Determining pellet penetration Determining pellet penetration depth using video diagnosticsdepth using video diagnostics

Tamás SzepesiKFKI-RMKI, Budapest, Hungary

21 April 2006

Contents

1. Motivation2. The pellet injector and the video diagnostics systems

3. Penetration depth

• Definitions

• Comparison

4. Future work and outlook

1. Motivation

Plasma edge control by ELM triggering

- ELM: MHD instability at the plasma edge→ fast (~ms) particle and energy loss→ type-I ELM: at strong heating, fELM ~ Pheat

- type-I ELMy H-mode: stable, robust→ WMHD, ne ≈ const.→ stable level impurity concentration ⇒ Prad ≈ const.⇒ candidate for reactor operation modeBUT: fELM · ΔWELM = const. and ΔW scales with machine size→ ITER: max. 5-10 MW / m2 peak load on divertor plates – critical [Lang, Nucl. Fusion 2004]

- ELM control (mitigation) by cryogenic D pellet injection

→ each HFS injected pellet triggers its own type-I ELM

→ sufficiently high injection frequency ⇒ full control over ELMs (no intrinsic ELMs)

→ the empirical formula seems to hold for triggered ELMs as well

[H. Zohm, PPCF 36, 105-128.]

1. Motivation

Aims

- to localize pellet when the ELM is triggered→ determination pellet trajectory (long exp. images)→ time stamps of the trajectory (multiple short exp.)

⇒ location of pellet caused perturbation can be found(ELM onset time from Mirnov signals)

- to study penetration depth and ablation of the pellets

→ extend the presently existing database [Belonohy] of HFS injected pellets (ASDEX Upgrade) with penetration depth calculations from video diagnostics

→ find correction formulae to calculate the „real” penetration from the ablation monitor signal

→ find a scaling of the penetration depth to extrapolate to larger machines e.g. ITER

2. The pellet injector systems

centrifuge system

• pellet speeds:

240, 600, 880 and 1000 m/s

• pellet mass: ~ 1020 D atoms

• max frequency: ~80 Hz

Leidenfrost-gun system

• pellet speed: 80 – 300 m/s

• pellet mass: ~ 1020 D atoms

• max. frequency: 100 Hz (with 90% reliability)

[Lang]

[Cierpka]

Leidenfrost-gun

[Kocsis EPS 2004]

2. The video diagnostics system

- up to 3 fast framing cameras may havethe same view

→ the same pellet can be observedwith different exposure schemes

- typical exposure schemes:

→ long exposure:‘bright’ imagewhole pellet trajectory visibleno time stamps of the path

⇒ penetration, calibration, pellet trajectory

→ multiple short exposure:‘dark’ imageparts of the trajectory visiblecarries temporal information

⇒ pellet speed, trajectory with temporal info

2. The video diagnostics system

side view, long exposure

injection lineinjection line

side view, multiple short exposure

HFS LFS HFS LFS

2. The video diagnostics system

top view, long exposure

injection lineinjection line

top view, multiple short exposure

HFS

LFS

HFS

LFS

3. Penetration depth

Definitions (1)

- penetration depth from the ablation monitor signal (λabl)

→ time-of-flight (TOF) measurement: λabl = vpel · Δt

☺ relatively easy to evaluate

☺ available for all pellets

assumes vp = const.

assumes straight pellet path

no spatial information

difficult to identify fragmentation

Time [s]

Ablation M

onitor [au]

Δt ~ 1 ms

[Belonohy diploma]

3. Penetration depth

injection line

injection tube end

separatrix

separatrix intersection

projected penetration

separatrix penetration

Definitions (2)

- penetration depth fromvideo diagnostics (λv)

complicated calibration → difficult to evaluate

not available to all pellets (max. 5 Hz repetition rate)

assumes no toroidal movement

☺ detailed spatial information on pellet trajectory in real and magnetic coordinates

☺ fragments easier to see (multiple short exp.)

3. Penetration depth

Comparison (1)

- fast pellets (1000 m/s)

→ often fragmented

→ fragmentation usually difficult to recognize (usually not detected)

→ straight trajectory

⇒ TOF measurement tends to give higher values

?

3. Penetration depth

Comparison (2)

- slow pellets (240 m/s)

→ less fragmentation

→ curved trajectory (acceleration!)

⇒ video measurement tends to give higher values (TOF does not account for radial acceleration)

⇒ a correction formula can be defined

projected penetration (+) in good agreement with the TOF measurements

0)()sin(v)(

1)cos(v)(

00

100

=+=

+++= +

ttztz

ta

btRtR a

θφ

φ

[Kálvin]

What is the ‘real’ penetration depth? – a physically relevant quantity should be defined

4. Future work

Pellet trajectory fitting

- to fit parameters of the curved pellet trajectory:

• a, b, φ

⇒ real path length of the pellet can be determined

⇒ to find the dependence of these parameters on the plasma parameters (esp. Te)

0)()sin(v)(

1)cos(v)(

00

100

=+=

+++= +

ttztz

ta

btRtR a

θφ

φ

[Kálvin]

4. Future work

Measurements with the Leidenfrost-gun

- top view very useful, but ~2m distance

→ lower resolution, less bright

⇒ more difficult to calibrate (distortion)

⇒ need compromise in calibration (or maybe optics upgrade)

Thank you for your attention!

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- How much for a beer?

The bartender looks at the neutron and answers: - For you? Free of charge!