lab 21 mA!

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External Antenna - Plasma Stability Studies
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External Antennas Hollow Anode E-gun

• Advantages
• Stabilize plasma for long pulses gt 100 ms
• Eliminate 13 MHz ignition system
• Enhance plasma density by electron injection

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1st Data External Antenna with Hollow Anode
E-gun
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High-Duty Factor Operation Heat Flux
Measurements
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New Cs Collars

• Motivation Current system is not optimized
  (indirect Cs transfer), does not meet the SNS
  requirement and only supports 3 cesiations.
• Progress We are developing Cs collars with
  enhanced geometry, temperature distributions and
  Cs sources
• Details RF Welton et al., AIP Conf. Proc. 763
  (2005) p 296

Enhanced geometry Integrated Cs Collar / Outlet
Aperture

• Employs a conical ionization surface which
  extends into outlet aperture
• Larger emittance observed

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New Cs Collars
Enhanced Temperature Distributions
Direct-Transfer Collar

• Design allows complete and independent
  temperature control of T1 T2 (30-650 C)
• Produced 50 mA with T1 211C and T2 585C at
  1.2 ms in spite of damaged antenna

Improved Cs source External Cs Reservoir
X-ray diffraction analysis shows a complete
absence of Cs chromate after 3 cesiations in
original LBNL Cs collar need a better Cs source!
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New Cs Collars Combining best features
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Improved Extraction Systems

• Motivation Detailed comparison between
  uncesiated performance of the DESY (40 mA) and
  SSC (35 mA) sources with the SNS (15 mA)
  revealed significant differences in the
  extraction systems which need to be explored.
• Identified two major differences (i) Point of
  Electron Separation (ii) Electric field at
  meniscus
• Details RF Welton, SNS Tech note 147
  www.sns.gov

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Improved Extraction Systems
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Improved Extraction Systems The DESY Experiment
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Helicon Plasma Generator for SNS Source

• Motivation Driven by higher current demands of
  the SNS Power Upgrade Project helicon coupled
  plasmas can increase plasma density by an
  order-of-magnitude! We observe that both beam
  current and electron density scale with RF power.

Original LBNL Ion Source plasma density and beam
current measurements
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Helicon Plasma Generator VASIMR Test System

• Helicon plasma generators can produce much higher
  densities at lower gas pressure and cooler
  electron temperatures.

• VASIMR Helicon System at ORNL-FED routinely
  produced electron densities of 1013-1014 e/cm3
  using H2

Courtesy of FF Chen
Courtesy of R. Goulding
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Helicon Plasma Generator

• Progress We have begun collaboration with ORNL
  Fusion Energy Division to retrofit the VASMIR
  helicon system to the SNS source utilizing their
  expertise. B-field calculations are currently
  being performed to explore the feasibility of
  this hybrid device.
• Details RF Welton, SNS Tech note 147
  www.sns.gov

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Outlook

• At low duty-factor (lt1), the ion source
  continues to meet the commissioning goals of the
  SNS Accelerator supporting the agenda of the
  physics group. The availability of the source
  steadily increased from 86 to 99 for the last
  commission periods.

• At full duty-factor (7), the new source
  conditioning procedure has resulted in an
  order-of-magnitude improvement in beam
  persistence allowing delivery of beams of excess
  of 33 mA for 16 days versus several hours
  achieved prior to this development. We are now
  much closer to the SNS operating goal of 40 mA
  for 21 days but not there yet!

• A multifaceted ion source RD program is in place
  with the goal of developing an ion source capable
  of meeting SNS operational requirement in 1-2
  years and upgrade requirements in 3-5 years.
  Risk is mitigated by simultaneously perusing
  parallel efforts. New Cs collars have already
  produced beams of 60 mA at full duty-factor 7,
  early external antenna development is also
  encouraging.