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Princeton Plasma Physics Laboratory

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Princeton Plasma Physics Laboratory

    Princeton Plasma Physics Laboratory

    NSTX Experimental Proposal Title: Study of the correlation between GAE activity and electron transport

    Effective Date:

    (Approval date unless otherwise stipulated) Revision: OP-XP-822

    Expiration Date:

    (2 yrs. unless otherwise stipulated)

    PROPOSAL APPROVALS

    Responsible Author: D. Stutman Date 06/18/08 ATI ET Group Leader: S. Kaye Date RLM - Run Coordinator: M. Bell Date Responsible Division: Experimental Research Operations

    Chit Review Board (designated by Run Coordinator)

    MINOR MODIFICATIONS (Approved by Experimental Research Operations)

    NSTX EXPERIMENTAL PROPOSAL

    TITLE: Study of the correlation between GAE activity No. OP-XP-

    and electron transport

     DATE: 06-18-2008

    AUTHORS:

    D. Stutman, L. Delgado-Aparicio, K. Tritz, M. Finkenthal (JHU) N. Gorelenkov, E. Fredrickson, S. Kaye (PPPL)

1. Overview of planned experiment

     A large increase in central electron transport with beam heating power was observed in NSTX H-

    modes, by changing the beam power at fixed q-profile (XP 612). This increase appears to be correlated

    with a quantitative and qualitative change in the Global Alfven Eingemode (GAE) activity. To strengthen

    and document this very important observation we propose to run an experiment in which we compare

    electron transport in plasmas with and without GAEs. We will use three different scenarios to make this

    comparison:

    (i) Create discharges heated by equal beam power, P but at different beam voltage, V bp(ii) Add to a baseline discharge with low GAE activity additional beam power at increasing V b

    (iii) Evaluate how RF heats the plasmas with most/least GAE activity, obtained in (i) and (ii) above

     These scenarios will be run in H-mode, the main NSTX operating regime.

     In addition, time permitting, we will attempt to use slow ramp L-modes in order to cross calibrate

    the density fluctuation amplitude measured by high-scattering with that measured by reflectometry.

    The estimated run time is one day.

    2. Theoretical/empirical justification

     Electron transport is the dominant loss channel in beam heated NSTX plasmas. An unusual effect

    is that the T profile flattens and the central ? strongly increases with increasing beam power in NSTX H-ee

    modes (Fig. 1a). The TRANSP sensitivity analysis, a host of perturbative electron transport experiments,

    as well as the recent FIDA data, indicate that the flattening is a genuine electron transport effect and not

    the result of a broadening of the heating (i.e., beam ion density) profile by MHD activity. Furthermore,

    the large central ? at high P in Fig.1a, together with a number of other observations, suggest that we are eb

    dealing with electron transport along stochastic magnetic field lines. (See link below for details).

    (nstx.pppl.gov/DragNDrop/Topical_Science_Groups/Meetings/transport_turbulence/2008/5-22-08/JHU_Joint_Meeting_v1.pdf)

    OP-XP- 2 / 7

     The main puzzle is thus what is causing fast electron

    transport in the central NSTX plasma in the absence of a significant 22??(m(m/s)/s)ee

    Tgradient (or for that matter of any other significant thermal e 100100

    gradient). A logical (if unexpected) answer may be that the free-6 MW6 MWa)a)4 MW4 MWenergy needed to drive this transport comes from the gradient of non-5050

    thermal particles. Indeed, the TRANSP analysis indicates that the 22

    fast ion density has by far the strongest gradient in the region r/a ?

    r/ar/a0.4-0.5.

    6 MW6 MW Furthermore, a good ‘mediating agent’ between the fast ion

    gradient and the electron transport may be expected to be the

    persistent Alfven Eigenmode MHD activity driven by the fast ions in GAEGAE

    NSTX. In particular, shear Alfven Eigenmodes (*AEs) have been TAETAE

    early on predicted to be able to induce electron transport through µ-4 MW4 MW

    tearing of the flux surfaces (see e.g., Lee, Okuda and Chance PRL b)b)

    1981).

