ORNL is managed by UT-Battelle, LLC for the US Department of Energy

Moderator Choices for SNS Second Target Station (STS)

Presented at the ENS2019 Workshop

PSI/Villigen/Switzerland

Franz X. Gallmeier, Igor Remec Neutron Technologies Division/STS Project

September 3-5 , 2019

2 2

Outline

• ORNL three source strategy • STS baseline parameters • Requirements • STS Moderator Suite and its performance • Tradeoffs • Outline of future work

3 3

ORNL Three Source Strategy HFIR reactor: highest-intensity continuous source of cold and thermal neutrons

FTS: optimized for high-resolution instruments decoupled thermal and cold moderators

STS: highest peak brightness for cold neutron beams towards small samples

4 4

FTS+STS Baseline Parameters

• PPU project will provide accelerator build-out to 2.8 MW power at 1.3 GeV proton energy and 60 Hz.

• Split repetition rates of 45 Hz to FTS and 15 Hz to STS

Parameter STS Short-Pulse

FTS Short-pulse

Power (kW) 700 2000

Repetition Rate (Hz) 15 45

Proton Pulse Length (µs) <1 <1

5 5

Choice of Moderators driven by instrument needs:

Charge: • Provide high intensity

long-wavelength neutron beams

• Serve 22 instruments

Met by new hydrogen moderators concepts: • Flat Cylinder

Moderator • Tube moderator

BL Instrument Description Moderator Type

1 SANS2 Kinetic SANS Tube Moderator 2 ZEEMANS High Magnetic Field Beam Line Cylinder Moderator 3 CYGNUS SANS Cylinder Moderator 4 TBD Cylinder Moderator 5 VBPR Variable Beam Profile Reflectometer Cylinder Moderator 6 SANS1 SANS Tube Moderator 7 BWAVES Broad-range Wide Angle V- Selector Cylinder Moderator 8 WASABI Wide and Small Angles with Big Intensity Cylinder Moderator 9 QIKR Quite Intense Kinetics Reflectometer Cylinder Moderator 10 TBD Cylinder Moderator 11 SANS3 SANS Tube Moderator 12 Neutron Spin Echo Tube Moderator 13 VERDI - Versatile Diffractometer Cylinder Moderator

14 HERTZ High Energy Resolution Terahertz Spectrometer Cylinder Moderator

15 TBD Cylinder Moderator 16 NeSCry Neutron Single Crystal Diffractometer Cylinder Moderator 17 CHESS Chopper spectrometer for Small Samples Tube Moderator 18 TBD Cylinder Moderator 19 TBD Cylinder Moderator

20 EWALD Enhanced Wide-Angle Laue Diffractometer Cylinder Moderator

21 MENUS Materials Engineering by NeUtron Scattering Cylinder Moderator

22 Imaging Tube Moderator

8 first prospective instruments to be built within STS project

6 6

Beamline layout

1 11 6

17 12

22

7 7

Renderings of MCNPX model Curtesy I. Remec

CDR Target Station Configuration

• Cylinder moderator: 16 beamlines • Tube moderator: 6 beamlines • Both coupled, para-H2 at 20K, with

H2O pre-moderator • Edge cooled beryllium reflector

8 8

Target and Proton Footprint Choices • Tungsten target, clad in

tantalum – Starting with stationary target

(stacked plates) – End with synchronous rotating

wheel tungsten target (edge-cooled segmented solid brick)

• Proton beam footprint – Starting with flat profile 30 cm2

proton beam – End with double gaussian with

90% in 62 cm2

9 9

Moderator and Reflector Choices • Sample sizes trend to smaller sizes driven by:

– increasing difficulty to synthesize complex materials,

– required higher resolution – limited sample environments sizes

• Sources of three centimeter dimensions are a good match to illuminate samples of 1 cm dimension (Zhao et al)

• Brightness gains can be harvested by reducing the moderator dimensions → High-brightness moderators at 3 cm height/diameter.

