Overview of Recent Experimental Results from theDIII–D Advanced Tokamak Program

S.L. Allen1 and the DIII-D Team2

1Lawrence Livermore National Laboratory, Livermore, California, USAe-mail: allens@fusion.gat.com

2General Atomics, P.O. Box 85608, San Diego, California 92186-5608, USA

Abstract. The goals of DIII-D Advanced Tokamak (AT) experiments are to investigate and optimize the upperlimits of energy confinement and MHD stability in a tokamak plasma, and to simultaneously maximize the frac-tion of non-inductive current drive. Significant overall progress has been made in the past 2 years, as the perfor-mance figure of merit βNΗ89P of 9 has been achieved in ELMing H-mode for over 16 τE without sawteeth. Wealso operated at βNΗ ~ 7 for over 35 τE or 3 τR, with the duration limited by hardware. Real-time feedback con-trol of β (at 95% of the stability boundary), optimizing the plasma shape (e.g., δ, divertor strike- and X-point,double/single null balance), and particle control (ne/nGW ~ 0.3, Zeff < 2.0) were necessary for the long-pulseresults. A new quiescent double barrier (QDB) regime with simultaneous inner- and edge- transport barriers andno ELMs has been discovered with βNΗ89P of 7. The QDB regime has been obtained to date only with counterneutral beam injection. Further modification and control of internal transport barriers (ITBs) has also beendemonstrated with impurity injection (broader barrier), pellets, and ECH (strong electron barrier). The newDivertor-2000, a key ingredient in all these discharges, provides effective density, impurity and heat flux controlin the high-triangularity plasma shapes. Discharges at ne/nGW ~ 1.4 have been obtained with gas puffing bymaintaining the edge pedestal pressure; this operation is easier with Divertor-2000. We are developing severalother tools required for AT operation, including real-time feedback control of resistive wall modes (RWMs) withexternal coils, and control of neoclassical tearing modes (NTMs) with electron cyclotron current drive (ECCD).

1. Introduction

The overall objective of the DIII-D research program is to establish the scientific basis foradvanced modes of tokamak operation [Advanced Tokamak (AT)] which will enhance the com-mercial attractiveness of the tokamak as an energy producing device. Reactor designs such asARIES-AT [1] have shown that AT goals should include: high power density (plasma pressure),high ignition margin (energy confinement τE), and steady-state operation with low recirculatingpower. High gain steady state operation requires a large ratio of the self-driven bootstrap currentto the total current, fBS. The AT is usually identified with active control of plasma profiles,particularly the current profile in shaped discharges. In addition, the divertor must simul-taneously provide power, particle, and impurity control. One measure of AT progress is theproduct of the normalized beta βN and the confinement enhancement factor H89P. Here βN =β/(Ip/aBT) where Ip is the plasma current, a is the plasma minor radius, and BT is the toroidalfield. The factor H89P is τE normalized to τ89P, the ITER89P scaling. ARIES-AT is envisionedto operate at βNH89P > 10 with fBS close to unity. In addition to these quantitative goals,progress in the development of several AT control tools must be made, including: non-inductiveoff-axis current drive with ECCD; internal transport barrier (ITB) control with NBI, impurities,and pellets; density, impurity, and heat flux control with the divertor; and active control of MHDmodes such as the resistive wall mode (RWM) and neoclassical tearing modes (NTMs). InDIII-D, an important current profile tool is electron cyclotron current drive (ECCD). A desire toinvestigate AT physics at a collisionality near that projected for next-step high-gain experiments,and the increased current drive efficiency at higher Te, lower ne, motivate operation in the densityrange of roughly ne/nGW ~ 0.3. In turn, this means that density control in shaped H-modedischarges is particularly important for these initial current drive experiments.

Recent significant progress has been made both in sustaining a higher numerical figure of meritfor a longer duration and in developing new AT control tools [2]. The product βNH89P has beenincreased to 9 for a duration of16 τE. Discharges with βNH89P = 7 have been maintained at 95%of the m/n = 2/1 stability boundary without a disruption for 35 τE, the limit of the hardwarecontrol settings [3]. A new quiescent double barrier regime (QDB) has been discovered(βNH89P = 7) [4], with both ion and electron transport barriers in the core and edge and aquiescent, non-ELMing edge. A key ingredient in achieving these advances has been density andimpurity control in high-triangularity (high-δ) plasmas with the new Divertor-2000 [5]. The

performance in the AT discharges with βNH89P ≥ 9 is limited by the resistive wall mode(RWM); the duration is limited by the evolution of the current profile and the growth of aneoclassical tearing mode (NTM). The very long duration discharges have q0 >~ 1 and are limitedby a NTM. Experiments using feedback-controlled external magnetic fields have made progressin controlling resistive wall modes RWMs [6]. ECCD has been used to stabilize NTMs, anddetailed multi-channel (36) motional Stark effect (MSE) measurements of currents driven withECCD show good agreement with theoretical models [7]. In these initial experiments, three1 MW class gyrotrons injected a total of 1.2 MW into DIII-D. In this paper, we will firstdiscuss the overall highlights of the DIII-D experimental program (high performance, longpulse, and QDB regime), and then examine the progress in four areas: confinement, power andparticle control, MHD stability, and profile control with ECCD.

2. DIII-D Progress Towards Improved-Performance, Long Duration Operation

A. ELMing H-mode

A plot of the numerical figure of merit —βNH89P as a function of τduration/τE —(Fig. 1) shows significant progress (largecircles) in sustained performance in thelast two years. Strong NBI injection dur-ing the plasma initiation phase(t < 300 ms) was used to obtain dis-charges with high q0 and low centralmagnetic shear, allowing achievement ofβNH89P ~ 9 for over16 τE (e.g. 98977 inFig. 2). This class of discharge has fBS ofover 50%, and 75% of the current isdriven non-inductively. The q-profile ismeasured by the magnetics and a 36channel MSE system. Typical behavior(98549) is that the q-profile has negativecentral shear (NCS) early in time, whichthen relaxes to a broad, flat profile withq0 > 1.5.

