Study of TJ-II Density Limit Dependence on Magnetic Configuration

Informes Técnicos Ciemat
1335
Diciembre, 2014
Study of TJ-II Density
Limit Dependence on
Magnetic Configuration
M. A. Ochando
E. Ascasíbar
A. Baciero
F. Castejón
D. López-Bruna
J. M. Fontdecaba
M. Liniers
K. J. McCarthy
F. Medina
I. Pastor
B. Sun
B. Zurro
and the TJ-II Team
GOBIERNO
DE ESPAÑA
MINISTERIO
DE ECONOMÍA
Y COMPETITIVIDAD
Centro de Investigaciones
Energéticas, Medioambientales
y Tecnológicas
Informes Técnicos Ciemat
1335
Diciembre, 2014
Study of TJ-II Density
Limit Dependence on
Magnetic Configuration
M. A. Ochando
E. Ascasíbar
A. Baciero
F. Castejón
D. López-Bruna
J. M. Fontdecaba
M. Liniers
K. J. McCarthy
F. Medina
I. Pastor
B. Sun
B. Zurro
and the TJ-II Team
Laboratorio Nacional de Fusión por Confinamiento Magnético
Toda correspondencia en relación con este trabajo debe dirigirse al Servicio de Información y Documentación, Centro de Investigaciones Energéticas, Medioambientales y
Tecnológicas, Ciudad Universitaria, 28040-MADRID, ESPAÑA.
Las solicitudes de ejemplares deben dirigirse a este mismo Servicio.
Los descriptores se han seleccionado del Thesauro del DOE para describir las materias que contiene este informe con vistas a su recuperación. La catalogación se ha hecho
utilizando el documento DOE/TIC-4602 (Rev. 1) Descriptive Cataloguing On-Line, y la clasificación de acuerdo con el documento DOE/TIC.4584-R7 Subject Categories and Scope
publicados por el Office of Scientific and Technical Information del Departamento de Energía
de los Estados Unidos.
Se autoriza la reproducción de los resúmenes analíticos que aparecen en esta publicación.
Catálogo general de publicaciones oficiales
http://www.060.es
Depósito Legal: M-26385-2011
ISSN: 1135-9420
NIPO: 721-14-056-6
Editorial CIEMAT
CLASIFICACIÓN DOE Y DESCRIPTORES
S70
THERMONUCLEAR REACTIONS; TOKAMAK DEVICES; MAGNETIC CONFINEMENT;
TJ-II HELIAC; TORSATRON STELLARATORS
Study of TJ-II Density Limit Dependence on Magnetic Configuration
Ochando, M. A.; Ascasíbar, E.; Baciero, A.; Castejón, F.; López-Bruna, D.; Fontdecaba, J. M.;
Liniers, M.; McCarthy, K. J.; Medina, F.; Pastor, I.; Sun, B.; Zurro, B. and the TJ-II Team
4 pp. 6 ref. 5 figs.
Abstract:
In all stellarator and torsatron devices, a non MHD-driven plasma density limit appears that depends on the absorbed
power density and the magnetic field. The dominant mechanism that limits the maximum attainable density in steady
state is the thermal collapse. As in the case of reverse field pinches (RFP) and opposite to tokamaks, no current disruption can occur when the thermal collapse develops and, moreover, the plasma can restart if the auxiliary heating
is maintained.
Depending on plasma composition, thermal instabilities may originate at some point of the plasma column and easily
provoke radiative collapses. The use of wall conditioning techniques with low-Z (C, B or Li) materials or divertors to
reduce the average impurity radiation strength, and the injection of pellets to centrally fuel the plasma, led to notable
extensions of the density limit. These facts reinforce the common belief that the density limit is also an edge phenomenon, as in tokamaks and RFPs. In TJ-II, a medium-size flexible heliac with moderate heating power capability (<
2.6 MW), the main action to control and increase the electron density has been wall coating with low-Z materials. The
change from boron to lithium walls has yielded an increment of a factor of two in tauE and 1.6 in maximum electron
density at low input power. During the last campaigns, further efforts to improve the plasma-wall interaction have been
made to produce more stable plasmas at full power and to widen the operative magnetic configurations map.
Estudio del Límite de Densidad en el TJ-II y su Dependencia de la Configuración Magnética.
Ochando, M. A.; Ascasíbar, E.; Baciero, A.; Castejón, F.; López-Bruna, D.; Fontdecaba, J. M.;
Liniers, M.; McCarthy, K. J.; Medina, F.; Pastor, I.; Sun, B.; Zurro, B. and the TJ-II Team
4 pp. 6 ref. 5 figs.
