High p experiments in JET and access to Type II/grassy ELMs

G Saibene

and JET TF S1 and TF S2 contributors

Special thanks to to Drs Y Kamada and N Oyama (JAERI-Japan)

Scope of JET small ELM experiments

• Obtain plasmas with: – High confinement (H98~1) with steady state core/edge

– Compatible with high density (n~0.8 nGR or more)– Acceptable ELM size (projected to ITER)

• Max loss to divertor ~4MJ in ITER 3-5% Wped ELM losses 5-10% Wped

– Identify access conditions & potential for extrapolation

• Compare JET results other experiments Asdex-U Type II ELMs and JT-60U “grassy” ELMs regimes

Small ELM regimes in Tokamaks: Type II ELMs in ASDEX-Upgrade

• Asdex-U: Type II ELMs found in standard H-modes– Quasi Double Null Configuration (QDN) [J Stober, NF 2001]

– Favored by high shaping (~0.4) and q (q95>4)

– High density (nped>70%nGR, *ped~1-2), N~1.8, p<1

– Also obtained at high Pin/p – very high n (*~0.8?) + high QDN– Change of MHD stability high n peeling/ballooning

=0.33, =2.3 50%ngr

=0.43, =3.2 83%ngr

=0.43, =3.5 88%ngr

[Sips, PPCF 2002]

Small ELM regimes in Tokamaks: Grassy ELMS – JT-60U

• JT-60U “grassy” ELMs: [Y Kamada PPCF 2002]

– Threshold in p (p>1.7)

– High (~0.4 ~0.6)/ high q95(6.54)

– H-mode edge + ITB & Low density (n<0.5nGR) - *ped~0.1– Strong Shafranov shift stabilizes Type I ELMs access

to second stability

JET Type II Studies [G Saibene EPS 2003, CP Perez PPCF 2004]

• Mixed Type I-II ELMs obtained in SN (& QDN) plasmas (~0.45-0.5) at q95<4 and N~ 2. H98~1, n/nGr~1

• Definition: No type I ELMs + Increase of inter-ELM power losses enhanced broadband fluctuations in magnetics and density (WB) Tped clamped, nped raise reduced

Type I Type I-II

nped,min~ 70% ngr

JET: QDN and q95

• Contrary to AUG results: high q95 reduces/closes access to mixed Type I-II regimes – QDN has no significant effect

p~ 0.7 – 0.8

JET: QDN and q95 (2)

• q95 : Type I III transition at low nped – no Type II

• q95 : average edge refuelling rate increases – not understood (both SN and QDN)

JET/AUG: is identity + QDN geometry the key to Type II ELMy H-modes?

• At identity parms: nped & Tped = constant! (ped*~2)- H98~1

• Low Pin: slow density peaking radiative collapse

• Increasing Pin Type I ELMs + steady state plasma core

• Increasing Ip/Bt at constant q operational space for Type II ELMs closed between L-H transition and Type I-III H-mode regime.

• Type II WB modes at ~10kHz + n fluctuations

No Type I ELMS, & pped=constant

Grassy ELMs: High p H-modes

• High configuration, QDN (sep <1cm) – standard H-mode scenario (li~1.1) – 1.5-1.2MA/2.7T for high p

• Results:– H98~1.2, n~0.9nGr and “grassy-like” ELMs obtained

(q95~6.8, the only value explored so far)

– Grassy ELMs are very small and irregular in size (H) and frequency (high)

• What makes these ELM small? (at high pped)– High p Shafranov shift stabilisation grassy ELMs?

• Comparison with standard ELMy H-modes at low p

• Comparison with high p, low li H-modes (qo>2, li~0.7, some with weak ion ITB)

Overview of p scan (high li)

• Standard Type I ELM activity up to p ~1.5 with H98~1

• From p ~1.6-1.7, regular H bursts disappear completely (H98 ~1.2)

• Irregular “grassy” H signature

High p (high li): MHD bursty activity (low frequency only)

• Grassy ELMs: small MHD bursts at low frequency, no washboard modes

62413 -p ~1.9 62406 - p ~1.35

• Type I: “Standard” MHD spectrum at lower p (broadband ELM signature + wb modes inter-ELM)

• Grassy ELMs MHD signature similar to Type I ELMs but MHD bursts extend very little in frequency

MHD spectra with Grassy ELMs

ASDEX-upgrade

JET

[J Stober, IAEA 2004]

p scan in low li H-modes - shapes

• Early heating scenario – no sawteeth (qo~2) – li~0.8-0.85

• Plasma shape: and ~ high li H-modes, but SN (note that sep <1cm but 2nd x-point is not in vacuum)