The discharge performance (β) was lim-ited by the RWM near the calculated no-wall ideal beta limit, which is approxi-mately βN ~ 4 li for a wide range ofDIII-D discharges. The RWM at thesehigh beta values grows slowly, and as theamplitude increases, the rotation de-creases, and there is a drop in beta to be-low the RWM limit resulting in stabiliza-tion of the mode. If beta is increased, theRWM grows to large amplitude and ter-minates the high performance phase. Ifwe avoid the RWM by regulating β, thedischarge duration is typically limited byneoclassical tearing modes, as a conse-quence of the evolution of the currentprofile. Often the NTMs become unstableas q(0) approaches 1.5, demonstratingthat current profile control is needed to in-crease the duration of these discharges.The very long-pulse discharges have

β N H

89p

τduration/τE

0 5 10 15 20 25 30 35 40

ELMy H-modeL-mode edgeQDB regime

ARIES-AT

ELM-free H-mode

DIII-DAT target

87977

98482

99441

103740

95983

96686

9897798965

98549

82205

96202

104205

104264

1042769656898435

93144

98760

96945

93149

89795

89756

ITER–AT

SSTR

ITER Base

0

5

10

15

Fig. 1. DIII-D performance, the large circlesindicate progress in the last two years. Key shotsinclude: 98977(high βNH = 9), 104276 (longduration at βNH = 7), and 103740 (QDB).

4

05

020

0

4 li

H89

2 sβN H89

βN H = 9 for 16 τE

βN

Time (s)1 2 3 40

Fig. 2. Discharge 98977 with βNH89P ~ 9 forover 16 τE and fBS = 50%. βN was sustainednear the no-wall limit (~4 li), no sawteethwere present, and q95 = 5.4.

q0 >~ 1 and with this current profile, increasesin beta first cause a NTM, as βN is below theideal or RWM limit.

To sustain A T discharges at high per-formance, we plan to replace the ohmic cur-rent profile with non-inductive current fromECCD. In Fig. 3, the total current J|| can bedetermined from the time history of the equi-librium reconstructions, and the ohmic currentcan be determined from the measured E|| (thetime derivative of the poloidal flux) along withthe plasma conductivity. Note that the ohmiccurrent is peaked at the half radius, andmodeling has shown that this can be replacedwith off-axis ECCD. In addition, as discussedin Section 6, the model predictions of ECCDefficiency continue to agree well with experi-ments. Localized current drive has alsorecently been demonstrated in ELMingH-mode discharges in the presence of MHDactivity, as discussed in Section 6.

Improved density control with the newDivertor-2000 and the DIII-D control system(controlling shape, β and ne) were instru-mental in obtaining the high performance,long duration discharges at ne/ngw ~ 0.3 withZeff < 2.0 (Fig. 4). To obtain these results, aDN plasma as in Fig. 10 was used and theNBI power was modulated to maintain a con-stant β level about 95% of the experimentally-determined m/n = 2/1 stability limit. The loca-tion of the inner (outer) upper divertor strikepoints was maintained near the entrance to theupper inner (outer) cryopumps of Divertor-2000 to maintain ne/ngw ~ 0.3. The divertoralso has new shaped graphite tiles in the upperdivertor area which minimize hot spots andhas reduced the core carbon concentration sothat Zeff < 2.0. The discharge reached a resis-tive equilibrium at about 3 s, as evidenced bythe flat traces in all 10 central MSE channels[8] (corresponding to the local magnetic fieldpitch angle) from 3 s until the end of the dis-charge. This equilibrium state was maintainedfor approximately 3 τR (the current diffusiontime), and the on-axis q value was 1.05.Sawteeth or other instabilities such as fish-bones were not observed. A low-level n=3,m=2 NTM was present during the ELMingH-mode phase. In the long-pulse shot, theplasma duration was ultimately limited byhardware control settings, only 1/2 of theavailable inductive flux swing was used. Asshown in Fig. 4(a) red, a controlled increase inβ resulted in the growth of a m/n = 2/1 NTM[Fig. 4(g)] which in turn caused a significant

0.0 0.2 0.4 ρ 0.6 0.8 1.0

J||JOH

100

80

Cur

rent

Den

sity

(A

/cm

2 )

60

40

20

0

Fig. 3. The ohmic current is determined fromthe measured E|| profile and the plasma con-ductivity. AT operation will replace thisohmic drive with off-axis ECCD. The remain-ing current is provided by NBCD and boot-strap current.

0

5

10

15

01234

Density = 0.36 x 1020 m–3

Normalized Beta βN = 2.7

Plasma Current Flattop 6.3 s

NBI Power (co-injection)

0

4

8

12

0.0

1.0

2.0

0 2000 4000 6000 8000Time (ms)

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80m/n = 2/1

0.0

0.4

0.8

1.2Dα

Neutron Rate

βN H89P ~ 7.5

0.0

1.0

2.0

3.0104266 104276

Fig. 4. The high performance (βNH89P ~ 7)long duration (3 τR) discharge sustainedwith feedback-control of β at 95% of thestability boundary and density control withDivertor-2000 at ne/ngw ~ 0.3. If the βN isincreased (red) a m/n = 2/1 mode growsand reduces βN .

decrease in confinement and β. In these discharges with q0 ~ 1, the β limit for the NTM is lowerthan that of the RWM.

In the early phase (t ≈ 500 ms) of discharge 104276, and similar discharges, further increases inbeta are limited by the RWM. At this time, q0 > 1.5 and the NTM are generally not observed.These plasmas have a shape suitable for pumping with Divertor 2000 (Fig. 10) and have slightlysmaller elongation and triangularity than high-performance plasmas before Divertor-2000 (i.e.,98549). With this less strongly-shaped plasma, the RWM boundary moves to a lower value ofβN consistent with expectations from ideal stability calculations [9].

