Design and operation of a novel divertor cryopumping system in Alcator C-Mod1

B. LaBombard, B. Beck, J. Bosco, R. Childs, D. Gwinn, J. Irby, R. Leccacorvi, S. Marazita,N. Mucic, S. Pierson, Y. Rokhman, P. Titus, R. Vieira, J. Zaks, A. Zhukovsky

Plasma Science and Fusion Center, M.I.T, Cambridge, MA 02139, USA

Download electronic copyof this poster: http://www.psfc.mit.edu/~labombard/APS07_Cryopump_Poster.pdf

1Supported by USDoE Coop. Agreement DE-FC02-99ER54512

Testing of Full-Scale Prototype Cryopump

Cryopump installation into Test Chamber

"Toroidal Loop" is assembled from three sections...

8 LHe and 4 LN2temperature sensors

... and "hung" from undersideof Test Chamber cover

Stainless steel baffles simulateupper divertor tiles and structures in C-Mod

'Dip-stick' measuresLHe level

Prototype Cryopump Test Results

Liquid Helium Tube Temperature (degrees K)

Liquid Nitrogen Shield Temperature (degrees K)

LiquidHeliumLevel (%)

ChamberPressure (mtorr)

2 mtorr readingduring H2 gas input

pressure riseduring regeneration

Cool-downto LHe

H2 Input Rate(torr-liters/s)

Total H2 Injected (torr-liters)

System PumpingSpeed, H2 (liters/s)

Time (minutes)300 2010

115120125130

5000

1000015000

0

0

100

2000

10

20300

20

40

0

80

40

50100150200

PlasmaShot

Warm-up andRegeneration

12,000 l/s

manually resetto zero priorto “shot”

Simulation of Multiple C-Mod Shot CyclesPrototype Test Chamber

- minimal LHe usage (2-3 liters/shot)

- 10,000 liters/s D2 High pumping speeds

- 12,000 liters/s H2

Excellent interface toC-Mod shot cycle

Efficient

Robust- survived up-to-air vacuum breach test with no damage to pump

- Cool-down/regen. ~8 min.cycle

4. Full-scale prototype tests3. Design

1031211027 EFIT: 1.000 Normalized Surface Heat Flux to Upper Cryo Tiles

0.45 0.50 0.55 0.60 0.65 0.70Major Radius (meters)

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

SecondarySeparatrix

-1

0

1

2

3

4

5

degr

ees

Field Line Angle

Non-beveled tiles Beveled tiles

Normalized Surface Heat Flux to Lower Divertor Tiles

0.00 0.05 0.10 0.15 0.20Arc Length (meters)

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Nor

mal

ized

Hea

t Flu

xN

orm

aliz

ed H

eat F

lux Normalized

Heat Flux

NormalizedHeat Flux

midplane λq = 3.0 mm

PrimarySeparatrix

-1

0

1

2

3

4

5

degr

ees

Field Line Angle

All beveled tiles

Reference: "AT-target plasma" - 1.0MA, 5.4 tesla DN discharge with 4.7 MW RF, steady H-mode, 3.5 mtorr upper div. pressure

Cryopump slots positionedclose to upper x-point

- Exploits flux expansion- Peak heat fluxes similar to lower divertor

Cryopump scheme will handlehigh-power USN discharges

- Allows maximal pumping speed to be utilized for physics experiments

0.50 0.55 0.60 0.65 0.70 0.75

0.30

0.35

0.40

0.45

0.50

0.55

0.60

B. LaBombard19-JUN-2006

C-Mod Cryopump Simulation - Case CH1

0.50 0.55 0.60 0.65 0.70 0.75Major Radius (m)

0.30

0.35

0.40

0.45

0.50

0.55

0.60

Verti

cal H

eigh

t (m

)

B. LaBombard19-JUN-2006

-0.05 0.0 0.05Toroidal Direction (m)

Atoms Launched=10000Trajectories Shown=300Atoms Leaked=883Atoms Pumped=431Atoms Ionized=8686Charge Exchange Events=8314CX Reflected into Slot=2626Pumped/Launched=0.043

Hits on Inner/Outer Connector=449Source= 2.64E+22 D atoms/s 373.1 torr-l/s D2

Baffle Width (meters)=0.063Profile Shot#1021002017:800 1MA:2MW:nebar= 2.5:H-modeEFIT: 1031211007:0.660 SSEP = -1.3 mm

Pumping= 1.14E+21 D atoms/s 16.08 torr-l/s D2Gas inventory = 8.5 torr-l D2

Performance estimate:

Input:Measured SOL profiles,Pump/tile geometry,EFIT equilibrium

Output:Atoms pumped perion incident on divertortile surface

Baffle

LSN H-mode withSSEP ~ -1.3 mm

Initial density e-foldingtime ~ 0.5 seconds

Heat Flux Handling of Upper ‘Flat-Plate’ Divertor Surface is Comparable to Lower ‘Shaped’ Divertor