Resumen:
En dispositivos de tipo stellarator y torsatron se encuentra una densidad límite que depende de la densidad de potencia absorbida y del campo magnético. El mecanismo dominante que limita la máxima densidad alcanzable en estado
estacionario no está relacionado con inestabilidades magneto hidrodinámicas, sino con el llamado colapso térmico.
Como en el caso de los dispositivos tipo reverse field pinch y al contrario de lo que ocurre en tokamaks, el colapso no
lleva asociadas disrupciones de corriente.
Dependiendo de la composición del plasma pueden originarse inestabilidades térmicas localmente, cuya evolución
desemboca en el colapso radiativo. Las técnicas para reducir el contenido de impurezas, como el acondicionamiento de
la pared de la cámara de vacío con recubrimientos de materiales de muy bajo número atómico (C, B o Li), la utilización
de divertores y la inyección de pastillas de hidrógeno para alimentar el plasma desde el centro, han permitido una notable
extensión del límite de densidad. En el TJ-II, un stellarator del tipo heliac, con potencia de calentamiento moderada
(hasta 2.6 MW), el recubrimiento con litio de la cámara de vacío previamente boronizada, ha permitido incrementar el
tiempo de confinamiento de la energía en un factor 2 y en 1.6, el valor de la densidad electrónica máxima. Asímismo,
se ha encontrado un régimen de operación en el que el plasma puede recuperarse tras el colapso y alcanzar mayores
densidades. La dependencia con la configuración magnética es muy débil.
.
.
41st EPS Conference on Plasma Physics
P2.074
Study of TJ-II density limit dependence on magnetic configuration
M. A. Ochando, E. Ascasíbar, A. Baciero, F. Castejón, D. López-Bruna, J. M. Fontdecaba,
M. Liniers, K. J. McCarthy, F. Medina, I. Pastor, B. Sun, B. Zurro and the TJ-II Team
Laboratorio Nacional de Fusión, CIEMAT, Madrid, Spain
Introduction.
In all stellarator and heliotron devices, a non MHD-driven plasma density limit appears that
depends on the absorbed power density and the magnetic field [1]. The dominant
mechanism that limits the maximum attainable density in steady state is the thermal
collapse. As in the case of reverse field pinches (RFP) and opposite to tokamaks, no current
disruption can occur when the thermal collapse develops and, moreover, the plasma can
restart if the auxiliary heating is maintained. Depending on plasma composition, thermal
instabilities may originate at some point of the plasma column and easily provoke radiative
collapses. The use of wall conditioning techniques with low-Z (C, B or Li) materials or
divertors [2, 3] to reduce the average impurity radiation strength, and the injection of pellets
to centrally fuel the plasma, led to notable extensions of the density limit. These facts
reinforce the common belief that the density limit is also an edge phenomenon, as in
tokamaks and RFPs. In TJ-II, a medium-size flexible heliac with moderate heating power
capability (< 2.6 MW), the main action to control and increase the electron density has been
wall-coating with low-Z materials [4]. The change from boron to lithium walls yielded an
increment of a factor of two in τE and 1.6 in maximum electron density [5] at low input
power. During the last campaigns, further efforts to improve the plasma-wall interaction
have been done to produce more stable plasmas at full power and to widen the operative
magnetic configurations map. After the recent installation of one liquid lithium limiter
(LLL), a marked reduction of carbon impurities has been achieved, especially in plasmas
with strong interaction with the vessel walls.
In this report, an updating of TJ-II database is presented, bringing to the foreground the
magnetic configurations that yield the larger increments over the Sudo limit [1].
Operation domain.
TJ-II is a four-period flexible Heliac with low magnetic shear and major and averaged
minor radii of 1.5 m and ≤ 0.22 m, respectively [5], and operated at B=0.95 T. For this
study, plasmas of different magnetic configurations were started with electron cyclotron
resonance heating (ECRH) (Pin ≈ 600 kW, 2 gyrotrons, at 53.2 GHz, 2nd harmonic, X-mode
41st EPS Conference on Plasma Physics
P2.074
polarization) and maintained with two tangentially injected neutral beams (co and counter)
delivering Pin ≤ 700 kW port through each. The considered ranges in the parameters of
interest are: input power density 0.33 - 1.55 MW/m-3; average electron density 2 - 8 x1019
m-3; minor radius 15.0 - 19.3 x10-2 m; iota2/3 1.33 - 1.63, and plasma volume 0.67-1.1 m-3.
