1 Research and Development Towards a Radiation-Cooled Target For Mu2e Peter Loveridge 6th High Power Targetry Workshop April 2016

2 Outline  Introduction to the Mu2e radiation cooled target design Target Test Programme  Thermal emissivity measurements  Ultra-high temperature thermal fatigue test  Target oxidation tests at high temperature and low pressure  Research into potential Silicon Carbide target coatings

3 The Mu2e Experiment – Part of Fermilab Muon Campus Proton Target lives inside solenoid here 8GeV, 8kW Beamline Beam kinetic energy8 GeV Main Injector cycle time1.333 sec Number of protons per spill8 Tp Average Beam Current1 μA Average Beam Power8 kW Beam spot shapeGaussian Beam spot sizeσx = σy = 1 mm Target MaterialTungsten

4 Mu2e Civil Engineering, Late 2015 g-2 Building Beamline Enclosure Beam Dump Plant Room Target Hall Detector Hall

5 Radiation Cooled Proton Target Concept 6mm Diameter Tungsten target Mounting ring End “hub” Tie rod (spoke) Leaf Spring Tensioning mechanism Mounting / Handling Features

6 Radiation Cooled Proton Target Concept Technical Challenges:  Continuous high temperature target operation  Fatigue life under continuous thermal cycling  Oxidation / chemical attack by residual gasses in the target environment Drivers:  No coolant plant required. Eliminates costs associated with design, hardware, plant room space, maintenance, etc.  Eliminating the need for an active coolant greatly simplifies the remote target exchange process.  Eliminates the risk of coolant leaks.  Minimise material for pion production 6mm Diameter Tungsten target Mounting ring End “hub” Tie rod (spoke) Leaf Spring Tensioning mechanism Mounting / Handling Features

7 Radiation Cooled Proton Target Concept Technical Challenges:  Continuous high temperature target operation  Fatigue life under continuous thermal cycling  Oxidation / chemical attack by residual gasses in the target environment Drivers:  No coolant plant required. Eliminates costs associated with design, hardware, plant room space, maintenance, etc.  Eliminating the need for an active coolant greatly simplifies the remote target exchange process.  Eliminates the risk of coolant leaks.  Minimise material for pion production 6mm Diameter Tungsten target Mounting ring End “hub” Tie rod (spoke) Leaf Spring Tensioning mechanism Mounting / Handling Features Address via Target Test Programme…

8 Thermal Emissivity Measurements

9 Target Operating Temperature  Target heats up until it is able to dissipate the average deposited power by thermal radiation  Equilibrium temperature depends on heat load, emissivity and surface area. Equilibrium temperature distribution For a beam power of 7.7 kW, 560W is deposited as heat in the target (FLUKA) Recall Tungsten T melt = 3400°C

10 Total Hemispherical Emissivity Measurement Concept

11 Typical Measurements Temperature distribution along the tungsten tube Temperature dependent emissivity deduced View through the optical window Digital pyrometer mounted on vertical linear slide

12 Thermal Fatigue Lifetime Tests

13 Thermal Stress in the Target  The beam cycle causes transient thermal stresses in the target rod  Thermal stress generated by radial temperature gradients in the rod  When beam is “on” radial temperature gradient and thermal stress increase because heat deposition is biased towards the centre of the rod  When beam is off the heat spreads by thermal conduction and the thermal stress decreases  Tensile stress at the surface, compressive stress in the core  ~24 Million cycles per year of continuous running on a sec cycle time  1 year target life requirement Below: Von-Mises Stress at a Z slice in the target rod near to the shower-max Above: The Delivery Ring beam intensity as a function of time

14 How to mimic beam induced thermal stresses without using a proton beam?  Use a pulsed power supply to heat specially shaped tungsten samples in a vacuum environment  Mimic the transient thermal gradients in the target  Control current pulse intensity and repetition rate  Closely match the target dimension, operating temperature, pulse temperature rise and thermal stress cycle in an accelerated lifetime test A Novel Thermal Fatigue Test for Mu2e How to make the samples? “Turn and Burn” wire EDM process at RAL precision development facility

15 Calculated Stresses in the Sample Von-Mises stress distribution before (left) and after (right) a current pulse Sample stresses back calculated using ANSYS Sample temperature recorded using digital pyrometer

16 Lifetime Test The sample survived 100 million cycles under conditions designed to mimic Mu2e Target operation. Equivalent to 4 years continuous operation. A failure was then induced by running the PSU “flat out” for a further 37 million cycles. Mimic Mu2e target operation PSU “flat out” Peak Current1900 A2300 A Repetition Frequency 16 Hz11.5 Hz ‘mean’ operating temperature 1750 °C2000 °C Measured ΔT at surface 44 °C73 °C Cumulative Number of cycles 100 million137 million Failure?NoYes