3D Monte Carlo Modeling provided design guidance and rough estimate of pumping performance

5. Cryopump Installation

C-Mod Cryopump Data 25-may-2007 - 26-may-2007

-40 -20 00

20

40

60 Liquid Nitrogen Inlet Pressure (psi)

-40 -20 0-150-100-50050 Nitrogen Exhaust Temperature (degrees C)

-40 -20 000.20.40.60.811.2 Liquid Helium Inlet Valve Setting (turns)

-40 -20 00

500

1000 Helium Exhaust Flow

-40 -20 0-200

-100

0Helium Exhaust Temperature (degrees C)

-40 -20 00

50

100 Liquid Helium Dipstick Level (%)

-40 -20 0-1e-701e-72e-73e-7 Linear IGT Pressure (Torr)

First cool-down of pump to LN2 and LHe temperatures

Liquid heliumon ‘dip-stick’

C-Mod base pressure drops an order of magnitudeTime (minutes)

LN2 cool-down

LHe cool-down

C-Mod Cryopump Data 25-may-2007 - 26-may-2007

440 450 460 470 48000.20.40.60.811.2 Liquid Helium Inlet Valve Setting (turns)

440 450 460 470 4800

500

1000 Helium Exhaust Flow

440 450 460 470 4800

50

100Liquid Helium Dipstick Level (%)

440 450 460 470 4800

1e-6

2e-6

3e-6 Linear IGT Pressure (Torr)

440 450 460 470 4800

10

20

30

40 TC3 Pressure (mTorr)

Seamless integration of cryopump operation with shot cycle (operated in a ‘manual’ mode for these shots)

RegenerationCooldownPlasma

Time (minutes)

6. First Operation in C-Mod

In-vessel orbital welding andleak testing of cryogenic tubing

LHe inlet

LHe exhaust

CryopumpLN2 shield

Halo currentRogowski coils

Cryopump

Hanger

10 instrumented ‘shim plates’30 sectors: shelf, plates, posts, baffles720 Moly tiles, fasteners, keepers, ...Probes, rogowskis & gauge instrumentation, ...

Gas Baffle

Gas Cuffs(for laser access)

Pumping Slots

Periscope

LHe transfer line fabrication

Cryogenic System on Top of C-Mod

LHe Dewar500 L

PLCRack

LHe stepper control valve

heatexchanger LHe line

LN2 line

Fabrication & Pre-assembly

External Cryogenics & Control System

Heliumtube

Inner LN2shield

Outer LN2shield

~1/30 Sector of Cryopump & Tile Structure

Installation of cryogenic loopin upper chamber

Finished installation of molybdenumtiles, support plates & baffles

Close-up of slot, gas baffle and in-vessel Penning gauge

Penninggauges

Gas BafflePumping Slot

Remote PLC Control Interface

C

G

D

C

G

D

C

G

D

C

G

D

C

G

D

- Guided by a series of passive pumping experiments, design evolved into a novel system of 30 'pumping slots'

- Pumping throughput insensitive to radial location of upper separatrix

Axisymmetric Concept “Pumping Slot” Concept

- Pumping throughput controlled dynamically via secondary separatrix

- Design accommodates vertical port laser access to Rmin= 0.62 m (Thomson, TCI, PCI,...)

=> Increased pumping

- Exploit SOL transport physics (large particle flux e-folding): Heat exhaust to lower div., pumping in upper div., employing near double-null discharges

1. ConceptC-Mod’s novel cryopump concept is driven by key challenges:

(1) Build an efficient, large-area cryopump in a C-Mod divertor with minimal impact on diagnostics and port usage

(2) Pump hydrogen (4.6 K surface temperatures)

(3) Minimize helium usage and cost/complexity of external cryogenic support systems

Ideas- Employ a full toroidal-loop cryogenic system => but this requires using the upper divertor!

- Use ‘pumping slot’ concept instead of axisymmetric design Improved particle throughput Reduced heat fluxes via poloidal flux expansion Diagnostic access aligned with pumping slots

2. Cryopump Project GoalsFull toroidal cryogenic loop in upper divertor chamber - one vertical port for feed lines

Upper divertor tiles/baffles arrangedfor optimal particle pumping -system of 30 'pumping slots'

All-welded design - in-vessel assembly using robotic 'orbital welder'

'Duplicate' of C-Mod pump fullyinstrumented and tested off-line in adedicated vacuum chamber

Liquid helium 'pot' design (4.2 K), capable of pumping hydrogen (4.6 K design target)