In summary, the basic control tools and discharge conditions required for evaluating ECCDsustainment of AT profiles have been established: (1) density control to levels needed forECCD, and (2) current profiles with the correct shape and composition such that replacement ofthe ohmic current at the half-radius with ECCD-driven current should sustain the discharge.With the increased gyrotron power planned in 2001, computational models indicate that nearlyfull non-inductive current sustainment of an AT mode is possible. As discussed in Section 5, weare also developing tools to control the RWM with external coils and the NTM with ECCD tofurther increase β and the duration of the high performance discharges.

B. Quiescent double barrier regime with counter NBI

A new attractive high performance operational regime, QDB, has been discovered on DIII-D thathas both an internal and external transport barrier, with good density and impurity control in theabsence of ELMs. Achievement of this mode relies on a combination of counter neutral beaminjection and divertor pumping. In the past, prompt beam losses with counter injection andresulting impurity influx hampered physics studies. Recently, careful conditioning and plasmacontrol have allowed counter injection experiments with impurity content similar to co-injectedshots. Discharges with the ∇Β drift down had less impurity influx, presumably because the fastions impinged on a better conditioned divertor floor, as opposed to a less well-conditioned upperdivertor baffle. In DIII-D, all neutral beam lines are in one direction, so counter-NBI experi-ments are done by reversing the direction of the plasma current, Ip, shown as negative inFig. 5(a). Although no experimental effort was made at optimizing the QDB regime, highperformance, βNH89P ~ 7, was sustained fordurations of several τE as shown in Fig. 1.

In the QDB regime, strong core and edgebarriers are formed (Fig. 6) in both the ionand electron channels, there are no ELMs orsawteeth, and density control is achieved. TheQDB profiles an ITB and a L-mode edge inFig. 6. The TRANSP transport analysisshows reduced transport out to ρ ~ 0.6 forboth the L-mode edge and the QDB. FIRscattering (Fig. 7, left) shows a sharp reduc-tion in the plasma turbulence at nearly all fre-quencies in the core of the plasma.

The QDB has an additional edge barrierwhich we call the quiescent H-mode or QH-mode. This edge barrier has an edge pedestalsimilar to that observed in ELMing H-mode,but does not have ELMs and hence does nothave the bursts of particles and heat to thedivertor plate. By locating the divertor strikepoints of the USN discharge close to thepump entrances of Divertor-2000, densitycontrol is achieved. (Shots without the NBIramp at 2-3 s as in Fig. 5 have a constant

NH89

N

β

β

%-M

A-T

/m%

-MA

-T/m (7)

IP (MA)

q0

qmin

Dα (a.u.)

<ne>(1019 m—3)

103740

m—

3

PNBI

MW

ELM-free

0 21 3 4Time (s)

1

3

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4

8

-2

-1

0

0

2

4

0

10

1

3

(a)

(b)

(c)

(d)

(e)

(f)

Fig. 5. A high performanceQDB dischargewith no ELMs.

density for the entire duration of thedischarge.) Hence, the edge serves as astrong energy transport barrier, butallows particles to be transported out ofthe plasma for plasma for density con-trol. The carbon density remainsunchanged during the QDB, and Zeff ~2-2.5. The edge QH-mode has beenstudied with a large array of diagnostics,including beam emission spectroscopy(BES) [10], phase contrast interferome-try (PCI) [11], reflectometry, and mag-netics. All of the diagnostics observe alow-frequency multi-harmonic spec-trum, as shown in Fig. 7 (right). Severalcoherent toroidal modes ranging fromn=1 to 9 are observed, and the fre-quency is proportional to the modenumber. The mode is localized to aregion near the separatrix and may con-tribute to controlling the edge pressuregradient. Qualitative comparisons havebeen made with the enhanced Dα modeobserved on C-Mod [12]. While theabsence of large pulses of energy lossto the divertor is common to bothmodes, many of the other signatures arequite different and will be explored inthe 2001 campaign.

Internaltransportbarrier

0

5

10

15

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1

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5

0.0 0.2 0.4 0.6 0.8 1.0ρ

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keV

1019

m—

3

Edg

e (

H—

mod

e)

barr

ier

Edg

e (

H—

mod

e)

barr

ier

L—modeedge L—mode

edge

QDB

QDB

0

2

4

6

8

1

2

3

4

5

6

Ti Te

ne

q

L—mode edge ITB (99849 1.12 s) QDB (103740 3.3 s)

Fig. 6. Transport analysis shows that the QDBregime has an core barrier in both the ions andelectrons, along with a edge transport barrierwithout ELMs; previous results from an L-mode edgewith only a core barrier are included. The QH-modeedge exhausts particles and impurities.

3. Understanding and Control of Internal Transport Barriers

Anomalous ion transport is believed to be a consequence of long wavelength (low-k, or kθ ~1-10 cm-1) turbulence in the plasma, usually attributed to the ion temperature gradient (ITG)instability. The ITB appears when these instabilities are suppressed in the core. Research inDIII-D seeks to establish control of the ITB by controlling various mechanisms connected withthis turbulence. Principle among these is the application of ExB shear [4]. Comparison of ExBshearing rates in similar discharges with co- and counter-NBI (Fig. 8) reveals that the region of

Freq

uenc

y (k

Hz)

1000

500

0

–500

–1000

Time (s)1.0 2.00.0 3.0 4.0

QH–modeedge}

0

4

Core turbulencesuppressed afterNBI initiation

FIR scatteringreduced at NBI turn-on

ñMulti-harmonic

edge modes(magnetics)

n = 18 kHz

n = 216 kHz

n = 324 kHz

n = 432 kHz

2.0 s 2.5 s0

40 kHz

Fig. 7. FIR scattering spectrum of QDB regime(left) shows dramatic reduction in fluctuations inthe core plasma at NBI turnon (onset of QDB regime). The multi-harmonic character of edgemultiharmonic mode is shown in the magnetics (right). PCI and BES also observe a multihar-monic spectrum.

