Design, installation and commissioning of the upper divertor cryopump …library.psfc.mit.edu/catalog/reports/2000/08rr/08rr006/... · 2008. 6. 3. · “Commissioning of the upper

Plasma Science and Fusion Center Massachusetts Institute of Technology

Cambridge MA 02139 USA This work was supported by the U.S. Department of Energy, Grant No. DE-FC02-99ER54512. Reproduction, translation, publication, use and disposal, in whole or in part, by or for the United States government is permitted.

PSFC/RR-08-6

Design, installation and commissioning of the upper divertor cryopump system in Alcator C-Mod

B. LaBombard, A. Zhukovsky, J. Zaks, R. Vieira, P. Titus, Y.

Rokman, S. Pierson, N. Mucic, S. Marazita, R. Leccacorvi, J. Irby,

D. Gwinn, R. Granetz, R. Childs, J. Bosco, B. Beck

1 _______________________________________________________________________

“Commissioning of the upper diveror cryopump system in Alcator C-Mod”, B. LaBombard, et al.

Design, installation and commissioning of the upper divertor cryopump system in Alcator C-Mod

B. LaBombard, A. Zhukovsky, J. Zaks, R. Vieira, P. Titus,

Y. Rokhman, S. Pierson, N. Mucic, S. Marazita, R. Leccacorvi, J. Irby, D. Gwinn, R. Granetz, R. Childs, J. Bosco, B. Beck

September, 2007 Over the past year, a new cryopump system in Alcator C-Mod’s upper divertor has

been successfully installed, operated and used as a research tool for initial investigations

on particle and density control. This report summarizes the activities that have led to the

completion of a C-Mod project milestone in FY07, “Investigate particle and density

control with the new upper divertor cryopump”.

1. Project overview 1.1 Design concept

The design of C-Mod’s upper divertor cryopump system is unique among the

world’s tokamaks, employing an array of gas-pumping slots that penetrate the upper

divertor target plate (see Fig.1). This geometry allows a single toroidal loop of liquid

helium to be used, greatly simplifying the cryogenic feed lines and providing the largest-

possible, continuous cryogenic surface inside a C-Mod divertor. The geometry also allows

the pump to be operated in an efficient low-flow or no-flow ‘quasi pool-boiling’ regime for

liquid helium. The toroidal loop (a 25.4 mm diameter inconel pipe with 0.25 mm wall) acts

as a helium vapor separator; it is divided internally into a series of compartments and is

only partially filled with helium, allowing helium to boil at atmospheric pressure with the

resulting vapor exiting via an exhaust tube. Under conditions of forced helium flow, the

compartments increase flow turbulence, wetting a larger internal perimeter of the pipe and

further enhancing the pumping speed (~10% increase). Bubble boiling of helium near

atmosphere pressures (4.2 K) is found to be effective in keeping a large fraction of the

cryopump surfaces to under the target value of 4.6 K. A superconducting level meter (‘dip-

stick’) is inserted into the exhaust tube, indicating the height of helium liquid in the pipe.

The helium pipe is surrounded by nitrogen-cooled and room-temperature surfaces,

shielding the pipe from ambient radiation, which includes room-temperature blackbody

radiation and plasma light. Molybdenum tiles protect the cryogenic structures from plasma

2 _______________________________________________________________________


contact. These tiles are arranged to provide an array of 30 ‘pumping slots’. A portion of the

neutral particle flux that is generated at the molybdenum tile surface undergoes charge-

exchange collisions with the plasma and scatters into the ‘gas box’ that surrounds the

cryogenic tubes (see Monte Carlo modeling of particle trajectories in Fig. 2). By varying

the mapping of the outer magnetic flux surfaces onto the tile surfaces, dynamic control of

the gas-pumping throughput can be achieved (see Fig. 3).

Figure 1 – Upper divertor cryopumping system concept in Alcator C-Mod. A portion of the neutral particles formed at the upper divertor target plate undergo charge-exchange collisions with the ambient plasma, causing them to pass through ‘pumping slots’ and enter into the ductwork surrounding the cryogenic surfaces.