7
0.016
6
lithium. This practice has enabled for the
!
0.012
5
Wdia (kJ)
tau exp (s)
Since 2008, TJ-II wall is regularly coated with
co + cnt NBI
cnt NBI
co NBI
co NBI + ECH
extension in plasma parameters range and
4
0.008
density
!
3
control.
comparison
2
Figure
between
1
shows
previous
the
[yellow
0.004
rectangle, see ref. 5] and present operation
1
00
00
20.004
4 0.008
6
0.012
8
0.01610
19
-3
tau
neISS04
(x10 xm0.5) (s)
!
Figure
1. Diamagnetic energy versus line
average electron density for the studied
discharges. The yellow rectangle represents the
last reported accessed operation region [5].
regions. Increments of about 1.5 in Wdia and
1.7 in ne have been obtained. The different
heating schemes for each shot are indicated in
the legend.
Comparison of TJ-II data with scalings.
Data from TJ-II were considered in a comparative inter-machine analysis to build the
unified scaling law for the energy confinement time in stellarators, the ISS04 [6]:
!
!EISS04 = 0.134a2.28 R0.64 P-0.61 ne0.54 B0.84 "-2/30.41!
tau exp (s)
0.016
In that analysis, data from ECRH-only
heated discharges were included, but with a
co + cnt NBI
cnt NBI
co NBI
co NBI + ECH
'high ripple' renormalization factor of 0.25.
For
0.012
the
present
set
of
NBI-heated
discharges, as can be seen in Figure 2, it is
0.008
apparent that the renormalization factor
0.004
must be relaxed in a factor of 2. Then, the
evident uneven behaviour of TJ-II with
0
0
0.004
0.008
0.012
0.016
tau ISS04 x 0.5 (s)
Figure 2. τE (exp) for NBI discharges vs. the intermachine energy confinement time τE (ISS04) x 0.5.
respect to other medium size stellarators,
ATF/CHS/H-E, [see Table 3 in ref. 6]
seems to be corrected.
This is due to the fact that the negative effect of large ripple on transport is reduced for
large collisionality. In fact, the comparison with the Sudo scaling, as is shown in Figures
3a and 3b, is rather good. However, it is observed that τE (exp) can surpass the limit for all
heating schemes, whilst ne (exp) seems to stay below the limit. Several plasma density
41st EPS Conference on Plasma Physics
!ESudo = 0.17a2 R0.75 P-0.58 ne0.64 B0.84!
a)
tau exp (s)
0.016
P2.074
b)
neSudo = 0.25(PB /a2 R)0.5
under fresh Li-coated
co + cnt NBI
cnt NBI
co NBI
co NBI + ECH
at constant magnetic
0.012
configuration and input
power to approach the
0.008
maximum density in a
0.004
controlled way. As an
0
0
0.004
0.008
0.012
0.016
tau Sudo (s)
example, the behaviour
Figure 3. Comparison of TJ-II data with Sudo limits [1] scalings: a) τE (exp)
vs. τE (Sudo) and b) ne (exp) vs. ne (Sudo). Note: ne’s are in 1019 m-3 units.
.
8
c)!
frequency (kHz)
7
nexp
6
5
4
4
3
3
2
2
1
1
H
1
2
3
4
5
6
L
7
8
nSudo
1.2
b)!
with
increasing
!"
100
50
density,
ι(a)=1.65,
and 390 kW co-injected
NB power is represented in
Figure 4. From densities
150
#"
100
50
labelled '3', plasmas with
this magnetic configuration
0
are prone to enter in H-
0.8
frequency (kHz)
nexp / ns
1
of a series of 15 shots
configuration
150
0
frequency (kHz)
a)!
scans were performed
0.6
0.4
0.2
150
$"
100
mode, characterized by an
increase of the confinement
50
0
0.5
1
time, likely due to the
1.5
tauexp / taus
Figure 4. a) Experimental vs. scaling
values for a series of increasing
density. b) Shots parameter location
in the Sudo scaling map. c) Mirnov
spectra from four of the shots with
increasing density (1-> 4). !
frequency (kHz)
0
0
150
%"
100
activity. It is seen that the
50
0
reduction of the MHD
1080
1100
1120
time (ms)
1140
1160
ratio τexp/τSudo ≥ 1, whereas
nexp/nSudo is kept ≤ 1.
Best performance: re-starting discharges.
