The most recent impurity transport studies in RFX-mod are presented. Results from enhanced confinement quasi-single helicity (QSH) and multiple helicity (MH) are compared. The transport parameters are determined by comparing a 1-dimensional collisional radiative simulation with experimental spectroscopic data. Transport parameters obtained for Ni particles injected by the laser blow-off method are described and discussed in relation to results from Ne injection and previous studies

A Alfier

A Fassina

F Auriemma

L Carraro

M E Puiatti

M Valisa

P Franz

S Menmuir

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#26607 (a) Bt (1,−7to−11) [mT]
         
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Impurity transport studies in multiple helicity and enhanced confinement 
QSH regimes in RFX-mod 
 
S. Menmuir, L. Carraro, A. Fassina, A. Alfier, F. Auriemma, P. Franz, M.E. Puiatti,  
M. Valisa 
 
Consorzio RFX, Associazione Euratom-ENEA sulla Fusione, Padova, Italy 
 
The most recent impurity transport studies in RFX-mod are presented.  Results from 
enhanced confinement quasi-single helicity (QSH) and multiple helicity (MH) are compared.  
The transport parameters are determined by comparing a 1-dimensional collisional radiative 
simulation with experimental spectroscopic data.  Transport parameters obtained for Ni 
particles injected by the laser blow-off method are described and discussed in relation to 
results from Ne injection and previous studies.   
Introduction The high plasma current discharges (Ip ≥1MA) in the reversed field pinch 
device RFX-mod are characterised by intermittent QSH phases, marked by the presence of a 
thermal island, that occupy a significant fraction of the discharge.  There is a strong increase 
of the electron temperature (Te) in the QSH island and steep gradients consistent with internal 
transport barriers [1].   
A series of experiments were carried out injecting Ni into the RFX-mod plasma through 
the laser blow-off (LBO) technique.  Signals from a number of spectroscopic diagnostics were 
monitored, including Ni XVII (249Å) and Ni 
XVIII (292Å) line emission and soft X-ray 
(SXR) time evolution and radial brightness 
profile reconstructions – normalised to a pre-
LBO profile to remove the C and O 
contribution (assumed constant during the 
LBO).   
In order to estimate the impurity (Ni) 
diffusion coefficient D and pinch velocity v, 
the experimental signals during the LBO pulse 
have been simulated with a 1-dimensional 
collisional-radiative model [2].  Experimental 
electron temperature and density are input to 
the code along with an influx time behaviour 
Figure 1: Discharge #26607 (1.55MA, LBO at 
85ms): (a) Bt m=1, n=-7 to -11; (b) ne; (c) Te; (d) 
central chord SXR and simulation (dashed); (e) 
NiXVII 249Å (thick) and NiXVIII 292Å line emission 
with respective simulations (dashed). 
36th EPS Conference on Plasma Phys. Sofia, June 29 - July 3, 2009 ECA Vol.33E, P-2.184  (2009)
Figure 3: Profiles of D and v 
used in the QSH case simulation.  
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Figure 2: Reconstructed experimental SXR 
profiles (*) for #26607 and their simulations.  
 
Figure 4: Discharge #26613 (1.3MA, LBO at 
85ms): (a) Bt m=1, n=-7 to -11; (b) ne; (c) Te; 
(d) central chord SXR and simulation (dashed). 
 
Figure 3: Profiles of D and v 
used in the QSH case simulation.  
         
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based on the Thomson spectrometer signal on a chord looking to the LBO port.  The 
simulation is repeated with different D and v radial profiles in order to find a reasonable 
reproduction of the experimental data. 
Ni transport in QSH   Figure 1 depicts a LBO during 
the flat-top phase of a long-lasting QSH period, 
allowing the whole Ni particle evolution to occur 
before the QSH crash.  The code is able to well 
reproduce the time evolution of the Ni line and central 
chord SXR emissions.  Figure 2 demonstrates that, in 
the simulation and experiment, immediately after 
LBO the SXR profiles are hollow and then return to 
being centrally peaked.  The determined profiles of D and v 
are shown in figure 3.  Calculating weighted residuals from 
the simulation of central SXR, Ni XVII 249Å and SXR 
profiles, simulation and experiment agree within ~15%.   
A significant feature is that v is directed outward over 
the whole plasma with an increased velocity barrier in the 
outer part of the plasma.  This velocity opposes the penetration of Ni particles into the plasma 
core causing the Ni to remain mainly near the edge.  Results from the SXR inversion confirm 
this with the initial edge peak due to Ni decaying without progressing into the core. 
A further feature of the profiles is that the low D, good confinement, region at the edge 
extends over nearly half of the plasma.  This is a change from previous results for C and O in 
lower plasma current [3] where the low D 
region at the edge was much smaller. 
A simple variation test shows that the 
presence of a barrier in v gives better 
consistency than simulations omitting the 
barrier or with an edge reversal (inflow).  
Similarly the transition in D should be located 
at the barrier, however, some uncertainty as to 
the exact values of D and v remains.   
Ni transport in MH  Other discharges were 
obtained with LBO during different phases of 
QSH and in MH conditions.  The latter case is illustrated in figures 4 and 5 where the 
36th EPS 2009; S.Menmuir et al. : Impurity transport studies in multiple helicity and enhanced confinement QS... 2 of 4
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MH
Figure 7: Edge Te profile for #26607 
(●) and #26613 (□).  
 