3 _______________________________________________________________________


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. LaBombard

19-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

Ver

tical

Hei

ght (

m)

B. LaBombard

19-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 D2

Gas inventory = 8.5 torr-l D2

Baffle

Figure 2 – Example of neutral particle trajectories (red lines) from a 3-D Monte Carlo code used to simulate the operation of the pump. The results from this numerical simulation were used to optimize the location and size of neutral baffles located at the large major radius side of the pumping slot (green).

1050303004 EFIT: 1.000 1050303023 EFIT: 1.000 1050303003 EFIT: 1.000

Figure 3 – The cryopump’s particle throughput can be changed dynamically during a discharge by varying the plasma flux that arrives on the upper divertor target. For example, varying the magnetic equilibrium from lower-single null dominant (left) to near-balanced double-null (right) increases the particle fluxes directed to the pumping slot tiles.

4 _______________________________________________________________________


The molybdenum tiles that form the divertor target plate and pumping slots are located

just above the nominal location of the upper x-point. This placement leads to significant

poloidal flux expansion at the divertor target, spreading the scrape-off layer heat fluxes

over a large surface area. Figure 4 shows a comparison of heat fluxes mapped to the upper

and lower divertor surfaces for a near-balanced double-null equilibrium. The power

handling of the upper divertor tiles is found to be comparable or even better than that of the

lower divertor surfaces (‘better’ because there are no ‘beveled tiles’ at the location of the

upper strike point, which lead to additional heat-flux peaking factors). Thus, this integrated

divertor target and cryopump design allows the pumping system to be fully utilized for

high-performance plasma physics experiments, including those that employ upper single-

null plasma equilibria.

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

deg

rees

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

No

rmal

ized

Hea

t F

lux

No

rmal

ized

Hea

t F

lux Normalized

Heat Flux

NormalizedHeat Flux

midplane λq = 3.0 mm

PrimarySeparatrix

-1

0

1

2

3

4

5

deg

rees

Field Line Angle

All beveled tiles

Reference: "AT-target plasma" - 1.0MA, 5.4 tesla DN discharge

Figure 4 – Poloidal flux expansion reduces peak heat fluxes on the upper divertor cryopump tiles to a level similar to that of the lower divertor. This enables C-Mod to operate high-performance plasmas in an upper single-null configuration with full utilization of the cryopump.

5 _______________________________________________________________________


1.2 Design goals

The cryopump team targeted a number of key specifications for the new upper

divertor and cryopump system:

- Divertor, tiles and cryogenic support structures designed to withstand 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 electromagnetic transients associated with a C-Mod disruption [1].

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

levels

- A full toroidal cryogenic loop (360 degrees) located in the upper chamber, occupying

only one vertical port for cryogenic feed lines.

- Upper divertor tiles/baffles arranged for optimal particle removal, using a system of

30 ‘pumping slots’; simple Monte Carlo modeling combined with C-Mod

operational experience 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.

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

- All vacuum and cryogenic connections inside the C-Mod vessel to be welded;

welding to be performed in-vessel using a robotic orbital welder system.

- Employ a partially-filled liquid helium ‘pot’ design at atmospheric pressure (4.2 K

surfaces), capable of pumping hydrogen gas (4.6 K design target).

- Build a full-scale, fully-instrumented prototype pump using the planned in-vessel

fabrication methods; test the prototype off-line in a dedicated vacuum chamber,

assessing its pumping performance in a geometry that simulates the C-Mod

divertor/tile structure.

- System pumping speed target: A minimum of 8,000 liters/s for room temperature D2

gas in an otherwise empty C-Mod vacuum vessel.

6 _______________________________________________________________________


2. Full-scale prototype testing

The testing and evaluation program for the prototype pump covered four principal

areas of operation: cool-down characteristics including cryogenic flow control and

stability, deuterium and hydrogen pumping speed and saturation behavior,

pumping/regeneration cycling, and resiliency to an up-to-air vacuum breach. Figure 5

shows the installation of the prototype pump into a toroidally shaped vacuum chamber,

custom built for this purpose. The testing program was completed in the fall of 2006 as

preparations were being made for the fabrication and installation of the final version of the

C-Mod cryopump.

2.1 Cool-down characteristics

As stated above, the cryopump scheme employs a unique ‘pot’ design with multiple

compartments for holding liquid helium, a helium inlet line (liquid) and a helium exhaust

line (vapor) with a helium level sensor (‘dip-stick’). This arrangement was found to exhibit

no cryogenic flow instabilities (helium or nitrogen). Cool-down to operating temperature

was accomplished simply by opening the cryogenic values. Improvements the LN2 lines

were identified and later implemented on the final version of the pump, including larger

diameter pipes (0.375” i.d.) fastened with a nearly continuous weld bead.