In many of the explored magnetic configurations and especially under high (Pin ≥ 1MW)
heating power, discharges that collapse at low densities can recover. Moreover, during the
second heating phase, the energy content and the average density increase in a factor of 2
and 3, respectively, whilst radiation only increases in a factor of 0.5 (see Figure 5a). The
emissivity profiles reveal the development of a radiative instability that begins at the edge
and propagates toward the core. Then, plasma cools down to <Te> ≈ 40 eV, and the density
profile becomes strongly peaked, conditions rather adequate to optimize heating beams
absorption. During the reheated phase, the plasma core can reach densities of 1 x 1020 m-3,
41st EPS Conference on Plasma Physics
P2.074
the profiles shape are the typical of the H-mode, the estimated Zeff can be as low as 1.2, and
the MHD activity in the whole plasma column is reduced with respect to H-mode.
200
2
Wdia
1160
1180
0.8
1200
time(ms)
1220
t = 1226 ms
emissivity (MW.m-3)
t = 1205 ms
0.3
t = 1186 ms
0.1
-1
-0.5
0
!
0.5
60
1
obtained with regular Licoating
!!"!#$% !
0.2
max=2.6
80
1160
1180
1200
1220
TJ-II
walls,
leading to a reduction of the
low NBI input power, co-
40
20
1140
of
plasma-wall interaction. For
60
injection
1160
1180
1200
1220
1240
time (ms)
1140
Higher density plasmas are
40
1240
0.5
0.4
80
20
0.7
0.6
max=2.2
0
frequency (kHz)
ne (10
100
SXR
Pnbi (MW)
0
1140
b)!
300
4
Conclusions
c)!
frequency (kHz)
ne
Prad
-19
-3
400
Shot # 33527
6
Prad (kW)
m ), Wdia (kJ), SRX (a.u.)
a)!
1240
time (ms)
Figure 5. a) Time evolution of representative plasma parameters. b) Radial
evolution of emissivity for discharges that collapse twice. Inset: Emissivity
profiles at collapses and restarting times, marked with arrows. c) Mirnov
and bolometer spectra from shot #33527.!
!
yields
better
performance than counterinjection, mainly due to
asymmetric fluxes.
ECH applied to co-injection
NBI or gas base dilution
with He does not help to increase plasma density. Although in this study the explored iota
range is rather narrow, our results suggest that the scaling dependence on that parameter
likely should be reconsidered.
MHD activity is strongly reduced approaching the maximum density, when the best
confinement is obtained, probably due to the reduction of ion radial orbits for high
collisionality.
For absorbed powers ≤ 1.55 MW.m-3, in this ripple dominated device, the maximum
attainable density, seems to be still limited by thermal collapse.
Acknowledgments
‘This project has received funding from the European Union’s Horizon 2020 research and
innovation programme under grant agreement number 633053. The views and opinions
expressed herein do not necessarily reflect those of the European Commission.’
References
[1] Sudo S et al 1990 Nucl. Fusion 30 11
[2] Giannone L et al 2003 Plasma Phys. Control. Fusion 45 1713
[3] Miyazawa J et al 2008 Nucl. Fusion 48 015003
[4] Tabarés F L et al 2008 Plasma Phys. Control. Fusion 50 124051
[5] Ascasíbar E et al Contrib. Plasma Phys. 50 594
[6] Yamada H et al 2005 Nucl. Fusion 45 1684
Study of TJ-II density limit dependence on
magnetic configuration
M. A. Ochando, E. Ascasíbar, A. Baciero, F. Castejón, D. López-Bruna, J. M. Fontdecaba,
M. Liniers, K. J. McCarthy, F. Medina, I. Pastor, B. Sun, B. Zurro and the TJ-II Team
Laboratorio Nacional de Fusión por Confinamiento Magnético, CIEMAT, 28040 Madrid, Spain
ne0.64
B0.84
τEISS04 =
0.134a2.28
R0.64
P-0.61
ne0.54
B0.84
7/4
12/7
1.7
@ Bt = 0.95 T
5/3
1.65
!/2"
13/8
1.6
8/5
11/7
14/9
1.55
1.5
0.016
0
0.2
0.4
0.6
0.8
1
Previous results
7
0.016
Nucl. Fusion 51 (2011) 094022
co + cnt NBI
cnt NBI
co NBI
co NBI + ECH
6
0.015
3
0.008
3
2
1
2
0.004
0
co + cnt NBI
cnt NBI
co NBI
co NBI + ECH
0.012
0.016
-3
200
2
Wdia
-19
ne (10
tau exp (s)
ne (experimental ) x 10
4
SXR
Pnbi (MW)
0
1160
1180
0
1200
time(ms)
1220
1240
thermal collapse
t = 1226 ms
-3
frequency (kHz)
Density scan at low power
8
n (experimental)
7
6
5
4
4
2
1
2
3
4
5
frequency (kHz)
2
L
6
7
8
n (scaling)
S
0.6
0.5
t = 1186 ms
0.3
0.1
-1
0.4
-0.5
0
0.5
0
0.4
1040
1
!
