Subject: Transport Experiments in the Scrape-Off Layer during Ohmic L- and H-mode using Gas-Injection
Plumes
From: S. Gangadhara, B. LaBombard
Group: Divertor/Edge
Date: 6/00
Approved by: Date Approved:
MP No.
276
Include immediate goal of the experiments, scientific importance and/or programatic relevance.
Refer to any relevant program milestones.
The goal of these experiments is to directly determine local impurity transport in the scrape-off layer of a diverted tokamak plasma during Ohmic L-mode and H-mode discharges by injecting trace amounts of gaseous impurities through a capillary mounted on a fast-scanning probe (a.k.a, the “fast-burping probe”) and viewing the dispersal of the impurities by visible imaging.
Discuss Physics basis of the proposed research, Prior results at Alcator or elsewhere, and any related work being carried out separately.
Impurity transport in the scrape-off layer (SOL) is of fundamental importance in tokamak plasmas. It is clear that the physical processes involved must be fully characterized and understood in order to predict core impurity levels in future tokamak reactors. A direct means of inferring information about impurity transport in the edge plasma is to inject impurities locally into the SOL and study the resultant impurity dispersal patterns
(“plumes”). Such studies have been previously conducted on Alcator C-Mod using fast piezo valves and the neutral injection array (NINJA) of capillary tubes, from which local parallel flows and cross-field drifts were assessed [1]. However, these studies were limited to plasmas in the far SOL, near the wall. In addition, impurity line emission was viewed with a single camera system, providing no information about the transport of impurities across flux surfaces.
A novel new design for the F-bottom reciprocating fast-scanning probe was developed prior to the 1997 run campaign, allowing for injection of gas through the probe tip [2].
With this design, impurity atoms can be injected locally at variable position in the SOL, including deep into the SOL, up to the separatrix. Plumes generated at different depths

in the SOL can yield information about profiles of local parallel and cross-field transport.
In addition, the local plasma density and temperature, and estimates of the local parallel and cross-field flows, are measured directly at the injection location via the probe. These data can be used in modeling the resulting impurity emission plumes, imaged from two near-perpendicular camera views.
During the 1998-99 run campaign the “fast-burping probe” was used to inject trace amounts of ethylene ( ∼ 5 x 10
16 molecules of C
2
H
4 per puff) into SOL plasmas for plume imaging studies. Parallel and cross-field transport were inferred from observed asymmetries and spreading (relative to the injection location) in both C
+1 and C
+2 emission [3].
However, information was unavailable on the (neutral) source profiles, leaving ambiguity on how to model these results. This has motivated the development of a beam-splitter system for aquisition of C
+1 and C
+2 plume data simultaneously. With a fully aligned and calibrated system, comparisons of parallel and cross-field profiles between the two charge states allows for direct estimation of transport parameters, and can be used as inputs for impurity transport modeling by the LIM code [4].
It has been observed on Alcator C-Mod that cross-field transport in the SOL increases with distance from the separatrix [5]. Coincident with this observation is one that at a fixed location in the SOL (in ρ space), transport (as parameterized by an effective D
⊥
) varies as a function of the local collisionality. Plume studies offer a unique means for confirming these observations, since plumes can be generated at various locations in the
SOL by varying the FSP injection depth. As the depth is varied, cross-field spreading of the plumes should also vary. Collisionality effects can be tested by varying, over a series of plasma discharges, the density into which gas injection takes place.
During H-mode it has been observed that plume emission is strongly dispersed in the cross-field direction, due to large E x B flows [3]. Also, as the H-mode character changes (i.e. from ELM-free to EDA) the character of this dispersal seems to change.
Measurements of this dispersal can be used to estimate the local value of E r
, and this value can be compared with estimates calculated using FSP data. Plume dispersal also indicates the directionality of E r
, simply based on the direction of the dispersal. Near the separatrix, where E r is expected to change sign, the plumes can thus serve as a clear indicator of this change, providing an advantage over the FSP, where E r must be inferred from local temperature and floating potential measurements, both of which vary strongly in this region in such a way as to make E r the difference between two large numbers. Thus, in H-mode the plumes can yield additional information on the local electric field structure.