7 _______________________________________________________________________


Figure 5 – Arrangement for testing the Alcator C-Mod prototype cryopump.

2.2 Deuterium and hydrogen pumping benchmarks

Stainless steel baffles were installed in the test chamber to simulate the divertor tile

structure in C-Mod (Fig.5). Pumping speeds were recorded for room temperature gases;

these became the benchmarks for the actual pump to attain in C-Mod. The overall system

pumping speed for room temperature deuterium and hydrogen was measured to be 9,700

and 12,000 liters/s respectively, under gas flow rates in the range of 25 to 50 torr-liters/s.

These numbers were found to be in-line with simple estimates based on gas conductances

and the active area of the cold surfaces. No saturation of pumping speed was observed up

to a tested inventory of 5,500 torr-liters of deuterium frost.

2.3 Pumping/regeneration cycling

A series of tests were performed to determine the best operational scenario for cool-

down, cryo-condensation and regeneration within the standard C-Mod shot cycle. Figure 6

8 _______________________________________________________________________


shows the behavior of the pumping system as a series of four ‘plasma discharges’ are

simulated. A simple operational sequence was identified, as illustrated by this figure:

1. Cool down to LN2 temperature before C-Mod run. This requires about 1 hour.

2. Start LHe flow (2 psi) just prior to a C-Mod shot (~7 minutes for first cool-down,

~2 minutes for subsequent).

3. The cryopump is ready for a plasma shot on the first indication of a LHe level

(>60%) on the 'dip stick'.

4. Take plasma shot while maintaining LHe flow.

5. Right after shot, stop LHe flow and begin flow of warm He gas.

6. Regeneration is completed after about 4 minutes of He gas flow.

Proceed to step 2 for the next C-Mod shot.

Liquid Helium Tube Temperature (degrees K)

Liquid Nitrogen Shield Temperature (degrees K)

Liquid

Helium

Level (%)

Chamber

Pressure (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

Pumping

Speed, H2

(liters/s)

Time (minutes)300 2010

115

120

125130

5000

10000

15000

0

0

100

200

0

10

20

300

20

40

0

80

40

50

100

150

200Simulation of Multiple C-Mod Shot Cycles using Prototype Cryopump Test Stand

PlasmaShot

Warm-up andRegeneration

12,000 l/s

manually resetto zero priorto “shot”

Figure 6 – Simulated pumping and regeneration cycles for a series of four C-Mod discharges.

9 _______________________________________________________________________


It was found that this cycle can be completed in about ~11 minutes for the first one and ~7

minutes for subsequent ones – an operational scenario that is ideally suited for the standard

C-Mod shot cycle (typically requiring a 15 to 20 minute time interval between successive

discharges). Pumping speed for H2 was reproducibly 12,000 liters/s. Only 2 to 4 liters of

LHe were consumed per simulated shot, which included a regeneration phase after each.

2.4 Up-to-air vacuum-breach test

For the final test on the prototype, an up-to-air accident was simulated. A 1-1/2”

diameter i.d. gate valve was promptly opened, exposing the test chamber to room

temperature atmosphere while the pump was filled with cryogenic liquids and operating

normally. Resultant overpressures in the range of 30 to 45 psi were detected on the LHe

exhaust line. Leak-checking revealed no evidence of any rupture or leaks in the nitrogen or

helium feed systems. Visual inspection also revealed no evidence of distortion to the pump

surfaces. Bellows sections that are used to connect among nitrogen and helium feed lines

were found to be unaffected.

3. Upper divertor and cryopump installation

Installation of the upper divertor tile structure and cryopump occurred over the time

period of January 2007 to April 2007. Figure 7 shows the process of in-vessel orbital

welding and an initial fit-up of the cryopump in the upper chamber, highlighting the

installed halo-current rogowski coils and the ‘hangers’ that are used to support the

cryopump body. This was a period of intense activity inside the C-Mod vacuum vessel,

involving not only the installation of the cryogenics system but also the installation of 30

sectors of ‘shelf plates’, ‘posts’ and baffles as well as 720 molybdenum tiles and associated

hardware. A view of the final installation can be seen in Fig. 8.