17 Lifetime Test The sample survived 100 million cycles under conditions designed to mimic Mu2e Target operation. Equivalent to 4 years continuous operation. A failure was then induced by running the PSU “flat out” for a further 37 million cycles. Mimic Mu2e target operation PSU “flat out” Peak Current1900 A2300 A Repetition Frequency 16 Hz11.5 Hz ‘mean’ operating temperature 1750 °C2000 °C Measured ΔT at surface 44 °C73 °C Cumulative Number of cycles 100 million137 million Failure?NoYes Sample can withstand many times the lifetime number of pulses ✔

18 Oxidation Tests at High Temperature and Low Pressure

19 At temperatures exceeding ~1300°C in vacuum, tungsten oxide will evaporate faster than it is formed. In this regime oxidation is realised as a surface recession, the rate of which depends strongly on temperature and oxygen pressure. Vacuum / Leak Test Residual Gas Analyzer Turbo Pump Vacuum Gauge Leak Valve 0.5mm diameter tungsten wire Surface recession of initially cylindrical tungsten rods heated in a low oxygen pressure

20 Vacuum/Leak Test Results Total Pressure (Torr) Recession Rate (mm/year) 1×10 -6 Few Microns 1× ×

21 Research into Potential Silicon Carbide Target Coatings

22 Target Coating Research The objective of a target coating is to improve the heat-transfer and chemical resistance of the target while at the same time making use of the excellent high-temperature mechanical properties of a tungsten substrate. The “ideal” target coating would have the following properties:  High emissivity for enhanced radiation heat transfer  Excellent resistance to oxidation  Non-porous  High continuous use temperature  Good CTE match to substrate  Excellent radiation damage tolerance  Commercially available Identified chemical vapour deposited (CVD) silicon-carbide (SiC) as a potential coating technology.

23 SiC Emissivity SiC Coating Bare Tungsten Literature values for total emissivity CVD SiC coating on a tungsten substrate 25μm wide grooves laser machined into a tungsten surface Bare Tungsten subject to various machining processes Comparison of tungsten emissivity vs surface finish

24 SiC Emissivity SiC Coating Bare Tungsten Literature values for total emissivity CVD SiC coating on a tungsten substrate 25μm wide grooves laser machined into a tungsten surface Bare Tungsten subject to various machining processes Comparison of tungsten emissivity vs surface finish Potential to significantly reduce target temperature ✔

25 Thermo-gravimetric Analysis (TGA) TGA In-House Test At RAL 6mm diameter x 1mm thick tungsten disks Before CoatingAfter Coating CVD Silicon Carbide Coated tungsten samples under preparation  Test oxidation resistance up to 1000°C in air  Oxidation in bare tungsten takes off after 500°C  SiC coated sample shows little/no response

26 Thermo-gravimetric Analysis (TGA) TGA In-House Test At RAL 6mm diameter x 1mm thick tungsten disks Before CoatingAfter Coating CVD Silicon Carbide Coated tungsten samples under preparation  Test oxidation resistance up to 1000°C in air  Oxidation in bare tungsten takes off after 500°C  SiC coated sample shows little/no response Good oxidation protection in atmospheric pressure air ✔

27 SiC Coated Wire Preparation for Vacuum/Leak Test SEM image of a SiC coated wire prior to heating Close-up photograph of the 50 μm thick silicon-carbide coating applied to a 0.5 mm diameter drawn tungsten wire Sample preparation, note the coated central portion and bare ends Calculated temperature distribution along the 50mm test section

28 Vacuum/Leak Test of SiC Coated Wire SiC coated tungsten wire shortly before heating. The sample showed no degradation after heating to 1250°C for 3 weeks in “good” vacuum of mbar. BEFORE AFTER After 1 week at mbar air the coating had completely burned off exposing the tungsten substrate The vacuum quality was then reduced to mbar by opening the leak valve. After a few days white deposits became visible near the ends.

29 Vacuum/Leak Test of SiC Coated Wire SiC coated tungsten wire shortly before heating. The sample showed no degradation after heating to 1250°C for 3 weeks in “good” vacuum of mbar. BEFORE AFTER After 1 week at mbar air the coating had completely burned off exposing the tungsten substrate The vacuum quality was then reduced to mbar by opening the leak valve. After a few days white deposits became visible near the ends. Poor oxidation protection in low pressure air ✘

30 J. Am. Ceram. Soc. 59 [9-10], 441,(1976). Passive and Active Oxidation Modes of SiC  Ironically the SiC coating does not provide an effective oxidation barrier if the pressure is too low Not suitable for the expected Mu2e target condition mbar 1300°C

31 Summary  The baseline technology choice for the Mu2e production target is a radiation cooled tungsten rod mounted in a support structure that resembles a spoked wheel.  Target test programme to address specific technical risks associated with continuous high temperature target operation:  Thermal emissivity measurements  Ultra-high temperature thermal fatigue test  Target oxidation tests at high temperature and low pressure  Research into potential silicon carbide target coatings