Describe the methodology to be employed; explain the rationale for the choice of parameters, etc. Describe the analysis techniques to be employed in interpreting the data, if applicable. If the approach is standard or otherwise self-evident, this section may be absorbed into the Experimental
Plan.
Experiments will be conducted during ohmic conditions, to obtain optimal FSP data.
We wish to investigate: (1) the dependence of cross-field spreading (i.e. cross-field transport) of C
+1 and C
+2 emission at fixed locations in the SOL as a function of density (or collisionality) during Ohmic L-mode, and (2) the dependence of cross-field transport on
2

SOL location ( ρ ) for fixed discharge conditions during both Ohmic L-mode and Ohmic
H-mode.
Detailed modeling of impurity plume dispersal and global impurity screening/transport experiments will be performed using the LIM impurity transport code [4]. Analysis of plume dispersal to extract the magnitude of cross-field and parallel transport rates will also be performed by developing an algorithm for performing a 3-D tomographic reconstruction of the plume data [3,6].
The upgraded imaging system will be tested first in a piggy-back run mode. This will allow us to optimize gas puff rates and camera exposure times to achieve non-perturbative injections with good signal-to-noise, and to determine if neutral density filters are required to achieve similar dynamic range for both CII and CIII emission viewed simultaneously on the same CCD. Once this parameter space has been mapped out, we will be ready for two
(2) dedicated runs, one an Ohmic L-mode collisionality scan and one an Ohmic H-mode investigation.
These experiments will also serve as a good opportunity for other diagnostics investigating transport in the scrape-off layer – in particular the new turbulence-imaging system looking at turbulent eddy and filament structure in the far SOL. As discharges conditions vary, changes in the plumes can be compared with changes in the eddy structure, as well as fluxes inferred from the FSP and ASP, providing a better understanding and characterization of transport in the edge plasma.
4.1 Machine and Plasma Parameters
Give values or range for :
Toroidal Field: 5.3 T for Ohmic L-mode run; ramping field for Ohmic H-mode run.
Plasma Current: 0.8 MA
Working gas species: deuterium
Density: 0.6 - 1.4 x 10
20 m
− 3
Equilibrium configuration ( if possible, refer to database equilibria
): Lower X-point diverted discharge similar to shot 960208015 with 2 cm inner and outer gaps, strike points on outer divertor probe #2 and inner divertor probe #1, and with optimized fast-scanning probe target position.
Pulse length, typical current & density waveforms, etc.
Refer to database or sketch desired waveforms
:
Current flattop to a minimum of 1 sec.
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4.2 Auxiliary Systems
RF Power, pulse length, phasing: none, but RF conditioning could be done near the end of the shot.
Pellet Injection (species): none.
Impurity blow-off injection: none.
Diagnostic Neutral Beam: none, but can run if they need to.
Special gas puffing: ethylene puffing through FSP NINJA plenum.
Other: none.
4.3 Diagnostics
List required diagnostics, and any special setup or configuration, e.g. non-standard digitization rate.
Fast-scanning probe (FSP)
(1) Setup FSP gas plenum to deliver C
2
D
4 to scanning probe drive.
(2) Adjust LCFS target, probe insertion depth.
(3) Setup East/West probes in sweeping mode to measure n e
, T e
, Mach flow and North/South probes in floating mode to measure fluctuations, fluctuation-induced fluxes.
F-top and F-side camera/beam-splitter systems
(1) Use ECDC to obtain camera alignment snaps before and after the run.
(2) Install CII, CIII filters in beam-splitter systems.
(3) Setup optical video storage system to record and archive images.
(4) Provide for external trigger and video synchronization.
A-side Scanning Probe (ASP)
(1) Setup East/West probes in sweeping mode to measure n e
, T e
, Mach flow and North/South probes in floating mode to measure fluctuations, fluctuation-induced fluxes.
Turbulence-Imaging Diagnostic
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Both sections must be filled in.
