Long Lifetime CW H- Ion Source for Project X

Long Lifetime CW H- Ion Source for Project X

Fermi National LaboratoryJuly 11, 2013

Evan Sengbusch, PhDJoe Sherman, PhD

Preston BarrowsDaniel Swanson

2

Project X Requirements and Proposed Solution

• > 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

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

Microwave Ion Source + Cesium Converter

Magnetron

AutotunerWaveguide

WaveguideBreak

Ground

-30 kV

PlasmaChamber

MagneticFilter

CesiumConverter Beam

Extraction

Faraday Cup/

Diagnostics

Ground

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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

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.

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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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PNL Neutron Generator Methodology


P-II

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

6Confidentiality 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

PNL Ion Source

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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

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

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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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

ENG – with spectrum tailoring & shielding

Trolley for horizontal scanning

Source side detector array

Top detector array“Transmission” side detector array

The detector array may consist of DDAA detectors, prompt fission neutron detectors, plastic scintillators, NaI detectors etc.,

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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 (1993-2002) 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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Theoretical H- Yield from H0 on W(Cs)

• Solid line is theory from H. L. Cui, J. Vac. Sci. Technol. A9, 1823 (1991).

• H0 thermal energy measurements (solid dots) from S. T. Melnychuk and M. Seidel, J. Vac. Sci. Technol. A9(3), 1650 (1991).

• kT is H0 temperature.• Work from Stevens Institute

of Technology, Hoboken, NJ.


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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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Cross Sections and Reaction Rates for Hyperthermal H0


Following discussion in Brian Lee’s thesis (Stevens Institute, 1993):

Dissociation cross section Reaction rate

Microwave Proton Source (MWS) as Driver for Hyperthermal H0


Neutrons

X-Rays

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.

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Proposed Driver – H- Production RegionsClassic Two Chamber H- Source


Ho Generator

Cesiated Mo

Ho

H-

Dipole filter field

slow electrons,

positive ions

Plasma electrode

Ho Generator

Cesiated Mo

Ho

H-

Dipole filter field

slow electrons,

positive ions

Plasma electrode

• 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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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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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.


Ion Source Design Overview

Plasma source Filter magnet Cesium converter Beam Extraction Beam diagnostics


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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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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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Ion Source Chemistry

Higher work function materials have lower conversion probability.

Mo: 4.36-4.95 eV W: 4.32-5.22 eV Cs: 1.8 - 2.14 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.

Negative ions can be generated by surface ionization of hydrogen ions and atomic hydrogen. [3]

H+ + 2e- → H-

H + e- → H-


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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 Cs2CrO4 + 10 Al → 12 Cs(g) + 5 Al2O3 + 5 Cr2O3

Impurity management critical due to high chemical reactivity of cesium with residual gas.

Cesiated surface electrically biased ~-100 V to promote deposition.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

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H- Converter and Extraction


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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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Magnetic Design - Filter


26Magnetic filter axial profile

Magnetic filter field lines

Thermal Design

295-625 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, 20-200 oC. Heated/cooled by pressurized air loop with inline

heater. H- ion production rate dependent on surface

temperature, optimum around 150-200 oC. Plasma heating effects to be determined experimentally and

adjusted for if necessary.Confidentiality statement: This document is the property of Phoenix Nuclear Labs and may not be


27

Thermal Simulations

300 W cartridge heaters, 100 oC air 500 W plasma heating, 100 oC air


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Mechanical Design


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Faraday cup/calorimeter

Pumpingstage

Csconverter

Magneticfilter

PlasmachamberDC

Waveguidebreak

AutotunerCirculator

Magnetron

Ground

-30kV

Driver and converter floated to -30 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.

Ground

Diagnostic Techniques


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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.

Faraday CupBackground

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

31Confidentiality statement: This document is the property of Phoenix Nuclear Labs and may not be


H0 Calorimeter


Beam Profile Measurement

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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


Optical Spectroscopy

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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.


Optical Spectroscopy

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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


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Neutrons

X-Rays

Optical SpectroscopyData 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.


Langmuir Probe

Types of Probes


Langmuir Probe

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Design Choice

Single Cylindrical Probe Linear Feedthru Glass Tube for the

insulating material Alumina for the main

probe section Tungsten Wire for data

collection


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Langmuir Probe Theory

Single Cylindrical Probe ]

»

can be found from the slope of vs.

ln ( I / I0)=eV p /kTe

ln ( I / I0)kTe Vp


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

41Confidentiality statement: This document is the property of Phoenix Nuclear Labs and may not be


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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


QUESTIONS?Evan Sengbusch, PhD, MBA

(608) [email protected]

mailto:[email protected]

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


Contingency Planning – Identify Design Areas That Could Be Challenging

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 45

• 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.

Long Lifetime CW H- Ion Source for Project X

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