10 _______________________________________________________________________


Test fit-up in upper chamber

Halo currentRogowski coils

Cryopump

Hanger

Completed welding of LHe lines

LHe inlet

LHe exhaust

CryopumpLN2 shield

Figure 7 – Welding and installation of the cryopump body into the upper chamber of C-Mod.

Gas Baffle

Gas Cuffs(for laser access)

Pumping Slots

Periscope

Figure 8 – Final installation of tile support plates, molybdenum tiles, gas cuffs and gas baffles. As shown in figure 9, the external components for the cryopump system consist

Penninggauges

11 _______________________________________________________________________


primarily of a 500 liter liquid helium dewar, vacuum-jacketed transfer lines (nitrogen and

helium) as well as a variety of control valves, sensors and an instrumentation rack,

containing the PLC control system. The installation of this system was completed in early

May 2007, allowing remote operation of the system from the C-Mod control room in a

‘manual’ mode.

LHe Dewar

PLCRack

stepper control valve

heatexchanger

LHe line

LN2 line

Figure 9 – Layout of cryogenic components and control system, mounted on top of C-Mod’s radiation-shielding igloo. A stepping motor is used to remotely control the flow valve on the 500 liter liquid helium dewar.

4. Cryopump startup and initial particle pumping tests

The first cool-down of the cryopump system to liquid helium temperature was

attained on May 25, 2007. In response, the base pressure in C-Mod dropped by an order of

magnitude (see Fig. 10). The cool-down and regeneration response times of the C-Mod

pump were found to be similar to that of the prototype (Fig. 11), allowing the cryopump’s

operation to be seamlessly integrated into the standard shot cycle.

A series of dedicated experiments were begun to characterize the performance of the

cryopump in C-Mod, following the program outlined in mini-proposal #475, “Cryopump

Startup” [2]. Figure 12 shows the results from a pumping speed benchmark test. The C-

Mod pump was found to attain a system pumping speed very close to the prototype, ~9,600

liters/s for room temperature D2 gas.

12 _______________________________________________________________________


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 (standard liters per minute)

-40 -20 0-200

-100

0Helium Exhaust Temperature (degrees C)

-40 -20 00

50

100 Liquid Helium Dipstick Level (%)

-40 -20 0-1e-70

1e-7

2e-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

Figure 10 –First cool-down of both nitrogen and helium cryogenic loops in C-Mod.

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 (standard liters per minute)

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)

Figure 11 – Repeated cycles of cryopump cool-down, plasma operation and cryopump regeneration, synchronized with the standard C-Mod shot cycle.

13 _______________________________________________________________________


20

40

60

To

rr-l

/s

D2 Injection Rate

0

2mto

rr

Torus Pressure

0 1 2 3 4seconds

0

5.0•103

1.0•104

1.5•104

liter

s/s

System Pumping Speed

Pumping speed = 9.62E+03 liters/s

Figure 12 – Measurement of system pumping speed for room temperature D2 gas. The C-Mod pump was found to essentially reproduce the pumping speed benchmark set by the full-scale prototype unit, ~ 9,600 liters/s. The performance of the pump for particle removal and density control was found to

be excellent in moderate-density upper single-null (USN) discharges, as show in Fig. 13. A

steady state particle removal rate of ~140 torr-l/s is demonstrated in this discharge with the

line density held constant at the programmed value. In order to achieve this rate of gas

injection (which exceeds the maximum available from C-Mod’s pulse gas valve system) a

supplemental gas puff of ~50 torr-l/s from the NINJA gas injection system was required.

The typical helium consumption for a full day of operation was found to be 60 to 90 liters.

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

Tor

r-l/s

Injection Rate

0.0 0.5 1.0 1.5 2.0seconds

0.0 0.5 1.0 1.5 2.0seconds

2

4

6

8

mto

rr

UDIV Pressure

0.0 0.5 1.0 1.5 2.0seconds

0

Figure 13 – Demonstration of excellent particle throughput and density control in a standard, moderate density, upper single-null discharge. A steady-state D2 removal rate of ~140 torr-liters/s is maintained over the duration of the plasma flat-top.