0.5
50
1040
1180
1200
1220
3
1240
Reheated plasmas are obtained in almost all explored
configurations; favoured by fresh Li-coating and full
power
max=2.6
80
60
40
20
1160
1180
1200
1220
1240
time (ms)
Mirnov and central bolometer chord spectra
0
frequency (kHz)
Entering in H-mode improves τexp
but nexp is limited by input power
1.2
1.0
Conclusions
150
3
100
Higher density plasmas are obtained with regular Li-coating of TJ-II walls and the use of LLL's
For low NBI input power, co-injection yields better performance than counter-injection NBI
50
0.8
ECH applied to co-injection NBI or gas base dilution with He does not help to increase plasma density
0
frequency (kHz)
0.6
0.4
0.2
0
0.5
1.0
tauexp / taus
1.5
4
100
MHD activity is strongly reduced approaching the maximum density
For absorbed powers ≤ 1.55 MW.m-3, in this ripple dominated device, the maximum
50
0
0
Rotational transform dependence likely should be reconsidered
150
1080
1100
1120
time (ms)
1140
1160
1080
1
time (
ρ = 0.1 1160
0
0
1200
1160
time (ms)
1140
2
<Te>rc0.2≥ <Te>tc0.05
0.4makes it very difficult to assess and Li sputtering, two distinct
and chemical bonding effects,
how the ratio of H to Li ejection should behave in reality. sputtering theory.
Finally, the release of glo
Furthermore, the ratio of neutral to ionic sputtering yields,
assumed to be 1/3 for metallic lithium [17], could be strongly a H plasma normalized to the
modified
by the presence of more electronegative species (such a shot to shot basis [16]. The
Co+counter
injection
as carbon and boron) in the mixture.
In figure 2, the release of and were obtained by the meas
0.3
1
2 density
3 together
4
5
6 667 nm)
7 in 8the plasma and
(at
H by He plasma (b) and the line average
with
19 -3
gas analyser (RGA)
the density-normalized Hα emission (a) are depicted.
fairly
nexp (xA10
m residual
)
constant yield of γ ∼ 0.03–0.04 is deduced for this particular of the residual gas in the vac
discharge. The analysis of other similar discharges yields figure, a transient phase chara
0.5 s (∼2 shots) is followed by
values between 0.02 and 0.06, in veryMHD
good agreement
activitywith
reduction
TRIM code estimates and making allowance for the intrinsic after a few shots. This behav
glow-discharge conditioning
uncertainties in the estimates.
max=2.2
and is interpreted
Figure 3 shows the dependence of the Li emission and a H plasma
the Hα signal, both density-normalized,
on the electron process, thus implying that r
80
temperature at the last closed flux surface, as measured by the which is He-depleted by H bo
He beam diagnostic in the60
same toroidal location [16]. Two place between shots. Experim
important facts are worth highlighting in the data shown in the between shots are now in prog
figure. First, a very similar threshold energy, corresponding to corresponding diffusion coeffi
40
A key ingredient for u
electron temperatures of Te ∼ 50 eV, seems to exist for both
processes, obviously higher than the expected ones from TRIM improvement is the radiation
20 Li sputtering by He+ ). Second,
Edge radiation can be kept sma
calculations (Eth ∼ 9 eV for
the same energy dependence seems to apply for H desorption that triggers the low radiation
Radial evolution of emissivity for discharges that collapse
twice. Inset: Emissivity profiles at collapse times.
100
1120
0.1
Figure 2. Left panel: time traces of Hα (lower trace), electron density (right) and ratio between them (upp
plasma on a hydrogenated Li wall. Right panel: deduced hydrogen desorption yield for He on Li : H for th
time (ms)
150
1080
0.2
1140
50
0.15
Impurity radiation reduction
2
1
100
0
nexp
3
3
1
1
H
150
Hα/ne
0.2
frequency (kHz)
emissivity (MW.m )
emissivity (MW.m-3)
(Sudo)x x0.5
10 (s)
tauneISS04
nexp / ns
100
0.7
0.01612
0.6
1
300
Prad
1140
2
3
H/α
ne
0.004
0.8
ne
6
4
10
2
0.25
1
0.6
0.8
80.012
1
line average de
-3
0.008
The renormalization factor needed to
fulfil the ISS04 scaling (H. Yamada et
al., 2005 Nucl. Fusion 45, 1684) is
relaxed to 0.5 for NBI heated plasmas.
