Recent Results and Plans for the Advanced Tokamak Program

Transcription

1 Recent Results and Plans for the Advanced Tokamak Program Program Advisory Committee Review February 6-7, 2002 MIT PSFC Presented by A. Hubbard Outline Overview Results from 2001and plans for Internal Transport Barriers - Scenario modelling and target development. Long term plans ( )

2 Long-term goal is a non-inductive, high β, long pulse plasma. Main physics components: 1. Current profile control via LHCD, at reactor-relevant densities. 2. Understanding, control and sustainment of Internal Transport Barriers, with T e ~T i and without momentum input. 3. Use non-inductive current drive (LHCD and bootstrap) to extend pulse length to near steady state (5 sec, 4-6 τ CR ) - divertor power handling and wall particle issues. 4. Increase β to MHD limit, and maximize through profile optimization, possibly stabilization. Program involves all physics areas (RF, transport, divertor, MHD) and has broad participation from the team.

3 Long Term Advanced Tokamak Program CY ITB studies Target devel. LH coupling Start LHCD j Profile control t > τ CR 4 MW LH Multiple N // Fully non- inductive CD 5 sec pulse b N > 3 AT Demonstration Scenario LH Phase I X-ray spectr. Cryopump LH Phase II 4-strap ICRH Divertor upgrade? MHD stabilization?

4 Safety Factor - q(r) ACCOME modelling indicates noninductive regimes are feasible. Example of an optimized target scenario. I LH =240 ka, I BS =600 ka MHD stable, β n =3. pulse length 5 sec (cf τ CR = 1.2 secs at 6.5 kev). Ip = 0.86 MA Ilh = 0.24 MA fbs = 0.7 Pressure (J / m3) Bo = 4.0 T βt = 2.68% βn = 2.86 J (MA / m2) q(0) = 5.08 q min = 3.30 q(95) = 5.98 r / a r / a

5 Internal Transport Barrier studies were a major success of the 2001 campaign ITB s were routinely triggered by off-axis ICRH, at r/a ~0.5. Core barriers co-exist with edge pedestal (EDA H-mode.) Stable conditions were reached for ~15 τ E, through addition of modest on-axis ICRH. This also increased, and modestly peaked, core T e, T i ne (10 20 m-3) ITB ETB Major radius (m) S. Wukitch, APS invited talk PRF [MW] off-axis W p [MJ] n e [10 20 m -3 ] n e0 /n e R DD [10 13 sec -1 ] T i0 [kev] B T = 4.5 T Ip=0.8 MA central Time [sec] 1 0.5

6 Barriers now formed with heating on high or low field side. Scenarios tested: F= 80 MHz, B=4.5 T F= 70 MHz, B=3.8 or 5.4 T Same condition r res /a = in all cases. With LFS heating at 70 MHz, used 80 MHz core heating to stabilize barrier. V Tor (10 4 m/s) n e (0)/n e (0.7) Resonance Location (r/a) MHz Toroidal Rotation Density Peaking B T (T) J. Rice, to appear in Nuclear Fusion

7 TRANSP analysis confirmed an energy transport barrier T i profiles measured with X-ray doppler. n e (r) by bremsstrahlung, TS and interferometer. Strong decrease in χ eff as well as D when barrier forms. ohmic ITB +off-axis ITB+off-axis+central With central heating, core χ eff increases somewhat but is still less than without barrier. Can control barrier strength, avoid impurity accumulation as well as MHD limits. Also made local measurements of χ e decrease in barrier, using sawtooth heat pulses (note q 0 < 1)

8 Internal Transport Barrier Issues and Experiments Understanding ITBs with off-axis ICRH. What is the physics trigger? Is there hysteresis? Role of rotation? What are the profile, time behaviour of χ, D? R/L T variation, B T scans, heat and impurity pulses. New X-ray views will give V tor (r) more routinely. Barrier location control. What determines the barrier location (usually r/a~0.5)? Can bootstrap current be expanded for more attractive AT scenario? Does it respond to changes in magnetic shear? Current scan, fast current ramps. Ohmic transport barriers. Most ohmic EDA H-modes have peaked density profiles. Insight into trigger, barrier location? How does central RF affect transport? Central RF into ohmic ITBs. Improving performance. How can we maximize energy confinement, bootstrap current? Higher B (5.4 or 6.2 T), higher power ITBs.