14 _______________________________________________________________________


5. Density control physics experiments

Experiments were performed to examine the relationship between density control and

plasma phenomena associated with magnetic topology, magnetic field direction and

confinement regime, initially as part of MP#475 and then in subsequent mini-proposals

that were designed to exploit the cryopump for physics investigations (for example,

MP#485, “H-mode density control and pedestal response to cryopumping” [3] and

MP#506, “H-modes with cryopumping in USN, normal field direction” [4]). These

experiments were performed during the last three months of C-Mod’s FY07 run campaign

(June-August, 2007).

Static pressures in the upper and lower divertor were studied as a function of

magnetic topology (USN/LSN) with both forward and reversed field. As shown in Fig. 14,

the static pressures in the upper chamber in USN were found to be very similar to those in

the lower divertor for LSN for otherwise similar discharges (both forward and reversed

magnetic field). This was taken as evidence that the toroidal slot geometry of the upper

divertor was functioning as intended, providing a ‘neutral compression’ that was

comparable to that produced by the closed, toroidally-symmetric lower-divertor geometry.

In other words, our choice of placing a large, toroidal-loop cryopump in the upper divertor

was a good one: The upper divertor slot/baffle arrangement provides the same neutral

pressure as the lower divertor – a location where we cannot install such a large cryopump

system.

As seen in Fig.14, the neutral pressures in the ‘active divertor’ (i.e., lower divertor

for LSN and upper divertor for USN) are systematically lower with reversed magnetic field

operation. This result appears to be connected to some well known ‘divertor asymmetries’

effects that are present in C-Mod and other tokamaks and associated with the magnetic

field direction (ExB drifts are thought to play a role). Since the pumping slots in the upper

divertor are connected to the outer divertor leg only, the divertor neutral pressure is higher

when a high-recycling condition is achieved in the outer divertor leg. Experiments have

shown that this condition is favored with forward B-field direction and USN – precisely

the situation for which the cryopump was optimized. Apparently, because the toroidal

‘pumping slot’ in the lower divertor samples both the inner and outer divertor legs (via the

private flux region), high neutral pressures there depend on attaining a high-recycling

15 _______________________________________________________________________


regime in the inner divertor leg, a condition that is favored with forward field direction and

LSN topology. Thus, the experiments that were performed in support of MP#475 have

helped to expose some of these plasma physics subtleties and form a good basis for

planning future experiments.

Upper Null - RedLower Null - Blue

Forward Field Direction

0.0

0.5

1.0M

A Plasma Current

0.00.4

0.8

1020m

-2

NL04

010

20

mto

rr Upper Divertor Pressure

10

20

mto

rr Lower Divertor Pressure

0.0 0.5 1.0 1.5 2.0seconds

Similar neutral pressures

Lower Divertor Pressure

Plasma Current

NL04

Upper Divertor Pressure

Reversed Field Direction

0.0

0.5

1.0

MA

0.0

0.4

0.8

1020m

-2

NL04 21

05

10

mto

rr

05

10

mto

rr

0.0 0.5 1.0 1.5 2.0seconds

Similar neutral pressures

1070720021 & 720002

1070525013 & 525006

Figure 14 – Comparison of static neutral pressures (i.e., without pumping) in the upper and lower divertors for upper single-null (USN) and lower single-null (LSN) magnetic topologies in ohmic L-mode discharges. Top panel is with forward magnetic field and current while the bottom panel is with reversed. The upper divertor pressure in USN is found to be similar to the lower divertor in LSN – evidence that the pumping slot geometry is working as intended. Peak divertor pressures are about a factor of two lower with reversed field direction.

16 _______________________________________________________________________


As the plasma density is lowered, both inner and outer divertor legs change over to a

low-recycling regime and, as a result, the plasma fluxes to the divertor surfaces drop faster

than linear with plasma density. This ‘divertor physics effect’ limits the particle-pumping

throughput of the cryopump at low densities. Figure 15 shows a comparison of a pumped

and non-pumped USN discharge in which the gas fueling was turned off at 0.7 seconds. An

initial fast density decay is followed by a slower pump-out of the plasma density. The slow

pump-out phase occurs after the change over from high- to low-recycling on the divertor

plate (see drop in divertor probe flux on Fig. 15).