the plasma with a strong shearing rate islarger with counter-NBI. This correspondsto an observed increase of the ITB’s spatialextent [4]. The difference can be understoodas follows: the shearing rate can beexpressed as a sum of terms representingcontributions from the pressure gradient andplasma rotation. In co-NBI discharges inDIII-D, the rotation term dominates and is ofopposite sign to the pressure gradient term,so that increasing the strength or extent ofthe barrier results in a reduction to theshearing rate. With counter-NBI, however,the pressure gradient term dominates, so thatincreasing the strength or extent of the bar-rier becomes a stabilizing influence.

Impurity injection has also demonstrated anability to increase the size of an ITB [4,13].Here, the initial introduction of the impurity(usually neon) reduces the turbulence growthrate and allows the gradients, and thereforethe ExB shearing rate, to become larger. Theincreased shearing rate then becomes thedominant effect, once again resulting in ITBswith larger radial extent. Another techniquethat has been shown effective in modifyingturbulence is pellet injection [14], which canform strong ITBs evident in all transportchannels.

The above tools primarily impact low-k tur-bulence, and therefore the ion channel.Electron thermal transport is believed to becontrolled not only by these instabilities, butalso by high-k instabilities such as the elec-tron temperature gradient (ETG) mode. Dueto the small spatial scales involved, ExBshear is not expected to affect ETG turbu-lence, hence the often observed result thatelectron transport is not reduced along withtransport in other channels. Recently, strongelectron ITBs have been observed in dis-charges with intense, localized, direct electronheating with ECH (Fig. 9) [4,7]. Theory-based simulations of these discharges indi-cate that the controlling physical mechanismhere is α-stabilization (often referred to asShafranov shift stabilization). Since α-sta-bilization can be effective in reducing bothhigh- and low-k turbulence, these dischargesare believed to also possess a nascent ionITB. This is supported by reflectometer mea-surements indicating significant reductions

0.0 0.2 0.4 0.6 0.8 1.0ρ0

1

2

3

4

5

105

s–1

ωE×Bγmax

co (87031 1.82s)ctr(99849 1.17s)

Fig.8. The calculated ExB shearing rate (solid)from CER data compared to the linear growthrate (dashed) for ITG modes in counter (blue)and co (red) injected NBI. The stabilizedprofile is broader in the counter NBI case.

0.0

0.1

0.2

0.3

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0.5

0.6Thomson

ECE0.110 s0.147 s0.210 s

0.0 0.2 0.4 0.6 0.8 1.0ρ0

2

4

6ke

V

MW

/m3

(ECH on at 0.1s)

Te

QECH

0.10 0.15 0.20Time (s)

0

0.2

0.4

0.6

0.1

1

10

m2 /s M

W

χi

PECH

χe

χineo

99696 ρ = 0.35

(Resistiveinterchange

collapse)

Fig.9. ECH has resulted in strong electronbarriers, and χ i is reduced to neoclassicallevels.

of low-k turbulence within the electron transport barrier. This regime offers the promise of anITB with equilibrated electrons and ions by applying heating power to the ions.

4. Density and Impurity Control with Divertor-2000

Power and particle control experiments focused on density control with cryopumping for ATplasmas, optimization of plasma shape, control of impurities with the new Divertor-2000, and ra-diative divertor operation. At the end of 1999, a new closed divertor with two cryopumps and adome in the private flux region was installed (Divertor-2000), shown in Fig. 10. The divertorshape is optimized for high-δ plasmas, and particle exhaust can be increased (decreased) bymoving either strike point towards (away) from the pump opening. Control algorithms weredeveloped so that each strike point could be controlled independently, and operation atne(core)/nGW ~ 0.3 in AT plasma shapes was routine. Up/down magnetic balance in double null(DN) plasmas was used to control the particle exhaust and the H-mode power threshold. Controlof the magnetic balance parameter dRsep of 1 cm was required [15]. dRsep is defined to be thedistance between the separatrices connected to the upper and lower null of a DN plasma mea-sured at the outer midplane. New shaped tiles that were carefully aligned (~0.1 mm gap) in theupper divertor reduced the carbon concentration in AT plasmas. At the end of one of the long ATpulses with 50 MJ of injected energy (104274, Fig. 10), the peak tile temperature from IRTVwas ~1000ºC, less than the threshold for carbon sublimation. As 50 MJ is close to the presentauxilliary heating capabilities of DIII-D, this operation is a good demonstration of an AT diver-tor for DIII-D.

Plasma shape experiments were carried out in ELMing H-mode plasma with several variations:(a) lower single-null (LSN), (b) upper-SN (USN), (c) balanced DN, (d) unbalanced DN,(e) several triangularities up to δ < 0.8, and (f) variations in the “secondary” divertor volume(e.g. the lower X-point in Fig. 10). With respect to up/down heat and particle sharing, magneti-cally unbalanced attached DN plasmas behave like SN plasmas except close to magnetic balance(-0.5 < dRsep < 0.5 cm). The divertor heat flux profile can be explained by a simple flux map-ping from the plasma midplane; the particle profile is broader due to local effects in the divertor.For detached plasmas, good heat flux sharing was obtained for a substantial range of unbalancedDN shapes. Finally, the presence of a second X-point (a “secondary divertor”, as in ITERFEAT) in unbalanced DN shapes did not degrade the plasma performance if it was sufficientlyinside the vacuum vessel. At modest densities ne(core)/nGW < 0.7, both core and pedestal per-formance increase with triangularity. In moderate density, unpumped plasmas, high δ increasedthe energy in the H-mode pedestal and the global energy confinement of the core, primarily dueto an increase in the margin by which the edge pressure gradient exceeded the value which wouldhave been expected had it been limited by infinite-n ideal ballooning modes. Previously, we havedemonstrated strong heat flux reduction with either deuterium puffing and pumping (P&P) orwith P&P and impurity injection at ne(core)/nGW ~ 0.5-0.9. A strong enrichment of argon (20)was observed [16]. P&P experiments with Divertor-2000 showed similar results.