5.1 Run sequence plan
Specify total number of runs required, and any special requirements, such as consecutive days, no Monday runs, extended run period (10 hours maximum), etc.
A total of three (3) run days are required. The first day will be for scoping studies and testing of the upgraded system, and can be carried out in piggy-back mode. One
(1) Ohmic L-mode run day (during which a density scan will be carried out) and one (1)
Ohmic H-mode run day are then required as dedicated runs.
5.2 Shot sequence plan
For each run day, give detailed specification for proposed shot sequence: number of shots at each condition, specific parameters and auxiiary systems requirements,etc. Include contingency plans, if appropriate.
Piggy-Back Run Tests
To optimize gas injection levels, camera exposure settings, and FSP insertion depths for simultaneous capture of CII and CIII using C
2
D
4 injection, a number of piggy-back discharges will be required. This will serve as a scoping study of the optimal operating space for obtaining plume data.
Run Day 1: Ohmic L-mode density scan
A density scan will be conducted to determine if plume character varies with edge collisionality. “Shallow” (in the far SOL) and “Deep” (near the separatrix) scans will be conducted to characterize transport variation with ρ .
I p
= 0.8 MA, B
T
= 5.3 T
Shot plan :
1-6 NL04=1.0e20, setup FSP target, gas, optimal camera exposures
7-8 NL04=1.0e20, “shallow” and “deep” FSP insertion, C
2
D
4 plumes
9-11 NL04=0.8e20, “shallow” and “deep” FSP insertion
12-14 NL04=0.6e20, “shallow” and “deep” FSP insertion
15 NL04=1.0e20, “deep” insertion of FSP
16-18 NL04=1.2e20, “shallow” and “deep” FSP insertion
19-21 NL04=1.4e20, “shallow” and “deep” FSP insertion
Run Day 2: Ohmic H-mode
At constant discharge conditions, the depth of injection will be varied to determine transport variation with ρ during H-mode. Timing of the injection will also be varied to catch various phases in the discharge, e.g. L-mode, L-H transition, ELM-free H-mode, EDA
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H-mode.
Set-up B-field ramp so as to obtain Ohmic H-mode plasmas similar to 1000518022, but staying at I p
= 0.8 MA. Start with NL04 = 0.8e20.
Shot plan :
1-6 Setup FSP target, gas, optimal camera exposures
7-12 Vary FSP timing, inject into far SOL
13-17 Vary FSP timing, inject into near SOL
18-20 Vary FSP timing, inject near and/or past separatrix
Discuss possible experimental outcomes and implications Indicate if the program may be expected to lead to publications, milestone completions, improved operating techniques, etc. Indicate if the experiments are intended to contribute to a joint research effort, or an external database.
Results of these experiments will allow us to:
(1) Measure parallel flows in the SOL by plume analysis and Mach probe
(2) Infer cross-field flows from plume analysis, Mach probe and E x B estimates
(3) Test parallel Langmuir-Mach probe theory
(4) Determine how cross-field transport varies with local collisionality by measuring crossfield spreading of CII, CIII emission at various values of collisionality
(5) Determine how cross-field transport varies with cross-field coordinate by measuring cross-field spreading of CII, CIII emission at various ρ
(6) Determine if E r becomes negative at the separatrix in H-mode plasmas
(7) Infer local D
⊥ and v
⊥ from 3-D reconstruction of plume emission
(8) Provide experimental data to constrain LIM transport modeling
Include references both to external and internal literature or communications which bear on this proposal. See Section 2.
[1] D. Jablonski, Doctoral Thesis, MIT Nuclear Engineering Department (May 1996),
PFC/RR-96-3.
[2] B. LaBombard, et al., PSI 1998.
[3] S. Gangadhara, et al., PSI 2000.
[4] P.C. Stangeby, C. Farrell, S. Hoskins, et al., Nucl. Fus. 28 (1988) 1945.
[5] B. LaBombard, et al., PSI 2000.
[6] C. MacLatchy, et al., APS 1999.
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