     We therefore searched for a correlation between changes in

    central ce and changes in AE activity. A quite compelling correlation

    2 MW2 MWwas found with the GAE (Global Alfven Eigenmode) activity. As

    recently shown at NSTX, these are high-n modes localized in the

    central plasma and have a substantial shear component [see e.g., N.

    Gorelenkov et al., E. Fredrickson et al.]. The correlation is illustrated

    in Fig. 1b, which shows that plasmas having high central ? have e

    also intense, broadband GAE activity, while plasma with lowest Fig. 1Fig. 1??behavior (a) and behavior (a) and eeGAE behavior (b) as a GAE behavior (b) as a transport is essentially GAEs free. Furthermore, the large ? increase efunction of beam heating function of beam heating

    power and at fixed-q. The power and at fixed-q. The for P>2 MW (Fig. 1a) suggests a threshold in the transition to bpower is stepped at 0.42s, power is stepped at 0.42s,

    from a steady 4 MW, to the from a steady 4 MW, to the stochastic electron transport, possibly correlated with a threshold in level indicated.level indicated.

    the GAE mode superposition, or ‘coalescence’, as seen in Fig. 1b

    also at P>2 MW. b

     Last but not least, the initial theoretical assessment of a possible GAE/electron transport

    connection is encouraging, in that multiple GAE modes seem to be able to induce stochastic transport of

    trapped electrons [N. Gorelenkov, preliminary]. If confirmed, this phenomenon could have deep

    implications for any burning plasma heated by a large population of fast beam ions and/or alphas.

OP-XP- 3 / 7

3. Experimental run plan

     The goal of the proposed experiment is to strengthen and better document the inferred connection

    between electron transport and GAE activity. To this end we propose to compare Tprofiles and electron e

    transport in three scenarios. They are primarily based on the fact that the GAEs, being excited by the high

    energy end of the fast ion distribution, have a threshold in beam voltage.

    (i) discharges heated by equal beam power, P but at different beam voltages, V bp

    (ii) baseline discharge with low GAE activity to which we add beam power at increasing V b

    (iii) RF heating of NBI plasmas with/without GAE activity

     Neon injection will be used in the first scenario to compare also particle transport.

     If possible, we will also try to obtain an estimate of the GAE density perturbation amplitude using

    high-k scattering, in conjunction with reflectometry. To this end we will cross-calibrate the high-k signal

    in terms of??n, using a comparison with reflectometry in slow-ramp, low density L-modes (see e.g., shot e

    112996). This calibration will then be used to estimate ?n in H-modes not accessible to reflectometry. e

     The XP will be run mostly in H-mode, for reasons of MHD stability at high power and for

    reproducibility. In addition, the elevated central-q typical of H-mode operation in NSTX, seems to favor

    GAE coalescence and rapid electron transport. As baseline condition we will use a recent, 4 MW, 0.9 MA,

    4.5 kG shot, nr. 129902. The run plan is as follows:

     Part I: Compare plasmas with same P but different V bb

    1. High V: A/90 + B/100 (2 shots) b

    2. Low V: A/90 + B/65 + C/65 (2 shots) b

    3. Decrease I ramp if q tends to reverse at low V (2 shots) pb

    4. Neon injection at high/low V (2 shots) b

     Part II: P step at increasing V bb

    5. Establish baseline A/90 + C/65 (2 shots)

    6. Step B at 450 ms: 60, 75, 90, 105 kV (8 shots)

     Part III: Compare RF heating in NBI deuterium plasmas with/w.o. GAEs

     7. Apply 2 MW RF to plasmas having largest difference in GAE content (4 shots)

     Part IV: High-k cross calibration with reflectometry (time permitting)

     8. High V slow-ramp L-mode (2 shots) b

    Total number of shots = 24

    OP-XP- 4 / 7

4. Required machine, NBI, RF, CHI and diagnostic capabilities

     (1) All neutral beams operational between 60 and 105 kV; required

     (2) RF with phasing optimal for heating in deuterium, P ?2 MW; required

     (3) High-frequency and low frequency Mirnov coils; required

    (4) MPTS at 16 ms spacing, with timing synchronized for a measurement at 450 ms; required