104276 4625.00

Old 1 mm

2.5 mm

New 0.1 mm

0.62 mm

1200

0

00 2000 4000

Time (ms)6000 8000

Peak Tile Temperature (¡C)

Heat input (MJ)50

ne/nGW ~ 0.3

Fig. 10. AT density and impurity control are accomplished with Divertor-2000,consisting of two cryopumps and precisely-aligned carbon tiles.

Design studies for high energy gain experiments and fusion power plants show the importanceof operation at high density while maintaining good confinement, in optimizing fusionperformance. Gas puffing with cryopumping (P&P) has resulted in high quality H-modes(H93P ≥ 1, or H89P ~ 2) at high density (ne(core)/nGW ~ 1.4) [17] (Fig. 11). This operation isalso favorable because the energy loss of the Type-I ELM (which can induce a large divertorheat flux) is a factor of five lower than that predicted by a multi-device scaling at lower density.The edge pedestal parameters Pe do not degrade in these high density discharges, and analysis isunderway to determine the exact pedestal requirements (e.g. the discharges are obtained morereadily with pumping) so that these favorable results can be duplicated in other devices.

5. Understanding and Control ofMHD Stability: RWMs and NTMs

In our high performance AT dischargesdiscussed above, the pressure, β, and per-formance near the no-wall stability limit(βN ~ 4 li) is usually limited by theRWM. The RWM is a low toroidalmode number kink mode occurringabove the ideal no-wall stability limit.Sufficient plasma rotation can stabilizethe RWM, but we observe that theplasma toroidal rotation decreases as theno-wall beta limit is exceeded, allowingthe RWM to destabilize. An example ofan RWM mode that produces smallreductions in β followed by a large col-lapse is shown in Fig. 12. Each increasein the mode amplitude is followed by areduction in β.

To control the growth of the RWM, wehave started feedback control experi-ments. On DIII-D, the slowly growingRWM modes were reduced using a sys-tem consisting of six sensor loopslocated outside the vessel at the midplaneand a six-element set of active controlcoils. In 2000, an array of 24 pickuploops was added, more capability wasadded to the control coil power supplies,and we developed a detailed systemmodel. The added power supplies wereimportant because the control coils sup-ply both the slowly varying error fieldcorrection for the plasma, and the fasttime response feedback control of theRWM. Several feedback schemes weretested, and two of these are compared inFig. 13: smart-shell with derivative gainand mode-control with derivative gain.(See Ref. [6] for an explanation of theseterms.) We found that the latter was themost effective in extending the durationof the phase with βN above the no-walllimit. Modeling with the VALEN 3-Delectromagnetic code for the presentsystem agrees well with the observed

0.5 1.0

TEXTORRImode

DIII–DAr ELMyH–mode

JT–60U ELMy H–mode

H93 × ne/nG

0.0 1.5

DIII–D L-modeWith Impurity

Injection

0.5

1.0

1.5

0.0

H = 1

DIII-D Puff and Pump ELMy H-mode

n /nGe

Fig. 11. Operation above the Greenwalddensity with puff and pump. RI-mode impurityinjection shots with increased confinement arealso shown. H93, the confinement enhancementfactor relative to ITER93 scaling is used.

1300

Max. δBr (G)

1400 1500 1600Time (ms)

1700 1800 19004.04.2

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300

δBr measured by saddle coils

200

Toro

idal

Ang

le (

deg.

)

β (%)

100

20

15

10

5

0

0

Fig. 12. An example of a RWM mode. (a) dBr fromsaddle loops shows a slowly propagating mode,(b) the mode amplitude, (c) each period of modegrowth is followed by a reduction in β.

o

x

C-coilSensorloops

C—coil sections

No FeedbackSmart Shell + Deriv. Gain

Mode Control + Deriv. GainRWM Amplitude

βNNo-Wall Limit(approx.)

Plasma Toroidal Rotationρ ~ 0.5

1100 1200 14001300 1500

Time (ms)

101951 101953 101956

(Gau

ss)

(km

/s)

100

200

0

300

20

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1

2

0

Feedback turn-on time

Fig. 13. The RWM is stabilized by externally applied magnetic fields in a feedback loop.

RWM stabilization. Model projections withsystem improvements (e.g. internal Bp sen-sors) indicate that beta can be increased 50%closer to the ideal wall limit (Fig. 14).Specifically, in the model case this is anincrease in βN from 3 (the predicted RWMonset), to 3.4 with the present system, tonearly 4 with internal Bp sensors. The idealwall limit in these model discharges wasβN ~ 5. Future plans include an 18 coil set (6coils above and six coils below thepresent coils), which is predicted to increaseβN to 80% of the difference between the ideal-wall limit and the no-wall limit (βN ~ 4.6 forthe model discharge).

While β is usually limited in the high per-formance DIII-D AT discharges by theRWM, current diffusion and the growth ofNTMs often limit the duration. In addition, wenormally observe that the NTM becomesunstable at a lower beta value than the RWMin discharges with q0 ~ 1. Shown inFig. 15(a) (red-blue curve) is the m=3/n=2

Ideal wallβ limit

No wallβ limit

Ideal kink

Resistivewall modeNo Feedback

AdditionalCoils

PresentSystem

InternalBp Sensors

Fig. 14. Predictions from the VALEN 3-Delectromagnetic code show increased β ispredicted with new internal (Bp) sensors.