SN low li – QDN high li p~1.9 for both plasmas

p scan in low li H-modes – overview of results

pi increased from ~1.0 to 1.9 Type I ELMs observed up to the highest p

Global parameters comparisons at high p

• Global confinement similar at high p • Grassy ELM onset confinement and pped are not degraded (cfr

Type III ELMs)• High-li, high p: higher pedestal collisionality for similar pped

• Lower limit for * for grassy ELMs existence not explored

ELM losses of grassy ELMs

• Grassy ELMs:– ELM energy losses <5% Wped

(~15% for low li high p )

– n/nped- T/Tped and W/Wped below typical H-mode values

ELM affected depth – reference

• Low p H-modes (p <1): Type I ELM affected depth (LELM) unchanged with ELM size (n and q95) - Depth smaller only for Type III ELMs

Low H-modes – n scan Low H-modes – q95 scan

[Loarte PPCF 2002, PoP 2004]

ELM affected depth – high p

• High p Grassy ELMs reduction of LELM Change of MHD?

• Correlation of LELM with p not observed for the low li H-modes,

High p/high li H-modes High p/low li H-modes

Jedge and Grassy ELM onset

• Link between Grassy ELMs and high p ?– Pedestal stabilisation by Shafranov

shift (s) both high & low li have similar s

– The current profile is much broader in the low li pulses, for the same p

• Higher edge current (or lower shear) at low li may change the pedestal MHD stability Type I

• Collisionality? High li *~0.4, Low li *~0.2 (JT-60U *~0.1)

• Caveat: high li equilibrium near to DN, the low li are pure SN

p – */q operational space

• AUG sim – Type II q95 =4.2

• QDN – SN Type I-II q95=3 – 3.6

• QDN grassy, high p – q95=6.7

• SN high p Type I – q95=7.7

AUG high p (q95~6.3) SN vs QDN

• ASDEX-upgrade equilibria – H-modes with p~2

AUG: “grassy” ELMs favored by QDN [J Stober, IAEA 2004]

SNmixed Type I-Grassy

SN

QDN

QDN pure Grassy ELMs

* ped similar for SN and QDN

Conclusions (1)

• High plasma shaping (, , QDN) common element to all small ELM experiments in JET:

• At low p, mixed Type I-II ELMs are observed in SN and QDN – increasing q95 closes off access to high nped and no Type II ELMs.

• Type II: MHD/n broadband fluctuations: WB modes increased transport

• High p: “threshold” similar to JT-60U, but

– Grassy ELMs in QDN – high nped (*~0.4) – high li– Type I ELMs in SN, lower nped (*~0.2) – lower li.

• Asdex-U results (improved H-modes and, more recently, high p H-modes) “continuum” between the two type of small ELMs (*, but MHD?) with QDN still essential.

Conclusions (2)

• Role of QDN to obtain steady state Type II ELM pedestal:

– Type II ELMs obtained in JET in an identity (= high QDN) with Asdex-U: Type II ELM phases correlated to enhanced MHD (and n) fluctuations (with *~2).

– Enhanced particle losses obtained – power losses still rather weak (Pin effect)

– Increasing Pin or Ip/Bt Type I ELMs come back

– High * and/or low Tped (resistive MHD) may be necessary to Type II ELMs onset and total Type I ELM suppression.

– Insight in ELM physics (in particular role of magnetic geometry) but no direct extrapolability to hot plasmas.

Conclusions (3)

• “Grassy” ELMs obtained at high p – ELMs with low energy losses obtained in high p H-modes – with

H98~1.2 and n/nGr~0.9 (demonstrated at q95~7)– The reduction of ELM size correlated to shrinking of the ELM

affected depth: change in MHD unstable modes?– In JET, high p is not sufficient to obtain grassy ELMs

• The operational space for Grassy ELMs still to be explored– Is high q95 a necessary condition? And QDN shape? – Low edge current/high shear required for Grassy ELM onset?– Difference in *: does it explain the low vs high li difference in

ELM behaviour observed in JET?• Future work:

– (Higher Ip) experiments at low *! as well as QDN SN – explore lower q95 scenarios and systematically investigate role of li.

Type II ELM Increased transport and MHD turbulence

• When pedestal ~identity (62430) long Type II ELM phase associated with increased MHD turbulence (low frequency)

• n fluctuations up as well – Similar to Asdex-Upgrade

Core MHD

Type II ELMs (62430) Type I ELMs (62428)