LH2 Absorber Design

Mary Anne Cummings

MICE Safety ReviewLBL

Dec 9, 2003

Hydrogen Absorber Design Principles

Muons can focus going through material! Absorber must handle significant head loads and maintain uniform density

and temperature (huge for muon colliders!) Beam heating must be minimized (high LR material in low )

Convection-driven liquid hydrogen absorber Internal heat exchange, no external hydrogen loop Need to monitor temperature, pressure and LH2 level

Thin window development Non-standard designs to minimize central thickness Must be sufficiently strong and have robust structural attachments

Location in center of high solenoidal magnetic field Concerns for quench forces on structural stability Heat stresses on windows below elastic tolerance

Additionally..

RF between LH2 absorbers to restore forward momentum:

The above goals drive the design of the absorber, its support structures, and placement in the cooling cell:

Liquid Hydrogen Safety

For safe operation, have designed with the redundant requirements:1)LH2 and O2 separation2)The avoidance of any ignition sources in contact with hydrogen.

The four key features of the design with respect to safety are:1)Window thicknesses specified based on safety factors of 4.0 for the

absorber and vacuum windows at maximum allowable working pressure (MAWP). Vacuum windows required to withstand 25 psi outside pressure without buckling.

2)Three layers of shielding between the outside atmosphere and the LH2; the outer surfaces are at room temperature to minimize the freezing of O2 on the absorber-system windows.

3)Separate vacuum volumes provided for the RF cavities, magnets, and LH2 absorbers.

4)Hydrogen evacuation systems using valved vents into external buffer tanks.

x x

z zP1

P2

LH2accelerator accelerator

LH2

P1

Multiple scattering

RF cavity RF cavity

dE/dx

+

Accommodating LH2

1. LH2-Air flammability limits: 4-75% ; detonability limits 18-59% conventional seals and vacuum vessels can provide sufficient shield between them

2. Sufficient clearance for LH2 venting into evacuation tanks (21 liters liquid 16548 liters at STP)

3. RF & Vacuum windows considered a “hazard/safe” barrier.4. All safety interlocks mechanical – based on expeditious venting of LH25. Two sets of windows, cooling channel containment (with Ar gas seals

at all flanges)

RAL safe LH2 operation

ISIS style safety scheme

Absorber details

LH2 Volume (at 20K) 21 litersAbsorber Vacuum volume 91 litersLH2 operating temp. 20.8KLH2 operating pressure 1.2 barsAllowable pressure range 1.05 – 1.7 bars Helium inlet temperature 14K

LH2

Absorber Vacuum Volume: “Dome”

“Magnet bore”

Absorber Window:

Vacuum Window:

He outlet:

He inlet:

He heat exchange

LH2

MLI

Ar

Ar

Absorber coil systemAmbient temperature on vacuum shields and outer channel wallQuench force on windows is smallStatic and quench forces are decoupled from the LH2 absorberHeat from quench and static sources are insufficient to cause boil-offClearance for possible LH2 rupture into vacuum volume sufficient to prevent cascading window rupture

C-He OUTC-He IN

L-H2 IN/OUT

24-M4

4-M6

24-M8

GATE VALVE

MLI

Thin Windows Design Progression of window profiles:“tapered” (1) and “bellows” (2 & 3)

If thinnest point not at the center, where?

Thin window and heavy flange of one piece … many options for absorber attachment

Certification:No closed form relation between pressure and largest stress. Need to accurately confirm:• Window manufacture • Performance under pressure tests

Two radii of curvature may not be concentric - critical to confirm minimum thickness

2 bolted options:

Photogrammetric measurements

Strain gages ~ 20 “points”

Photogrammetry ~1000 points

CMM ~ 30 “points”

Non-contact method~ 1000 pts. measured simultaneouslyHigh accuracy for pressure tests and shape measurementsDetailed enough data to confirm FEA calculations for extreme performance situations, and to confirm window manufactured as designedPortable system

Solid Absorber Changeover

1) None of the shielding requirments for LH22) Ambient and beam heat deposition require no special cooling

requirements3) Can be mounted and secured in existing absorber shell4) Quench hazard?

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(basic) Barrier

Power+

Signal

Return

Ground

“safe circuit”

fuse

Absorber Instrumention

1. All electronics will have to conform to European safety standards w.r.t. flammable gas.

2. Current absorber readouts: Temperature probes Pressure gauges Level sensors

3. Can port through LH2 or vacuum: example: MDC Vacuum Products Corp

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UPS

PC w/16 chan ADC

FISO

Cryo(temp)

IRM

Barrier(s)

network

ACNET

Sealed Conduit(s)

Intrinsically safe signal conditionersand transmitters

power

Hazard Safe

Intrinsically safe

Concerns for absorber electronics: Power per channel Feedthroughs Seals

Basic barrier

Wire+shielding concerns:Two twisted pairs – not grounded, details depend on overall MICE grounding schemeCommon mode surges due to magnetsNoise/sensitivity issues

Experiment certification

Windows certification done separatelyCertification of windows-absorber assemblyEstablish correct placement of absorber inside coilPressurization tests of absorber/coil vacuum systemNon-LH2 cryogenic test of absorber/coil system and instrumentationAssembly into channel: final purge before hydrogen fill

LH2 absorber

Coil

Tranverse absorber/coil removal from channel

Exception Situations

Concern based on the potential hazards of LH2Fire:

Large range of flammable and detonable limits Burning velocity at STP: 265-325 cm/s – MACC’s time in 200

meter run: 28.5 s = 702 cm/s Minimum energy for ignition in air: 0.02 mJ

Explosion: detonation velocity (STP): 1.48-2.15 km/s Extreme cold – all the usual cryogenic hazards

Hazard analysis based system parameters:Pressure change

Leaks (rupture, seal failure, incorrect cryo. connections Plugs (frozen LH2) Overcooling/Undercooling of LH2

Temperature changeGas concentration

Safety failsafes based on 2 independent system failure