NTM stability boundary from the Rutherford equation. The important quantities are the poloidalbeta and the width of the magnetic island compared to a threshold island width. This is ametastable situation: if a seed island is present and the perturbation is greater than a thresholdsize it can grow to saturation (right side of the curve). Scaling of the NTM threshold beta withcollisionality and ρi

* , and the role of island rotation have been studied on DIII-D [18] and othermachines [19]. Complete stabilization of the m/n = 3/2 NTM by ECCD (predicted by theory)and demonstrated experimentally [17]) has been achieved on DIII-D as shown in Fig. 15. Theexperiments on DIII-D highlight the localization of current drive even with the presence ofMHD. The ECCD resonance location was varied by a 1.4% scan of the toroidal field during theflat-top of the discharge; the radial location of the driven current has been calculated withTORAY-GA [Fig. 15(d)]. Only one location of the driven current, [the red curve in Fig 15(d)],results in a complete reduction of the mode amplitude [red curve in Fig. 15(b)]. There is also acorresponding increase in the βΝ at this same time, giving a rough indication of the performancedegradation of the NTM mode for this discharge.

6. Progress With ECCD in ATTarget Discharges

In our high performance AT target dis-charges (e.g. 98549) with qmin > 1.5 and75% non-inductive current drive, theremaining inductive current is peaked atapproximately the half radius. Toincrease the duration of these dischargesto steady state, we will use ECCD toreplace the inductive current at the halfradius (Fig. 3). A basic theoreticalunderstanding of localized ECCD istherefore necessary to predict therequired ECH power for a particular ATscenario. Previous analysis of experi-ments showed that the EC-driven currentwas at the radius predicted by theory, butthe profile inferred from magnetic recon-structions was broader than the calcu-lated profile. However, recent direct cal-culations of the multi-channel MSE sig-nals from ECCD theory are in agreementwith the raw MSE data, validating thestrong spatial localization feature ofECCD. Demonstration of localizedcurrent drive in ELMing H-mode isshown in Fig. 16, which compares themeasured current (MSE) with TORAY-GA calculations. To obtain the datashown Fig. 16, we calculate dBz/dR as afunction of radius from adjacent MSEchannels. The change in this quantitybetween two identical discharges with-and without-ECCD is proportional to thelocal current density. Because of the highspatial resolution of the MSE, the radiallocation of the current can be determinedaccurately. The two peaks in Fig. 16 aredue to the two ECH resonance locationsin the plasma. Note that the location ofthe driven current from the TORAY-GAagrees well with these experimental mea-surements. Furthermore, other experi-ments have indicated that the magnitudeof the driven current is in good agree-ment with the model, giving us confi-dence that our predictions of the locationand magnitude of driven current are rea-sonable.

In 2000, we started the commissioning ofthree gyrotrons; this work progressed toan injection of 1.2 MW into the plasma.A scan of the polarization of each systemwas completed and X-mode injection wasverified. A new, steerable launcher (move-

10

8

6

4

2

03

2

1

01500 2000 2500 3000 3500 4000

“Knee”in βθ vs w

4500Time (ms)

×10–

4

n = 2 Mirnov (T)

βN

1.5

1.0

0.5

0.00

βθ

1

√

2

w/wpol

3

3

4 5

Unstable, w > 0

βθ, min

Irf/Ip = 0

Seed

Irf/Ip = 0.02

Saturated

0.50 0.55 0.60

No effectStabilize

No effect

0.65 0.70 0.75 0.80r/a

0

5

10

15

20

(am

ps/c

m2 )

W3/2 ~ 7 cmfrom ECE radiometer

r3/2~

(a)

(b)

(c)

(d)

Fig. 15. Stabilization of m=3/n=2 NTM with ECCD.(a) The ECCD raises the predicted stabilityboundary (red trajectory), (b) the n=2 Mirnovshows NTM stabilization (red curve), (c) the βNrecovers (red) for the stabilized case, (d) thelocalized ECCD (TORAY-GA) stabilizes the NTMwhen applied at the proper radius (red). A BT rampwas used to sweep the location of the ECCD.

able between shots) was used to compare co-and counter-ECCD on subsequent shots.Similar off-axis current was driven in eachcase. This level of operation was adequate forthe NTM stabilization and electron ITBexperiments, but not adequate for sustainmentof AT modes. We are currently installingthree new long-pulse gyrotrons and we planto have a total of six for the 2001 campaign.

7. Summary

Since the last IAEA in 1998, DIII-D researchhas advanced our understanding of keyphysics issues in the areas of transportbarriers, densiity and impurity control, MHDstability, and current sustainment in high-performance plasmas. We have also madeprogress relative to numerical goals and indevelopment of AT control tools.

1.6

∆

∝∆

J φ (

MA

/m2 )

1.8 2.0

ECCD(Toray — GA)

2.2 Major Radius (m)

dBz

dR

—0.4

—0.2

0.0

0.2

0.4

Fig. 16. The radial location of the ECCDfrom MSE measurements (red) agrees wellwith calculations from TORAY-GA modeling.

A brief summary of the progress on numerical goals is presented in Table 1:

Cases βNH89P τdur /τE fBS ne/nGW qmin Zeff

1. AT pressure equilibrium 9 16 0.5 0.6 1.5 2.52. Long pulse resistive

equilibrium7 >35 ~0.30 0.3 >1.0 1.7

3. QDB 7 >5 ~0.45 0.3 1.5 2-2.5

1. This is an example of an AT target maintained for pressure relaxation time. The currentprofile has the ohmic current at half radius, which can be replaced by ECCD. For theECCD available on DIII-D, the density must be reduced. These discharges were obtainedbefore the installation of Divertor-2000.

