1. Long Lifetime CW H- Ion Source for Project X
Evan Sengbusch, PhD
Joe Sherman, PhD
Preston Barrows
Daniel Swanson
Fermi National Laboratory
July 11, 2013

2. Project X Requirements and Proposed Solution
Microwave Ion Source + Cesium Converter
> 10 mA CW H- beam current
Beam emittance < 0.2 pi- mm-mrad at RFQ entrance
Extracted at 30 kV
Lifetime > 1 month (4-6 months preferred)
High gas efficiency
Hi power efficiency
Magnetic
Filter
Plasma
Chamber
Cesium
Converter
Beam
Extraction
-30 kV
Ground
Ground
Waveguide
Break
Faraday
Cup/
Diagnostics
Autotuner
Waveguide
Magnetron
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3. Phoenix Nuclear Labs
Founded in 2005; 14,000 ft2 lab (including two shielded bunkers) located in Madison, WI
Multidisciplinary team of PhD scientists, engineers (nuclear, electrical, mechanical), and technicians
PNL core mission is to design, build, and commercialize high flux neutron generators
PNL has demonstrated neutron production of 3x1011 n/s (D-D) CW and anticipates a 5x1011 n/s demonstration in 6-12 months
Funded primarily by VC’s/angels and several DoD / DoE contracts:
$50M NNSA cooperative agreement - isotope production
4 DoD Contracts – Neutron radiography, IED detection, nuclear survivability, and neutron diffraction
DoE – Ion source development for high energy physics
JIEDDO – Pending contract to study stand-off detection of IED’s
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4. PNL High-Flux Neutron Generator
Technology base: 300 kV deuterium beam incident on deuterium or tritium gas target
Up to 5x1011 DD n/s or 5x1013 DT n/s emitted isotropically
Key innovations:
Gaseous target increases neutron yield and device lifetime
Very high current achieved by novel ion source and beam extraction design
2 prototypes have been built and are operating
P-I: Radiography system (US Army)
P-II: Medical isotope production (Nat Nuclear Security Admin)
2 in design phase
P-III: IED detection (US Army)
P-IV: Medical isotope production (Nat Nuclear Security Admin)
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5. PNL Neutron Generator Methodology
P-II
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6. PNL Microwave Source Performance
122 hour (99.99% uptime) CW operation demonstrated at 50 mA, 45 kV
> 90 mA deuterium extracted at 260 kV
60kV, 65mA Beam on calorimeter
PNL Ion Source
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7. Medical Isotope Production
PNL is a subcontractor on a $50M+ cooperative agreement with the National Nuclear Security Administration (NNSA) and SHINE Medical Technologies for domestic production of the medical isotope Moly-99
Moly-99 is used by 55,000 patients each day in the US for nuclear medicine procedures
US Gov has made a non-HEU domestic source of Moly-99 a high priority
Eight subcritical fission assemblies, utilizing an aqueous solution of LEU, will each be driven by the PNL intense neutron generator to produce half of the total global demand for Moly-99
Starting in 2016, 8 neutron sources per year (5x1013 DT n/s each) will be delivered to the SHINE isotope production facility and will be maintained and serviced by PNL
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8. Neutron Radiography
Orders-of-magnitude increase in neutron yield allows for practical implementation of non-reactor thermal neutron radiography for:
Artillery shells – system delivered to US Army
Critical aircraft and spacecraft components
Composite materials
Fast neutron radiography is of interest for cargo screening at sea- and airports
Requires high neutron yield to be practical
Provides elemental information complementary to X-rays
Dual X-ray/Neutron radiography systems being implemented in China, Australia (CSIRO/NuTech)
Rapiscan recently requested information about the PNL neutron source for fast neutron radiography
Neutrons
X-Rays
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9. Component Testing & Evaluation
Army Phase I SBIR has been awarded to PNL to evaluate using PNL neutron generator to irradiate critical components
Air and spacecraft operate in high-radiation environments and must be tested and hardened
Current testing done at HEU-based reactors – high cost and security/regulatory burden
PNL’s neutron source can simulate nuclear environments without HEU
Air Force Phase I SBIR has been awarded to PNL to evaluate aircraft components using neutron diffraction
Neutron diffraction is a proven technique for bulk residual stress analysis
Presently only available at reactors and spallation sources
PNL’s high neutron yield could allow this important measurement technique to take place in laboratory/factory settings
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10. Neutron-Based IED/SNM Detection
Neutrons interact with explosive elemental constituents or fissile material
High energy gamma rays and/or neutrons are emitted and detected to signal the presence an IED or SNM
With very intense sources, detection is possible at operationally significant standoff distances; elemental composition information available also
PNL is being funded by the Army and JIEDDO to miniaturize its neutron generator for mobile and/or vehicle-mounted detection
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11. Ion Source Overview
Technical - historical account of Stevens Institute (Hoboken, NJ), and their analysis of H- production by hyperthermal H0 on cesiated molybdenum surface (1993).