9 Target Development and Scenario Modelling. Experimental target profiles New experiments, Optimized parameters Modelling w RF, LHCD Sensitivity studies. How should target be improved? Modelling is used to assess wave accessibility, damping, and CD efficiency, and guide target plasma development toward more optimal scenarios.

10 Case 1: Double Barrier Mode Density profile approximates an ITB discharge with off-axis ICRH. T e has been increased to 3 kev ACCOME predicts 60% bootstrap current (470 ka) - But, at radius smaller than optimum. ( I BS ~100 ka in actual discharge). Expts aim to expand barrier, increase T e. Electron Density (m-3) Bo = 4.5 T Ip = 0.79 MA Plh = 3.0 MW fbs = 0.60 Ilh = 0.11 MA βt = 1.96% βn = 2.55 J (MA / m2) Ioh = 0.20 MA Safety Factor - q(r) q(0) = qmin = q(95) = 6.85 r / a

11 Case 2: Current ramp, low density L-Mode. Based on an expt with T e0 > 5 kev, 1.8 MW ICRH. Model predicts I LH = 390 ka, strong shear reversal. Lower n e cases gave I LH up to 660 ka. Planned Expt: Try early off-axis ICRH. Will we get an ITB?

12 Electron Density (m-3) Case 3: EDA H-mode Actual density profiles, n e,ped ~ 2x10 20 m -3. Low ICRH power, T e0. LH waves are accessible. I LH smaller (~80 ka) some rays damp near axis; modelling still in progress. Bo = 5.4 T βt = 0.67 % βn = 1.07 Modelling Plans : Vary launched N // to optimize CD. Planned experiments: - Raise T e (higher ICRH) Improve off-axis wave damping, CD efficiency. - Reduce density Increase driven current.

13 Near Term Plans for Modelling RF Sensitivity studies of LHCD current profile using ACCOME (vary n e, B, T e, N // ) (Collaboration with R. Dumont, PPPL) 2-D (V perp, V // ) Fokker Planck simulations of LHCD (R.W. Harvey CompX, P. Bonoli) - preliminary results show increased CD LHCD efficiency, distribution simulations, X-ray diagnostic design (Y. Peysson, Cadarache, A. Bers, J. Decker, MIT). Full Wave simulations of LHCD (1-D) and IBW (3-D) (R. Dumont, and C.K. Phillips, PPPL, P. Bonoli). TRANSPORT Couple current drive and transport modelling using TRANSP in predictive mode (building on Bonoli s previous work). Use χs obtained from analysis of ITBs, simplified criteria for barrier formation. (MIT grad students, with PPPL support). Gyrokinetic analysis (GS2) of existing ITB discharges. (M. Redi, PPPL) MHD Low n and ballooning stability analysis of modeled scenarios with PEST-2, Keldysh code and MARS (J. Ramos, MIT PSFC) Explore opportunities for collaboration with AT modelling, theory effort at GA.

14 Longer Term Plans for Modelling Full Wave LH simulations in 2-D (R. Dumont and C.K. Phillips, PPPL, P. Bonoli, part of SciDac effort) Use evolving capabilities of TRANSP for more theory-based predictive modelling. Eg. assessing ω ExB vs γ ITG. Develop and incorporate improved particle transport modelling (critical for ITB simulations).

15 Engineering Support of AT program LHCD preparation activities have progressed very well, both at MIT and PPPL. On schedule for March 2003 delivery of first launcher. Details in talks by Ron Parker, Jim Irby. Long pulse tests demonstrated operation for 3 seconds 5 T, 800 ka. L and H-mode discharges, with ICRF. Low densities were maintained. ~ 4x10 19 m -3 (L), 2x10 20 m -3 (H) T e0 ~ 2 kev t pulse ~ 15 τ CR (would be ~ 4 τ CR at 5 kev). Details by Jim Irby.