> 8,000 liters/s system pumping speedfor room temperature D2

Minimal interruption of line-of-sight access through C-Mod’s vertical ports

Divertor, tiles and cryogenic support structures designed to with-stand eddy current and halo current loads corresponding to a full-performance 2.0 MA (plasma current), 9 tesla (toroidal field) upward-going plasma disruption with a total halo current of 0.5 MA; detailed analysis to be performed with the aid of an ANSYS model of the elec-tromagnetic transients associated with a C-Mod disruption [1]

Rogowski coils embedded into the tile support structures to monitor halo current levels

Simple Monte Carlo modeling combined with C-Mod operational expe-rience to be used to optimize design

Gas pressure gauges (Penning and baratron type) and an embedded Langmuir probe array to measure divertor neutral pressures and plasma fluxes to the target plates

[1] Titus, P., et. al, "C-MOD cryopump design and analysis," presented at the 21st IEEE/NPSS Symposium on Fusion Engineering, Knoxville, TN, USA, 2005.

Test fit-up of full toroidal loopEnd view of 1/3 loop section

Aggressive puff/pump scenariosdemonstrated

Excellent L-mode density control at moderate densities

SS fueling at ~140 torr-l/s20,000 l/s pumping speed at pump entrance... may be important tool for H/D control during startup

We now can produce steady-statedischarges with continuous,non-recycling impurity injection

Example: Steady Ar injection (MP#502)

Shot# 1070830032Plasma Current (MA)

0.00.20.40.60.81.0

Line Density

05.0•1019

1.0•1020

ICRF Power (MW)

01234

Argon Gas Puff

026

10

Radiated Power (MW)

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.40.00.20.61.0

seconds

10705250260.0 0.5 1.0 1.5 2.0seconds

0.0 0.5 1.0 1.5 2.0seconds

0.20.40.60.81.0

1020

m-2

NL04

0.0 0.5 1.0 1.5 2.0seconds

0.0 0.5 1.0 1.5 2.0seconds

100

200

Torr-

l/s

Injection Rate

0.0 0.5 1.0 1.5 2.0seconds

0.0 0.5 1.0 1.5 2.0seconds

2468

mto

rr

UDIV Pressure

0.0 0.5 1.0 1.5 2.0seconds

0

Effectiveness of cryopump for density control is determinedby plasma transport physics

Particle removal ratedrops dramatically asdivertor changes fromhigh to low-recyclingregime

0.0 0.5 1.0 1.5 2.0seconds0.0

0.5

1.0

MA Plasma Current

0.0 0.5 1.0 1.5 2.0seconds0.00.20.40.60.81.01.2

1020

m-2

Line Density

0.0 0.5 1.0 1.5 2.0seconds

050

100150

Torr-

l/s

Gas Injection Rate

0.0 0.5 1.0 1.5 2.0seconds

0.0 0.5 1.0 1.5 2.0seconds010

20

mto

rr

Upper Divertor Pressure

0.0 0.5 1.0 1.5 2.0seconds02

46

amps

Divertor Probe Flux

0.0 0.5 1.0 1.5 2.0seconds

Pump on

Pump off

C-Mod’s intrinsicwall-pumping takesover role of ‘density control’at the lowest densities

Prompt L-H density riseset by pedestal formation

Cryopump does not alter thisphysics (nor does strong puffing!)

=> evidence of ‘stiff profiles’ and controlling ‘critical gradients’

Cryopump impacts pedestal densities over long time-scales (~0.5 s)

1070601030 & 290.0 0.5 1.0 1.5 2.0seconds

0.0 0.5 1.0 1.5 2.0seconds0.00.51.01.52.0

1020

m-2

Line Density

0.0 0.5 1.0 1.5 2.0seconds

0.0 0.5 1.0 1.5 2.0seconds

0

10

mm

Upper Null

0.0 0.5 1.0 1.5 2.0seconds

L-mode reference plasma

L-H density rise

Lower Null X-point Balance

Cryopump is found to affect the toroidal distribution of neutrals

“G-Side” pressure is LOWER“CB-Side” pressure is HIGHER

Puffing with cryopump ON...