Review of LEDA (LANL) H+ injector performance ( ) based on the microwave proton source (MWS), and why this source appears to be an excellent cw H0 driver for cesiated converter source.
Simulation for 10mA, 30keV H- beam extraction. Meets Project X requirements.
Practical realization of long lived H- source.
Uses experience from the Chalk River Lab and the Los Alamos LEDA MWS technology.
This H- source is based on the U.S. Spallation Neutron Source (SNS) Cs converter, the Lawrence Berkeley National Lab (LBNL) magnetic filter, and the Cs H- converter technology from Novosibirsk.
Third talk is on H- source design details. (Preston Barrows)
Confirmation of MWS plasma properties optimal for Cs converter H- production
High electron temperature (kTe) in the driver region, and effective kTe reduction in the H0 converter.
Observation of hyperthermal H0 (kTH0 > 1eV).
High H0 flux from MWS driver.
H- beam current and noise characteristics.
Fourth talk is on H- source diagnostics. (Dan Swanson)
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12. Theoretical H- Yield from H0 on W(Cs)
Solid line is theory from H. L. Cui, J. Vac. Sci. Technol. A9, (1991).
H0 thermal energy measurements (solid dots) from S. T. Melnychuk and M. Seidel, J. Vac. Sci. Technol. A9(3), (1991).
kT is H0 temperature.
Work from Stevens Institute of Technology, Hoboken, NJ.
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13. Production of Hyperthermal H0
Hyperthermal H0 defined as H0 energies > 1eV.
High electron temperature H2 plasmas leads to direct H2 dissociation to hyperthermal H0.
The electron energy threshold for direct dissociation of state II in the adjoining figure is 8.8eV.
The minimum dissociation energy of state II is 2.2eV.
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14. Cross Sections and Reaction Rates for Hyperthermal H0
Following discussion in Brian Lee’s thesis (Stevens Institute, 1993):
Dissociation cross section
Reaction rate
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15. Microwave Proton Source (MWS) as Driver for Hyperthermal H0
What do we know about the MWS from H+ production (CRL, LANL)?
kTe ~ 20eV (from Chalk River Lab Langmuir probe measurements)
J+ = 0.26A/cm2
Ne = 1.2 X 1012 e/cm3
N(H2) = 7.1 X 1013 H2/cm3 (molecular flow)
H+ fraction 90% at ~ 1 kW 2.45GHz microwave power
Continuity equation for H0 flux based on volume production (V) and surface (A) loss
NeN(H2)<sve>V = nHovHoA/(4a)
fH0 = nH0vH0/4 = 6.6 X 1018 H0/(cm2-s) (MWS)
*Interesting observation: Based on 4.1sccm H2 flow rate in LANL MWS the neutral flux density effusing from the MWS is 4.7 X 1018 neutrals/(cm2-s) -> all H2 dissociated to H0!
For 20% conversion efficiency (H0 -> H-), 15% solid angle efficiency, jH- = 24mA/cm2
remis = 0.4cm, IH- = 12mA, erms,n = (remis/2)(kTH-/mc2)1/2 = .065 (pmm-mrad), kTH-= 1eV
No optimization of MWS for H0 production assumed, or, possible contribution to H- production from slow positive ions.
Neutrons
X-Rays
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16. Proposed Driver – H- Production Regions Classic Two Chamber H- Source
H0 generator is MWS
Dipole filter for reducing hot electrons
Cesiated molybdenum converter (H0 -> H-). Cone exit aperture radius = 0.5cm
Plasma electrode has
remis = 0.4cm
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17. 30kV H- Extraction System
PBGUNS simulation using H- plasma meniscus option. 10mA extracted current.
Extraction gap = 27.2mm, emission aperture radius = 4mm,
extraction aperture radius = 3.2mm
kTH- = 1eV, erms,n (PBGUNS) = 0.10 (pmm-mrad)
Co-extracted electrons separated from H- beam after extraction electrode by a dipole separation magnet.
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18. Expected H- Source Lifetime
MWS discharge (2.45GHz, 875G ECR) can run very long time in cw mode (months). PNL has gained expertise in reducing EMI while developing 300keV positive ion accelerators.
The MWS is most gas and power efficient cw H+ source known.
PNL H- injector design will place most sensitive electronics at ground level, thus minimizing EMI problems (minimal equipment on 30kV deck).