Despite the large gas throughputs that are generated by a combination of puffing and

pumping, the local divertor conditions (density, temperature, target flux) were found to

remain similar for otherwise similar core plasma conditions. Figure 15 illustrates this

point. For times between 0.5 and 0.7 seconds, the trace labeled “divertor probe flux” is

found to be unchanged in going from “pump off” to “pump on” (a change from zero to

~100 torr-l/s throughput). While this result may seem surprising at first, it is actually

consistent with our expectations since the cryopumping system removes, at best, only a

small fraction (~4%) of the incident plasma flux striking the divertor surfaces.

The physics of H-mode, with its prompt formation of a ‘density pedestal’ at the L-H

transition, also impacts the effectiveness of the cryopump for core plasma density control.

Figure 16 shows a comparison of and L-mode and an H-mode discharge, both with strong

cryopumping applied after 0.75 seconds. In these cases, the cryopump is effectively

‘turned on’ by programming a step change in plasma equilibrium from LSN to USN (see

“x-point balance” trace). The fast density rise associated with the first L-H transition (and

others) is seen to be much faster than the time scale for the pump to remove particles from

the vacuum vessel. However, the pump does gradually deplete the plasma density in both

discharges over a longer time scale. When the H-mode discharge momentarily transitions

back to L-mode, it has a density similar to that of the L-mode discharge. These results

indicate that despite the occurrence of transient L-H transitions, the overall particle pump

throughput is relatively insensitive to these events.

17 _______________________________________________________________________


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

0

50

100150

Tor

r-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

10

20

mto

rr

Upper Divertor Pressure

0.0 0.5 1.0 1.5 2.0seconds0

2

4

6

amps

Divertor Probe Flux

0.0 0.5 1.0 1.5 2.0seconds

Pump on

Pump off

Figure 15 – Comparison of a pumped and non-pumped discharge in which the gas fueling is turned off at 0.7 seconds. The prompt density decay after 0.7 seconds is followed by a slower ‘pump-out’ of the plasma density – a time when the divertor changes from a high to a low-recycling regime.

1070601030 & 29

0.0 0.5 1.0 1.5 2.0seconds

0.0 0.5 1.0 1.5 2.0seconds0.00.5

1.0

1.5

2.0

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 NullX-point Balance(EFIT SSEP parameter)

-10

Figure 16 – Comparison of pumped L- and H-mode plasmas. The magnetic topology is changed from lower-null dominant to upper-null dominant at 0.75 seconds, causing the upper divertor cryopump to ‘pump-out’ the discharge after that time. The cryopump’s particle removal rate cannot complete with the prompt density rise associated with an L-H transition. However, independent of confinement mode, the pump does provide a gradual ‘pump-out’ of the plasma density over a longer time scale.

18 _______________________________________________________________________


An interesting and unanticipated consequence of the upper divertor cryopump

operation is its effect on the toroidal distribution of neutrals in C-Mod. As shown in Fig.

17, gas is injected into the C-Mod torus at the location of “B-side”. With the cryopump

turned on, a large gas puff is needed to maintain a constant plasma density. Since the

cryopump exhausts the torus in a toroidally symmetric way, this combination of pumping

and puffing leads to a change in the toroidal distribution of neutral pressures. As shown in

Fig. 17, neutral pressures at the location of “G-side” decrease with strong puffing/pumping

while pressures at “CB-side” increase. This redistribution of neutrals was found to be

beneficial for the operation of the J-port ICRF antenna during this run campaign, due to an

RF break-down limit at high neutral pressures.

Finally, the cryopump has demonstrated its utility in producing controlled, steady-state

plasmas with a high radiation level, maintained by the continuous injection of a non-

recycling impurity gas. Figure 17 shows the time traces from an USN discharge in support

of MP#502 [5], which combined the tools of strong cryopumping, strong D2 gas puffing

and steady-state argon injection. This discharge represents a new research capability for C-

Mod – a direct consequence of the successful implementation of the upper divertor

cryopump system.