• Beryllium reflector is a standard at spallation neutron sources

Zhao et al, Rev Sci. Inst. 84 (2013)

10 10

STS Cylinder and Tube Moderators: Brightness metrics

FTS moderator for comparison as of Jan 2019: • 1.4 MW, 60 Hz, 1.0 GeV • IRP2 with heavy-water cooling • Aluminum proton beam

window • 30% ortho hydrogen • 20% loss due to power-

induced degradation

25 4 6

FTS

STS

11 11

STS Cylinder and Tube Moderators: Pulse shapes

• d

Besides higher peak brightness STS moderators also excel in faster pulse decay

12 12

STS Cylinder and Tube Moderator: Gains over FTS

• Time-integrated and peak brightness gains compared to FTS coupled moderator

Time-averaged Brightness Peak Brightness

power-normalized power-normalized

per proton per proton

13 13

STS Moderator Summary

• Cylinder and tube hydrogen moderators with critical dimensions of 3 cm pushed to the almost-full para state were chosen for STS to produce high-intensity cold neutron beams and to serve as much as 22 instruments

• Reducing dimensions to about 3 cm in height/diameter provides brightness gains over the FTS coupled moderators and match well to instruments of centimeter-size samples

• Gains in time-averaged and peak brightness of 5-7 and 23, respectively, over FTS materialize from compact design and good coupling to target

14 14

What are our tradeoffs?

• The size of the moderator viewed area determines the brightness but also the beam intensity – conflicting quantities.

• The moderator depth is not fixed and impacts the ratio of time-averaged and peak brightness.

• Instrument specific metrics are needed for refined moderator optimization

15 15

Figure-of-Merits so far used for optimizations

• Peak-brightness integral E< 5meV (used for Cylinder moderator)

• Time-integral brightness E< 5meV (used for Tube moderator)

16 16

Impact of FOM on optimizing Cylinder Moderator:

Tint FOM

Peak FOM

Parameter (mm) (mm)

PBH 40 40 MBR 104 41 PMLR 29 21 PMLB 28 23 PMLT 22 21 TZO -172 -88

Dimensions of premoderator thicknesses and moderator radius are greatly affected by choice of FOM as does the neutronics performance

Peak-FOM optimized Tint-FOM

optimized

17 17

Performance Comparison Cylinder moderators

• solid curve: optimized to time-averaged brightness

• dashed curves: optimized to peak brightness

18 18

What tube diameter to chose for Tube Moderators? • Tube Moderator with diameters of 2

and 3 cm were studied. • Both resulted at best time-averaged

brightness at dimensions: – Tube length 16-17 cm – Premoderator thickness of 3.2 cm

0

2e+14

4e+14

6e+14

8e+14

1e+15

1.2e+15

1.4e+15

0.0001 0.001 0.01 0.1 1 10 100

tint b

right

ness

(n/c

m2 /e

V/s

r/s)

energy (eV)

STS-tube D2-1STS-tube D2-2STS-tube D3-1STS-tube D3-2

STS-cyl 3x3STS-cyl 3x6

STS-cyl-big 3x3STS-cyl-big 3x6

0

5e+16

1e+17

1.5e+17

2e+17

2.5e+17

3e+17

3.5e+17

4e+17

0.0001 0.001 0.01 0.1 1 10 100

peak

brig

htne

ss (n

/cm2 /e

V/s/

sr)

energy (eV)

STS-tube D2-1STS-tube D2-2STS-tube D3-1STS-tube D3-2

STS-cyl 3x3STS-cyl 3x6

STS-cyl-big 3x3STS-cyl-big 3x6

• Gains in time-integrated brightness over cylinder moderator for both diameter options

• Gains in peak brightness for 2-cm-diameter tube.

19

Tube moderator optimization towards Pulse Peak-Brightness and Time-integrated Brightness

• Here for 3-cm-diameter tubes.

• Moderator characteristics vary most with tube length

• Tint and Peak FOMs are conflicting requirements

20

Tube moderator: tube length tradeoffs

• Trade-offs in – time-integrated brightness, – pulse width, – pulse shape

21 21

Outlook

• The reflector will require a detailed look: sizing, material alternatives

• Instrument designs will mature and will define refined instrument needs.

• Engineering design will set engineering constraints on moderator designs.

• All pieces of information will feed into at least another round of moderator optimization to arrive at a moderator performance tailored to the needs of the instruments.