Recent work at the U.S. Spallation Neutron Source (SNS) has indicated a single cesiation of the converter cone may last two weeks or more without detriment to H- production. For this reason, the PNL design follows the SNS converter developments as closely as possible. The Cs oven proposed here will contain enough Cs for many Cs applications.
There is good reason to suspect that the proposed source could operate at 10mA, 30keV in cw mode for several months.
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19. Ion Source Design Overview
Plasma source
Filter magnet
Cesium converter
Beam Extraction
Beam diagnostics
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20. Design Goals
Stable ECR plasma driver capable of producing high density and high temperature plasma for long run times.
Adjustable electron temperature in Cs conversion region by use of filtering magnets.
Efficient conversion of high temperature H+ ions and neutrals into H- ions though surface reactions with low work function materials.
Extraction and acceleration of high quality beam.
Incorporation of diagnostic instruments.
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21. Magnetic Design - Driver
Frequency of cyclotron motion given by
For 2.45 GHz microwaves and electrons, resonance match when B = 875 G [2]
Best performance when resonance zones located near front and rear of plasma chamber.
Field leakage outside driver reduced with iron/steel shunts.
Minimize B in waveguide to reduce unwanted ionization.
Minimize axial B in conversion region to improve magnetic filtering.
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22. Ion Source Chemistry
Negative ions can be generated by surface ionization of hydrogen ions and atomic hydrogen. [3]
H+ + 2e- → H-
H + e- → H-
Higher work function materials have lower conversion probability.
Mo: eV
W: eV
Cs: eV
Cesium work function as low as 1.3 – 1.7 eV at thickness of about 0.6 monolayers. [1]
Low binding energy (0.75 eV) of additional electron is beneficial to neutralization, but also makes H- ions vulnerable.
Plasma parameters and background gas in conversion section are critical. 22
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23. Cesium Source Commercially available alkali-metal dispenser.
Cesium is stored in a stable chemical compound.
Controlled release of pure Cs through decomposition reaction of compound and reducing agent.
SAES Cs dispenser contains cesium chromate (Cs2CrO4), zirconium and aluminum.
Production and release of pure Cs. Temperature driven rate above 625 oC
4 Cs2CrO4 + 5 Zr → 8 Cs(g) + 5 ZrO2 + 2 Cr2O3
6 Cs2CrO Al → 12 Cs(g) + 5 Al2O Cr2O3
Impurity management critical due to high chemical reactivity of cesium with residual gas.
Cesiated surface electrically biased ~-100 V to promote deposition. 23
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24. H- Converter and Extraction24
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25. Magnetic Design - Filter
A magnetic filter field cools the plasma before converter surface to reduce the destruction of negative ions by electron stripping.
Electron temperature of ~10 eV in driver. Target electron temperature of 2 eV at converter surface.
Difficulty: high-permeability plasma aperture plate to contain driver fields tends to shunt filter magnet away from desired location.
Aperture chamfered to add distance between plate and filter while still containing driver fields.
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26. Magnetic Design - Filter
Magnetic filter field lines
Magnetic filter axial profile
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27. Thermal Design
oC minimum temperature for cesium dispenser, depending on compound.
Cesium dispensers driven by small cartridge heaters or direct current.
Thermally isolated with stainless or ceramic standoff.
Cesiated surface cooled to selectively enhance deposition rate, oC.
Heated/cooled by pressurized air loop with inline heater.
H- ion production rate dependent on surface temperature, optimum around oC.
Plasma heating effects to be determined experimentally and adjusted for if necessary.
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28. Thermal Simulations
300 W cartridge heaters, 100 oC air W plasma heating, 100 oC air
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29. Mechanical Design Driver and converter floated to -30 kV.Cs
converter
Driver and converter floated to kV.
Microwave hardware, diagnostics, and driver solenoids at ground.
Use proven and existing PNL technology when possible.
Modular design.
Simple dis/assembly.
Inclusion of diagnostics.
Flexibility for contingency plans.
Plasma
chamber DC
Waveguide
break
Magnetic
filter
Pumping
stage
-30kV
Ground
Ground
Autotuner
Circulator
Faraday cup/
calorimeter
Magnetron
Confidentiality statement: This document is the property of Phoenix Nuclear Labs and may not be copied, used, or disclosed for any reason except as authorized by PNL

30. Diagnostic Techniques
Calorimeter
Atomic Flux Measurements to confirm the flux from the Driver is high enough.
Faraday Cup
Beam Current and Noise Measurements for assisting in determining the necessary strength of the filter magnet.
Video Camera Mounted on the conflat cross
Used for Beam Profile Measurements to visually verify the cross section of the beam.
Optical Spectroscopy
Plasma Density and Temperature Measurements to further help understand the plasma source and possibly detect impurities.
Langmuir Probe
Plasma Velocity, Temperature, and Flux Measurements to further assist in determining the necessary filter magnet strength.