6. Summary

A new upper divertor cryopump system has been successfully installed and

operated in Alcator C-Mod. The system is a fully integrated divertor and cryopump design

that employs a number of unique features including a ‘pumping-slot’ style divertor target

and a full toroidal cryogenic loop that acts as a compartmentalized liquid helium ‘pot’ (or

as a turbulized pipe for the case of forced liquid helium flow). The system is found to

function as intended, providing C-Mod with a new high-throughput particle exhaust tool

(exceeding 9,000 liters/s for room temperature D2 gas) while minimizing helium

consumption and avoiding a complex external cryogenic system. Dedicated experiments

were performed during the FY07 run campaign to explore the capability of the pump for

particle exhaust and plasma density control as a function of magnetic topology, plasma

density and confinement regime. These experiments have provided valuable information,

demonstrating the pump’s utility for future physics experiments and for expanding the

research capabilities of Alcator C-Mod.

19 _______________________________________________________________________


The activities outlined in this report satisfy the Alcator C-Mod’s FY07 project

milestone, “Investigate particle and density control with the new upper divertor

cryopump”.

Acknowledgements

The first author wishes to thank the entire cryopump team for the Herculean effort that

was required to make this pumping system a reality in Alcator C-Mod. This work is

supported by U.S. D.o.E. Coop. Agreement DE-FC02-99ER54512.

A

B

C

D

E F

G

H

J

K

B

G

CB

0.0 0.5 1.0 1.5 2.0seconds

0.0 0.5 1.0 1.5 2.0seconds0.0

0.4

0.8

1.2

1020

m-2

Line Density

0.0 0.5 1.0 1.5 2.0seconds

0

50

100150

To

rr-l

/s

Gas Injection Rate

0.0 0.5 1.0 1.5 2.0seconds

0.0 0.5 1.0 1.5 2.0seconds0.0

0.2

0.4

0.6

mto

rr

‘G-Side’ Neutral Pressure

0.0 0.5 1.0 1.5 2.0seconds

0.0

0.5

1.0

1.5

mto

rr

‘CB-Side’ Neutral Pressure

Pump on

Pump off

“G-Side”

“CB-Side”

“B-Side” Gas Injection

Figure 17 – Strong cryopumping combined with gas-puffing from the “B-side” fueling valve leads to a change in the toroidal distribution of neutral pressures.

20 _______________________________________________________________________


Shot# 1070830032Plasma Current (MA)

0.00.20.40.60.81.0

Line Density

0

5.0•1019

1.0•1020

ICRF Power (MW)

01234

Argon Gas Puff

02

6

10

Radiated Power (MW)

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.40.00.2

0.6

1.0

seconds Figure 18 – Example of a steady-state, radiative plasma with continuous injection of argon. Such discharges represent a new research opportunity in C-Mod, made possible by the successful implementation of the upper divertor cryopump system.

References

[1] Titus, P., Zaks, J., Vieira, R., Gwinn, D., and Labombard, B., "C-MOD cryopump design

and analysis," presented at the 21st IEEE/NPSS Symposium on Fusion Engineering, Knoxville, TN, USA, 2007.

[2] 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, “Cryopump Startup”, Alcator C-Mod mini-proposal #475, http://www.psfc.mit.edu/research/alcator/program/cmod_runs.php

[3] J.W. Hughes, B. LaBombard, M. Greenwald, A. Hubbard, B. Lipschultz, J. Terry, D.

Whyte, W.L. Rowan, R. McDermott, K. Marr, I. Bespamyatnov, M. Reinke, “H-mode density control and pedestal response to cryopumping”, Alcator C-Mod mini-proposal #485, http://www.psfc.mit.edu/research/alcator/program/cmod_runs.php

[4] J.W. Hughes, B. LaBombard, M. Greenwald, A. Hubbard, B. Lipschultz, J. Terry, D.

Whyte, W.L. Rowan, R. McDermott, K. Marr, I. Bespamyatnov, M. Reinke, “H-modes with cryopumping in USN, normal field direction”, Alcator C-Mod mini-proposal#506, http://www.psfc.mit.edu/research/alcator/program/cmod_runs.php

[5] M. Reinke, A. Ince-Cushmann, J. Rice, B. Labombard, A. Graf B. Lipschultz and W.

Rowan, “HIREX-SR Calibration Plasmas”, Alcator C-Mod mini-proposal#502, http://www.psfc.mit.edu/research/alcator/program/cmod_runs.php

Documents

Design, installation and commissioning of the upper divertor cryopump …library.psfc.mit.edu/catalog/reports/2000/08rr/08rr006/... · 2008. 6. 3. · “Commissioning of the upper