16 Other Planned AT Preparation Experiments. Long term schedule requires cryopump design information be obtained this year. Top priority. Will cryopump in top of vessel give good pumping? Run unbalanced double null plasmas. Upper Cyropump Secondary Scrape-off Layer Investigate current sustainment by bootstrap. can already run pulses > τ cr. Propose using high power RF, switching off ohmic feedback to see how much current is sustained, for how long. (Perkins) Lower Cyropump

17 Experiments deferred to late 2002 Even with the high priority given to AT experiments, we cannot carry out all good proposals in the 8-week summer 2002 campaign. Some experiments for late campaign: Long pulse demonstrations at higher RF power, density. Other proposed ITB physics investigations. Thorough exploration of high T e startup scenarios with early RF. Further iteration on LH target development, following modelling of summer experiments.

18 Long Term Advanced Tokamak Program CY ITB studies Target devel. LH coupling Start LHCD j Profile control t > τ CR 4 MW LH Multiple N // Fully non- inductive CD 5 sec pulse b N > 3 AT Demonstration Scenario LH Phase I X-ray spectr. Cryopump LH Phase II 4-strap ICRH Divertor upgrade? MHD stabilization?

19 2003: Commission LHCD Phase I (1 launcher) Increase power, assess power density limit (3 MW source available). impurity issues? Focus on LH coupling, wave physics studies. Measure coupling efficiency, reflectivity vs density, launcher and limiter position. SOL density profiles. LHCD and heating efficiency and deposition profile studies vs density, N //. Imaging Hard X-ray Spectrometer and MSE will be very important. Initial investigation of ITB interaction with LHCD. Transmission Coefficient TRANSMISSION COEFFICIENT n = Density (cm-3) n = 3 S. Bernabei, PPPL

20 2004: Exploit LHCD Phase I with ICRH Run LCHD together with 5-6 MW ICRF. Study interaction issues. Use cryopump for density control to maximize driven current. Demonstrate current profile control via N // variation. Maximize LH plus bootstrap current. Goal is 50% non-inductive current. Investigate divertor power handling, impurity accumulation, wall saturation effects, and other long-pulse issues (for ~3 secs). Study ITB formation and sustainment, in optimized shear scenarios. Initial exploration of β limits at reduced current. Use MHD spectroscopy to diagnose instabilities. Try stabilization of NTM using LHCD and/or MCCD.

21 2005: Begin LHCD Phase II (2 nd launcher) Commission second launcher, bringing power up to 3 MW launched (4 MW source). Repeat coupling, efficiency, CD profile studies. Use flexibility of two launchers to create spectrum with two N // peaks, optimize CD localization and efficiency. Modelling and other experiments (eg ASDEX) show that a high N // component increases off-axis absorption. Commission new 4-strap ICRF antenna (needed to free up a port, maintain total power). Initial investigation of mainly non-inductive CD. Assess need for further divertor upgrades to handle 5 second pulse, at full power.

22 : Exploit full power LHCD and ICRH for long pulse, high β AT operation Use full power to maximize non-inductive current drive: Goal is fully non-inductive current, from LHCD plus bootstrap current. Extend pulse length to 5 seconds (~6 τ CR at 5 kev). Reach and experimentally document β limits, at B T = 4.0 T. Adjust RF mix, parameters to control current and pressure profiles, maximize β limits. Goal is β N =3.0. Assess need for, and feasibility of, active MHD stabilization. (This design activity will have started during previous years). If it seems attractive, add part-way through this period. This could enable β N well above the no-wall limit.

23 Summary Many interesting results were obtained in 2001, especially extension and analysis of our unique ITB regime experiments will address outstanding questions. Scenario modelling and experimental discharge development are closely linked, and will be a focus of this year s preparation for LH in Long term program leads progressively to a non-inductive, steady state, advanced tokamak demonstration in a unique, reactor-relevant regime, all RF drive, B T = 4-6 T, T i ~T e. Plenty of work, and exciting physics, for the next 7+ years!