Confidentiality statement: This document is the joint property of Phoenix Nuclear Labs and may not be copied, used, or disclosed for any reason except as jointly authorized by PNL. 30

31. Faraday Cup Background
Measure 30keV H- current; electron suppressor either electrostatic and/or magnetic (Electrostatic Shown)
Beam noise measurement; expect bandwidth ~ 10 MHz
Working with e/H- separation magnet (located immediately after 30kV extractor), deduce e/H- ratio
Faraday Cup entrance aperture diameter (molybdenum plate) designed on the basis of the PBGUNS predicted divergence, and known drift distance to the Faraday Cup entrance 31
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32. H0 Calorimeter
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33. H0 Calorimeter
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34. Beam Profile Measurement
Design Concept
Mounting a Video Camera on the conflat cross for viewing the beam profile
There is a Window on the conflat cross for the Camera to view the beam through without being damaged
We can assume we have an axisymmetric beam, so one Video Camera is sufficient
If the coextracted electrons are seperated from the H- beam in the horizontal plane, it would be interesting to mount the camera in the vertical plane so the seperation of the two beams would be visible 34
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35. Optical Spectroscopy Background
Used for Plasma Density and Temperature Measurements
Can also be used for detecting impurities and leaks in the system
The change in wavelength at fwhm of an emission peak is due to Doppler broadening
For the 656nm hydrogen line, this is about .15nm for 10eV and .05nm for 1eV.
The resolution of the monochromator needs to be below these values.
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36. Optical Spectroscopy Light input options Lenses
Potential exists for better performance
Can require more sophisticated mounting and alignment hardware
Needs transmission through a vacuum window and guarding against stray light
Monochromator needs to be physically located as close to the vacuum wall as possible
Fiber Optics
No need to set and maintain precise alignment of components
Vacuum feedthru is a stock part and creates no concern of external light noise 36
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37. Optical Spectroscopy Data Collection Classical Monochromator
3737
Optical Spectroscopy
Data Collection
Classical Monochromator
Has a single detector which measures the intensity of a single wavelength of light over time.
Single wavelength is selected by mechanically shifting elements.
This style is slower but has better resolution in .01 nm or better.
Extra resolution provided here is not necessary for this application.
Newly designed CCD collector
Samples the entire available spectrum at once.
Faster data collection is limited only by the required exposure time.
Faster feedback allows for easier characterization of source plasma temperatures over a wide range of operating parameters.
Neutrons
X-Rays 37
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38. Langmuir Probe Types of Probes 3838
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39. Langmuir Probe Design Choice Single Cylindrical Probe Linear Feedthru
Glass Tube for the insulating material
Alumina for the main probe section
Tungsten Wire for data collection 39
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40. Langmuir Probe Theory can be found from the slope of vs.
4040
Langmuir Probe
4040
Theory
Single Cylindrical Probe ]
can be found from the slope of vs.
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41. Diagnostics Summary Multiple Diagnostic Tools being used Calorimeter
Video Camera
Faraday Cup
Optical Spectroscopy
Langmuir Probe
Multiple Values to be obtained
Atomic Flux
Beam Current, Noise, and Profile Measurements
Plasma Density, Temperature, and Velocity 41
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42. Conclusions
PNL has demonstrated high current, long lifetime CW operation with a positive deuterium microwave ion source
There is a good reason to believe that coupling this source with a Cs conversion cone will result in a high performance CW H- source with a long lifetime
Preliminary designs have been completed
Next step is pursuit of Phase II SBIR funding to build and test the H- ion source
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43. QUESTIONS? Evan Sengbusch, PhD, MBA (608) 210-3060

44. References
[1] Handbook of Ion Sources, Bernard Wolf, CRC Press, Inc., 1995
[2] NRL Plasma Formulary, Naval Research Laboratory, 2011
[3] Work function measurements during plasma exposition at conditions relevant in negative ion sources for the ITER neutral beam injection, R. Gutser, C. Wimmer, and U. Fantz, 2011
[4] Fusion Physics, IAEA, 2012
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45. Contingency Planning – Identify Design Areas That Could Be Challenging
PM instead of electromagnet driver source. Better control of kTe in the converter region and Cs oven temperature control. May have complications of the electromagnet and dipole filter fields.
Modification to Cs oven, converter cone, and tube for:
Thermal loading surprises
Hyperthermal H0 incident angle on Cs converter cone
Coextracted electron dump options
Weak or strong dipole magnet after 30keV beam formation? Present design is for weak field so H- beam direction correction is minimal. Preferred option.
Dump coextracted electrons on electrode with intermediate potential. Seems unattractive for cw beam reliability to dump electrons in the extraction field.
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