2. This is an example of a long pulse ELMing H-mode discharge that is maintained inresistive equilibrium. Beta feedback on the NBI power was used to maintain 95% of m/n =2/1 stability limit with no sawteeth. Divertor 2000 was used for density and impuritycontrol and the correct density was achieved for DIII-D AT operation with ECCD. Am/n = 3/2 NTM is present, which can be controlled by ECCD in future experiments.

3. This is the new QDB transport regime with strong ion and electron barriers in the core andthe edge. There is a quiescent H-mode edge plasma with edge harmonic oscillations thattransports particles but has no ELMs.

We have also made progress on the development of AT control tools:

1. Divertor 2000 has achieved the control of density (ne/nGW = 0.3), impurities (Zeff < 2), andheat flux (~1000° carbon tile temperature with 50 MJ input) in the DIII-D AT regime.

2. We have developed several ITB control tools, including counter NBI, impurity puffing, andpellet injection. Experiments and modeling continue to support ExB stabilization.

3. RWM modes have been stabilized with feedback-controlled external coils. An improvedsystem in 2001 is predicted to allow high β operation.

4. NTMs have been stabilized with ECCD. The modified Rutherford equation modelcontinues to be consistent with the experimental observations. Increased gyrotron powerwill allow development of control tools that can be more routinely used.

5. Detailed MSE measurements have verified the localized deposition of ECCD, andcomparisons with TORAY-GA show consistent results. These same results predictsufficient ECCD for sustainment of AT modes with adequate gyrotron power whenoperated at the proper density.

6. We have commissioned three gyrotrons for the experiments in 2000. We plan to add threeadditional long-pulse (10 s) gyrotrons for experiments in 2001.

Acknowledgments

This is work was supported in part by the U.S. Department of Energy under Contract No. DE-AC03-99ER54463, W-7405-ENG-48, and DE-AC05-00OR22725

References

[1] NAMJABADI, F., et al., these proceedings, paper FTP 2/15.[2] TAYLOR, T.S., and the DIII-D Team, “Results from the DIII-D Scientific Research

Program,” Fusion Energy 1998 (Proc. 17th Int. Conf. Yokohama, 1998), IAEA, Vienna(1999).

[3] LUCE, T.C., et al., these proceedings, paper EX4/3.[4] DOYLE, E.J., et al., these proceedings, paper EX 6/2; BAKER, D.R., et al., these

proceedings, paper EX 5/03.[5] ALLEN, S.L., et al., “Experiments and Computational Modeling Focused on Divertor and

SOL Optimization for AT Operation on DIII-D,” to be published in J. Nucl Mater., 2000[6] NAVRATIL, G.A., these proceedings, paper EXP 3/4 (R); GAROFALO, A.M., EXP3/01.[7] PRATER, R., et al., these proceedings, paper EX8/1.[8] RICE, B.W., et al., Fusion Energy 1998 (Proc. 17th Int. Conf. Yokohama, 1998), IAEA,

Vienna (1999).[9] TURNBULL, A.D., et al., Nucl. Fusion 38, 1467 (1998).[10] McKEE , G.R., et al., these proceedings, paper EX6/5.[11] ROST, C., et al., private communications.[12] MARMAR, E.S., et al, these proceedings, paper EX 2/5.[13] MURAKAMI, M., et al., these proceedings, paper EX 5/1[14] BAYLOR, L.R., et al, these proceedings, paper EX2/4.[15] FENSTERMACHER, M.E., et al, these proceedings, paper EX2/4.[16] WADE, M.R., et al., Nucl. Fusion 38, 1839 (1998).[17] MAHDAVI, M.A., et al., these proceedings, paper .EXP1/04.[18] LA HAYE, R.J., et al., these proceedings, paper EXP 3/05.[19] ZOHM, H., et al., these proceedings, paper EX3/1.

DIII–D TEAM

S.L. Allen,a) P.M. Anderson,b) M.E. Austin,c) D.S. Baggest,b) W. Baity,d) D.R. Baker,b) D.E. Baldwin,b)

G. Barber,d) R. Bastasz,e) C.B. Baxi,b) L. Baylor,d) S. Bernabei,f) J. Bialek,g) J.A. Boedo,h) I.N. Bogatu,i)

A. Bondeson,j) A.S. Bozek,b) R. Bravenec,c) B.D. Bray,b) D. Brennan,k) J.D. Broesch,b) N.H. Brooks,b)

K.H. Burrell,b) J. Burrus,b) J. Callen,l) R.W. Callis,b) T.N. Carlstrom,b) E. Carolipio,m) B. Carreras,d)

W.P. Cary,b) T.A. Casper,a) V.S. Chan,b) M. Chance,f) J. Candy,b) L. Chen,n) E. Chin,b) H.K. Chiu,b)

S.C. Chiu,b) M. Chu,b) R.J. Colchin,d) S. Combs,d) K. Comer,l) W. Davis,f) J.C. DeBoo,b) J.S.deGrassie,b) S. Delaware,b) R. Deranian,o) J.L. Doane,b) E.J. Doyle,p) D. Edgell,i) R. Ellis,q) R. Ellis III,f)

D. Ernst,f) T.E. Evans,b) R. Feder,f) M.E. Fenstermacher,a) C. Fenzi,l) J.R. Ferron,b) D. Finkenthal,r)

R. Fonck,l) E. Fredrickson,f) J. Freeman,b) M. Friend,b) C. Fuchs,s) S. Galkin,t) A. Garofalo,g )

G. Garstka,q) G. Giruzzi,u) P. Gohil,b) A.A. Gootgeld,b) I. Gorelov,b) F. Grantham,b) D. Gray,h

M. Gryaznevich,v) J.M. Greene,b) K.L. Greene,b) C.M. Greenfield,b) N. Greenough,f) R.J. Groebner,b)

S. Guenter,s) T.S. Hahm,f) M.J. Hansink,b) T.E. Harris,b) R.W. Harvey,w) T. Hatae,x) C. Hegna,l) W.W.Heidbrink,m) F.L. Hinton,b) J. Hogan,d) E. Hollman,h) K.L. Holtrop,b) R.-M. Hong,b) J. Hosea,f)