This effect has been found to aid operationof J-port ICRF antenna

...because gas injection isat “B-Side”

A

B

C

D

E F

G

H

J

KB

G

CB

0.0 0.5 1.0 1.5 2.0seconds

0.0 0.5 1.0 1.5 2.0seconds0.00.4

0.81.2

1020

m-2

Line Density

0.0 0.5 1.0 1.5 2.0seconds

050

100150

Torr

-l/s Gas Injection Rate

0.0 0.5 1.0 1.5 2.0seconds

0.0 0.5 1.0 1.5 2.0seconds0.00.2

0.40.6

mto

rr

‘G-Side’ Neutral Pressure

0.0 0.5 1.0 1.5 2.0seconds

0.00.5

1.01.5

mto

rr

‘CB-Side’ Neutral Pressure

Pump on

Pump off

“G-Side”

“CB-Side”

“B-Side” Gas Injection

Cryopump has also aided operations inreducing base pressure (’fixing leaks’)

1070525018 & 21

0.0 0.5 1.0 1.5 2.0seconds0.0

0.5

1.0

MA

Plasma Current 18

0.0

0.5

1.0Plasma Current 21

0.0 0.5 1.0 1.5 2.0seconds0.00.20.40.60.81.01.2

1020

m-2

NL04 18

0.00.20.40.60.81.01.2NL04 21

0.0 0.5 1.0 1.5 2.0seconds-500

50100150

Torr-

l/s

Injection Rate 18

-50050100150Injection Rate 21

0.0 0.5 1.0 1.5 2.0seconds12

14

16

mm

SSEP 18

12

14

16SSEP 21

0.0 0.5 1.0 1.5 2.0seconds010

2030

mto

rr

UDIV Pressure 18

010

2030UDIV Pressure 21

0.0 0.5 1.0 1.5 2.0seconds02

46

amps

UDIV P8 18

02

46UDIV P8 21

0.0 0.5 1.0 1.5 2.0seconds0

1

2

amps

UDIV P10 18

0

1

2UDIV P10 21

0.0 0.5 1.0 1.5 2.0seconds

0.0

0.5

1.0

amps

UDIV P11 18

0.0

0.5

1.0UDIV P11 21

C

G

D

Upper DivertorProbe Array

1 5 9 14

Cryopump Operation Does NOT Change Relationship Between Upper Divertor Plasma Conditions and Machine Parameters (Ip, ne,...)

Black Traces = Pump ONColor Traces = Pump Off

At same line density with pump OFF/ON: - Upper divertor Ion saturation currents are identical

Ion Saturation Currents

7. Performance Tests

20

40

60

Torr

-l/s D2 Injection Rate

0

2mto

rr

Torus Pressure

0 1 2 3 4seconds

0

5.0•103

1.0•1041.5•104

liter

s/s

System Pumping Speed

Pumping speed = 9.62E+03 liters/s

- Chamber ~ 3.5 mtorr

- Pumping speed ~ 9,600 liters/s

Cryopump Easily Exceeds D2 System Pumping Speed Target (> 8,000 l/s)

- D2 injection at ~35 torr-l/s

Pumping slot/baffles work as plannedUSN upper divertor pressures ~ LSN lower divertor pressures

- However, divertor pressures and pumping throughputs are sensitive to magnetic field direction

H-mode Response=> Upper pumping slots are just as effective in ‘compressing’ neutrals as C-Mod’s closed lower divertor structure

Lower Divertor Pressure

Plasma Current

NL04

Upper Divertor Pressure

Reversed Field Direction

0.0

0.5

1.0

MA

0.00.40.8

1020 m-2

NL04 21

05

10

mto

rr

05

10

mto

rr

0.0 0.5 1.0 1.5 2.0seconds

Upper Null - RedLower Null - Blue

Similar neutral pressures

Forward Field Direction

0.0

0.5

1.0

MA Plasma Current

0.00.40.8

1020 m-2

NL04

01020

mto

rr Upper Divertor Pressure

1020

mto

rr Lower Divertor Pressure

0.0 0.5 1.0 1.5 2.0seconds

Similar neutral pressures

1070720021 & 7200021070525013 & 525006

Embedded Langmuir Probe Array

Full toroidal loop in upper divertor, maximizing pumping speed

Gravity-feed liquid helium 'pot' design, capable of pumping hydrogen (4.6 K surfaces)

Minimal LHe usageNo LHe recirculation system

9,600 liters/s pumping speed demonstrated for D2

Optimized tile surface geometry for high-power USN dischargesUnique toroidal array of ‘pumping slots’ and gas baffles

=> In-vessel inspection: no significant melt damage occurred during 2007 run campaign, which had many USN discharges, ICRF > 3 MW

Embedded halo-current rogowski instrumentationTiles and structure designed for 2.0 MA, 9 tesla disruption

Embedded Langmuir probes for particle flux optimization

60-90 liters of LHe consumedduring full C-Mod run day

8. SummaryA novel upper divertor cryopumping system has been successfullycommissioned in Alcator C-Mod

Excellent density control in moderate density plasmasSteady-state puff & pump throughputs of ~140 torr-l/s demonstrated

Toroidal slot geometry ‘compresses’ neutrals effectivelyStatic neutral pressures similar to those attained in closed lower divertor

Dynamic control of pumping throughput via secondary separatrix

=> Additional experiments revealed effect to be caused by in/out divertor asymmetries

‘jog’ in x-point balanceinitiates pumping