    (5) CHERS operational and synchronized for frame starting at 450 ms; required

     (6) MSE operational and synchronized for measurement starting at 450 ms; required

    (7) High-k scattering taking data at R?115 cm, if possible in ‘interferometric mode’; required

    13(8) Reflectometer 44.5 GHz frequency (3 10 cm-3 cutoff) ; required

     (9) Three-color tangential optical SXR array; required

    (10) USXR arrays in two-color configuration: Hor. Up Be10, Hor. Down - Be100; required

    (11) FIDA; desired

5. Planned analysis

    TRANSP, multi-color SXR, impurity transport, ORBIT, GS2.

    6. Planned publication of results

    Contributions to international conferences and in refereed journals.

    OP-XP- 5 / 7

    PHYSICS OPERATIONS REQUEST

    TITLE: No. OP-XP- AUTHORS: DATE: Machine conditions (specify ranges as appropriate)

    I(kA): 4.5 kG Flattop start/stop (s): -0.02/1.0 s TF

    I (MA): 0.9 Flattop start/stop (s): 0.12-0.22/0.8 sP

    Configuration: DN

    Outer gap (m): 0.05-0.10 Inner gap (m): 0.01-0.06

    Elongation ?: 2.25 Upper/lower triangularity ?: 0.6/0.6

    Z position (m):

    Gas Species: D, Ne Injector(s):

    NBI Species: D Sources: Voltage (kV): 60-105 Duration (s): 1s

    ICRF Power (MW): 2 Phasing: TBD Duration (s): 0.3

    CHI: Off Bank capacitance (mF):

    LITER: Off

    Either: 121135, 121172

    Or: Sketch the desired time profiles, including inner and outer gaps, ?, ?, heating,

    fuelling, etc. as appropriate. Accurately label the sketch with times and values.

    OP-XP- 6 / 7

    DIAGNOSTIC CHECKLIST

    TITLE: No. OP-XP-

    AUTHORS: DATE:

    Note special diagnostic requirements in Sec. 4 Note special diagnostic requirements in Sec. 4

    Diagnostic Need Want Diagnostic Need Want

    ? MSE Bolometer tangential array ?

    NPA ExB scanning Bolometer divertor

    NPA solid state ? CHERS toroidal ?

    Neutron measurements ? CHERS poloidal ?

    Plasma TV ? Divertor fast camera

    Reciprocating probe Dust detector

    Reflectometer 65GHz ? EBW radiometers

    Reflectometer correlation Edge deposition monitors

    Reflectometer FM/CW Edge neutral density diag.

    Reflectometer fixed f ? Edge pressure gauges

    Reflectometer SOL Edge rotation diagnostic

    RF edge probes Fast ion D_alpha - FIDA ?

    Spectrometer SPRED ? Fast lost ion probes - IFLIP ?

    Spectrometer VIPS ? Fast lost ion probes - SFLIP ?

    SWIFT 2D flow Filterscopes ?

    Thomson scattering ? FIReTIP

    Ultrasoft X-ray arrays ? Gas puff imaging

    Ultrasoft X-rays bicolor ? H? camera - 1D

    Ultrasoft X-rays TG spectr. High-k scattering ?

    Visible bremsstrahlung det. ? Infrared cameras

    X-ray crystal spectrom. - H Interferometer - 1 mm

    X-ray crystal spectrom. - V Langmuir probes divertor

    X-ray fast pinhole camera Langmuir probes BEaP

    X-ray spectrometer - XEUS Langmuir probes RF ant.

     Magnetics Diamagnetism ? Magnetics Flux loops ? Magnetics Locked modes Magnetics Pickup coils ? Magnetics Rogowski coils ? Magnetics Halo currents Magnetics RWM sensors Mirnov coils high f. ? Mirnov coils poloidal array Mirnov coils toroidal array Mirnov coils 3-axis proto.

OP-XP- 7 / 7

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