W. Houlberg,d) C.L. Hsieh,b) D.A. Humphreys,b) A.W. Hyatt,b) H. Ikezi,b) A. Isayama,x) R.C. Isler,d)

G.L. Jackson,b) N. Jaluka,y) J. Jayakumar,a) T.H. Jensen,b) T. Jernigan,d) R.D. Johnson,b) L. Johnson,f)

D.H. Kaplan,b) K.M. Keith,b) A.G. Kellman,b) D.H. Kellman,b) R. Khayrutdinov,z) J.S. Kim,i) J.F.Kinsey,aa) R.J. La Haye,b) G. Labik,f) L.L. Lao,b) C.J. Lasnier,a) J. Latchem,b) E.A. Lazarus,d) R.L.Lee,b) R.A. Legg,b) A.W. Leonard,b) J.A. Leuer,b) Y.R. Lin-Liu,b) L. Lodestro,a) J.M. Lohr,b) T.C.Luce,b) S. Luckhardt,h) V. Lukash,z) J.L. Luxon,b) M.A. Mahdavi,b) R. Maingi,d) C.C. Makariou,b) M.A.Makowski,a) J. Mandrekas,bb) J. Manickam,f) B.B. McHarg Jr.,b) G.E. McKee,l) A. M. Messiaen,cc) W.H.Meyer,a) K.R. Middaugh,b) R.L. Miller,b) P.K. Mioduszewski,d) J. Moller,a) R.A. Moyer,h )

M. Murakami,d) A. Nagy,f) G.A. Navratil,g) A. Nerem,b) W. Nevins,a) Y. Nikolski,b) D.E. Nilson,a)

M. Okabayashi,f) Y. Omelchenko,b) R.C. O'Neill,b) J. Ongena,cc) T.H. Osborne,b) L. Owens,d) P.B.Parks,b) A.W. Peebles,p) B.G. Penaflor,b) Q. Peng,b) F.W. Perkins,f) P.I. Petersen,b) T.W. Petrie,b) C.C.Petty,b) J.C. Phillips,b) D.A. Piglowski,b) R.I. Pinsker,b) P.A. Politzer,b) D. Ponce,b) M. Porkolab,dd)

G.D. Porter,a) R. Prater,b) S.G. Pronko,b) A. Punjabi,y) S. Raftopooulos,f) A. Ramsey,f) E.E. Reis, Jr.,b)

D.E. Remsen,b) M.E. Rensink,a) C.L. Rettig,p) T.H. Rhodes,p) B.W. Rice,a) J.I. Robinson,b )

G. Rolens,b) M. Rosenbluth,b) D. Ross,c) C. Rost,dd) D. Rudakov,h) N. Sauthoff,f) R.I. Savercool,b) J.M.Schachter,b) M.J. Schaffer,b) D.P. Schissel,b) G. Schmidt,f) J.T. Scoville,b) T.C. Simonen,b) S.M.Skinner,b) R.T. Snider,b) P.B. Snyder,b) H.E. St John,b) W. Stacey,bb) G.M. Staebler,b) B.W. Stallard,a)

R.D. Stambaugh,b) P.G. Stangeby,ee) E.J. Strait,b) E. Synakowski,f) H. Takahasi,f) H. Takenaga,x) P.LTaylor,b) T.S. Taylor,b) T. Terpstra,f) D.M. Thomas,b) S. Turgarinov,z) A.D. Turnbull,b) B. Unterberg,ff)

R. Vernon,l) S. VonGoeler,f) M.R. Wade,d) F. Waelbroeck,c) M.I. Walker,b) R.E. Waltz,b) W.R.Wampler,e) J.G. Watkins,e) G. Watson,k) J.C. Wesley,b) W.P. West,b) J. Whaley, Jr.,e) D.G. Whyte,h)

H. Wilson,v) N. Wolf,a) C.P.C. Wong,b) K. Wong,f) S.K. Wong,b) F. Yin,n) B. Zaniol,gg) L. Zeng,h)

M. Zerbini,ee) C. Zhangn)

a)Lawrence Livermore National Laboratory, Livermore,CA.

b)General Atomics, San Diego, CA.c)University of Texas at Austin, Austin, TX.d)Oak Ridge National Laboratory, Oak Ridge, TN.e)Sandia National Laboratories, Livermore, CA.f)Princeton Plasma Physics Laboratory, Princeton, NJ.g)Columbia University, New York, NY.h)University of California, San Diego, CA.i)FARTECH, San Diego, CA.j)Chalmers University, Götteborg, Sweden.k)ORISE, Oak Ridge, TN.l)University of Wisconsin, Madison, WI.m)University of California, Irvine, CA.n)ASIPP, Hefei, Peoples Republic of China.o)University of Wales.p)University of California, Los Angeles, CA.q)University of Maryland, College Park, MD.

r)Palomar College, San Marcos, CA.s)IPP, Garching, Germany.t)Keldysh Institute, Moscow, Russiau)Assoc.-EURATOM-CEA, Cadarache, France.v)UKAEA-Culham, Abingdon, UK.w)Comp-X, Del Mar, CAx)Japan Atomic Energy Research Insitute, Naka, Japany)Hampton University, Hampton, VA.z)TRINITI, Troitsk, Russia.aa)Lehigh University, Bethlehem, PAbb)Georgia Tech, Atlanta, GAcc)Assoc-EURATOM-Ecoloe Royole Militaire Academy,

Brussels, Belgiumdd)MIT, Cambridge, MA.ee)University of Toronto, Toronto, Canadaff)Assoc.-EURATOM, Kernsforschunganlage, Jülich,

Germanygg)University Pahwa, Padua, Italy.