PERSISTENT SURVEILLANCE FORPIPELINE PROTECTION AND THREAT INTERDICTION

Long term monitoring of DIII-D wall conditions following a boronization

W.P. Westa, M. Grothb, A.W. Hyatta, G.L. Jacksona, M.R. Wadea

And the DIII-D Team aGeneral Atomics

b LLNL

Presented at theDSOL ITPA Meeting

Toronto

November, 2006

Motivation: Reduce frequency of boronizations

• Each boronization cost manpower and run time – ~ 1 full day of plasma operation required to desorb embedded

helium from a fresh BZN film– Several person-days of engineer/technician time required

• Long pulse, high duty cycle, superconducting devices not amenable to frequent boronizations – Previous typical plasma-time between BZNs on DIII-D ~ 103 s.– On next generation tokamaks (East, K-Star, ITER) this is the order

of a fresh BZN between every (or a few) discharge.

• Advent of strong divertor pumping on DIII-D leads to indications that mean time between boronizations might be extended– BZN/3 weeks was like an apple a day– Little or no effects on plasma ops if a BZN was skipped.

Tools for evaluating need for BZN

• A daily reference shot (DRS) was devised to monitor long term changes in parameters that reflect wall conditions.– Spectroscopy

• Impurity influx/content• Zeff, Radiaited Power

– Gas Balance Measurements• Fueling• Exhaust• Recycling• Density Rise/Loss

• High performance discharges were repeated after a long campaign without BZN. – Fusion Gain (N*H89/q95

2)

– H89

– Neutron production

Daily reference shot (DRS) provides relevant data under consistent operating conditions

• Shot taken as first shot each morning– Also serves as a check on beam duct conditioning

• High LSN shape compatible with most commonly used f-coil patch panels – Not compatible with USN patch panel=>skip those

days• Three phases in shot

– L-mode during rampup and early flat-top– Slow beam power ramp until an L-H transition is

followed by an ELM-free period– Final period of increased beam power to induce

ELMing H-mode phase.• A total of 40 usable shots obtained over a 3

month operating period.

DRS L-mode database indicates a minor upward trend in core impurities

• Edge lines indicate no long term trend in influx at midplane

• CVI CX and NiXXV lines indicate some upward trend in core impurities

• Visible Bremsstrahung and O VIII CX show no long term trend

6800 Plasma-seconds between BZN events

DRS H-mode database shows no clear trend in edge or core impurities

• Edge lines indicate no long term trend in influx at midplane

• CVI CX and NiXXV lines: possible slight upward trend in core impurities

• Visible Bremsstrahung and O VIII CX show no long term trend

Advanced tokamak (AT) and hybrid discharges show no degradation in performance across 2006 campaign

• High performance hybrid and AT discharges were occasionally repeated throughout campaign.

• Discharges repeated just before and after last BZN.

• Just before last BZN, seven strongly-pumped hybrid discharges were sequentially repeated without helium glow discharge cleaning between each discharge.

• No degradation in performance (H89, G=NH89/q95

2, neutron production) was observed.

• Hybrid discharges were very repeatable without between shot helium glow.

Up to 5800 plasma-seconds since last BZN, AT discharges keep on performing

• Shot 126472 taken after 5800 plasma.seconds of operation

– 122 major disruptions since BZN on June 10th

• Shot 126763 taken after 320 plasma.seconds of operation

– Taken after BZN on September 16

• Performance very repeatable

AT fusion gain, carbon fraction, and neutron production constant across campaign

• These high AT discharges extrapolate well to the ITER Q=5 steady state scenario

• Nickel, a very minor player in contamination, increases with total operation time after BZN

Hybrid performance constant over several discharges with no between shot helium glow

• These performance levels are typical of hybrid shots throughout the 2006 campaign.

• Strong pumping is key to maintaining good graphite wall conditions (R. Maingi, et al., Fusion 36, (1996) 245)

Discussion: High performance more robust with graphite walls compared to high Z metal walls

• Tokamaks with metal walls require routine BZN for high performance– C-MOD with molybdenum walls (Lipschultz, PSI 2006)– AUG with mostly tungsten walls (Neu and Kallenbach, PSI

2006, Hefei)– Both cases routine boronizations are required to reduce high

Z contamination and associated high radiated power in attempts to produce high performance discharges.

• DIII-D used mostly beam heating, C-Mod mostly RF heating– On C-Mod parasitic effects of RF power used for plasma

heating is found to be a source of plasma contamination by Mo.

– On AUG, similar effects are seen, yet central RF heating also has been used to reduce W contamination of the core plasma.

Conclusions

• In DIII-D with an all graphite wall and strong divertor pumping capability, we have demonstrated the ability to reproduce ITER relevant high-performance discharges over 6000 plasma-seconds of operation with no intervening boronizations or bakes.

• Over a short period (~ 50 plasma-seconds) the ability to maintain hybrid operation without between shot helium glow discharge cleaning has been demonstrated.

• Multi-phase daily reference shots are useful in assessing long term trends in wall outgassing/pumping and wall impurity sources.

DRS provides data on wall release of deuterium and impurity line emission

• More wall release means less gas input needed to reach programed density

• More wall release tends to increase dne/dt in ELM free phase and increase density at first ELM

• More wall release tends to increase pumping plenum pressure early in shot

DRS wall source indicators shown no strong trend throughout 2006 campaign

• Net density rise and peak rate of rise constant across campaign

• Caveat: Particle balance may be dominated by large gas input during l-mode phase

• ELMing H-mode radiated power and PRAD relatively constant throughout campaign

Hybrid performance constant throughout campaign and without between shot glow

N, H89, and fusion gain maintained over 5800 plasma seconds without BZN

• Performance and density maintained over seven sequential shots with no between shot helium glow conditioning

Spectroscopic and performance data from many shots used to assess effect of wall conditions

• Data from every plasma shot has a lot of scatter– 1600 discharges (8000 plasma-seconds) with a wide

variety of operating parameters

• Daily reference shot data shows specific long term trends under well controlled conditions– 40 discharges with repeated operating parameters

• Reproduction of specific high performance discharges – Advanced tokamak and hybrid discharges with high N

and high H-factor chosen for demonstration.

• 3 months, ~1400 discharges, 6800 plasma-seconds between BZNs.