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reconstructed SXR profiles immediately following the 
LBO are not hollow as in the QSH case.  The central 
SXR and Ni line decays are slower – a 12ms time 
constant exponential decay compared to 8ms for the 
QSH case.   
Different impurity transport coefficients as in 
figure 6 are needed.  The main difference in the MH 
parameters is that the good confinement edge region 
does not extend as far into the plasma.  The velocity 
barrier is weaker and is also shifted outwards.  The more 
external D transition in MH is likely reflective of the 
different Te profile – figure 7 – which is flat to a large 
radius and then drops rapidly; in the QSH case Te profile 
was more peaked.  
The Ni total density profiles at steady-state have been 
calculated by extrapolating the best simulations of the QSH and MH cases, assuming in both 
cases constant Te and ne (as measured at LBO in the 
QSH discharge) and constant Ni influx.  The profiles 
(figure 8) show that there is a very effective barrier to 
the Ni penetration in the QSH case whilst in MH the 
particle level is higher in the core and extends further 
out.  The resulting SXR time evolutions are very 
alike suggesting the experimental differences may be 
partly due to the very different plasma parameters in 
the two cases.  We plan to increase our understanding 
of this by extending our database of LBO discharges in 
the next campaign.  
The experiments seem not to indicate a significant 
difference in the confinement in the core for the QSH 
with respect to the MH regime.  For the main gas there 
are indications of better confinement inside the hot 
helical region.  The different behaviour of the two 
species finds a correspondence in the Monte Carlo ORBIT code results which attribute the 
Figure 8: Stationary case total Ni 
population for QSH (full) and MH 
(dashed) D and v. 
Figure 5: Reconstructed experimental 
SXR profiles (*) for #26613 and their 
simulations. 
Figure 6: MH case D and v. 
36th EPS 2009; S.Menmuir et al. : Impurity transport studies in multiple helicity and enhanced confinement QS... 3 of 4
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 (c) ne [1019m-3] 
Figure 9: Discharge #26361, Ne puffing at 140ms: (a) central SXR; (b) Ne radiated 
power, both with simulation (blue); (c) time evolution of ne profile. 
reproduction of the differences to the very different collisionalities [1,4].  At this moment 
there is only qualitative agreement with the ORBIT Ni results due to the order of magnitude 
difference in D obtained from experiment – more extensive analysis of this is ongoing. 
Ne impurity transport  Complementary experiments using neon gas puffing (figure 9) were 
also carried out at RFX-mod.  Ne is a recycling impurity and so remains in the plasma 
throughout the 
discharge. The 
QSH regime is 
initially disturbed 
as Ne causes the 
density and 
radiated power 
to climb, however, the typically intermittent QSH behaviour is quickly re-established.  Using 
the Ni D and v from the QSH case above, we are able to reproduce the step increase in the 
SXR and radiated power signals.  The density profile before and after the injection are 
reproduced.  Ne is also confined in the edge region.   
Conclusions The latest results of impurity transport analysis on RFX-mod are presented.  
Spectroscopic measurements from Ni laser blow-off and Ne puffing experiments were 
simulated with a 1-D impurity transport code.  For Ni LBO into QSH the good confinement 
lower diffusion coefficient region at the edge extended further into the plasma than in 
previous analyses.  An outward pinch velocity with large barrier in the external plasma and no 
reversal was found which acts strongly against Ni reaching the core.  The transition region in 
D and v was determined to be closer to the edge for the MH plasma and the barrier weaker.  
The position of the barrier appears to be linked to that of the Te gradients.  Particle transport 
and confinement seemed to be less influenced by the magnetic topology than by the plasma 
conditions of temperature and density.  The Ni transport parameters of QSH regime were also 
able to reproduce the Ne transport within acceptable margins.  That the same transport can be 
acceptable for both Ni and Ne impurities despite their mass difference suggests that we can 
also extend this to the intrinsic C and O impurities.  In QSH the impurities do not accumulate. 
Acknowledgement This work was supported by the European Communities under the contract of Associations 
between EURATOM/ENEA.  One of the authors (S.M.) gratefully acknowledges the financial support from 
Vetenskapsrådet.  
References [1] Carraro et al., Nuclear Fusion 49 (2009) 055009 [2] Mattioli et al., PPCF 44 (2002) 33 
[3] Carraro et al., 33rd EPS Conf. Rome (2006) P5.083  [4] Carraro et al., IAEA (2008) EX/P3-9  
 (a) central SXR [W/m2]  (b) Ne radiated power [MW]  
36th EPS 2009; S.Menmuir et al. : Impurity transport studies in multiple helicity and enhanced confinement QS... 4 of 4


Impurity transport studies in multiple helicity and enhanced confinement QSH regimes in RFX-mod

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Impurity transport studies in multiple helicity and enhanced confinement QSH regimes in RFX-mod

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