6
0.008
0
1.5 increment in Wdia and 1.7 in ne
Shot # 33527
radiative collapse
4
5
Figure 1. Maximum value of the plasma energy content (left) and energy confinement time (right) versus
(black) and Li-coated walls (red symbols). Crosses correspond to ECR and circles correspond to NBI hea
400
6
0.008
2 0.004
4
m )
0.004
neSudo = 0.25(PB /a2 R)0.5
00
00
3
line average density (1019 m-3)
Best performance: re-starting discharges
0.008
0
0.012
8
2
4
tau ISS04 x 0.5 (s)
10
m )
1
5
0
co + cnt NBI
cnt NBI
co NBI
co NBI + ECH
0.01610
0
0.000
0.012
tautau
ISS04
x 0.5
Sudo
(s) (s)
0.016
4 0.008
6 0.012 8
-3 (s)
ISS0419x 0.5
tau
ne (x10
0.016
0.016
12
20.004
0.009
0.006 (+)
ECRH
B( )
0.003
Li ( )
19
0.012
0.012
0.012
tau_E (s)
4
Prad (kW)
0.008
0.008
(Ascasíbar et al. CPP 2010)
0.012
5
00
00
0.004
0.004
0.004
15.00
17.76
18.04
18.20
18.37
18.66
18.77
19.08
19.25
17.11
16.10
15.63
15.16
19.20
19.16
0.666
0.934
0.964
0.981
0.999
1.031
1.043
1.079
1.098
0.867
0.768
0.723
0.680
1.092
1.087
Updated results
0.41
ι-2/3
0.004
0.004
00
00
1.372
1.517
1.539
1.553
1.567
1.593
1.609
1.630
1.650
1.628
1.627
1.628
1.628
1.676
1.668
#
Hα, Hα/ne
tau exp (s)
0.008
0.008
1.299
1.423
1.447
1.457
1.467
1.492
1.510
1.534
1.551
1.556
1.562
1.564
1.571
1.575
1.591
electron density (10
tau exp (s)
0.012
0.012
a (cm)
rotational transform profiles of the
most used magnetic configurations
1.45
m ), Wdia (kJ), SRX (a.u.)
co + cnt NBI
cnt NBI
co NBI
co NBI + ECH
iota(a) Vol. (m3)
3/2
1
0.016
0.016
iota(0)
H Desorption Yield (a. u.)
P-0.58
1.75
Prad/Pabs
R0.75
Co, counter, co + counter NBI and ECH + co NBI injection
W_dia (kJ)
ISS04
0.17a2
Input power scan: 0.33 MW/m-3 < Pin < 1.55 MW/m-3
frequency (kHz)
τESudo =
Rotational transform and magnetic well scans in H and H+He plasmas
tau exp (s)
Comparison with scalings
SUDO
Operation domain
Wdia (kJ)
ABSTRACT
In all stellarator and torsatron devices, a non MHD-driven plasma density limit
appears that depends on the absorbed power density and the magnetic field. The
dominant mechanism that limits the maximum attainable density in steady state is
the thermal collapse. As in the case of reverse field pinches (RFP) and opposite to
tokamaks, no current disruption can occur when the thermal collapse develops and,
moreover, the plasma can restart if the auxiliary heating is maintained.
Depending on plasma composition, thermal instabilities may originate at some point
of the plasma column and easily provoke radiative collapses. The use of wall
conditioning techniques with low-Z (C, B or Li) materials or divertors to reduce the
average impurity radiation strength, and the injection of pellets to centrally fuel the
plasma, led to notable extensions of the density limit. These facts reinforce the
common belief that the density limit is also an edge phenomenon, as in tokamaks
and RFPs. In TJ-II, a medium-size flexible heliac with moderate heating power
capability (< 2.6 MW), the main action to control and increase the electron density
has been wall coating with low-Z materials. The change from boron to lithium walls
has yielded an increment of a factor of two in tauE and 1.6 in maximum electron
density at low input power. During the last campaigns, further efforts to improve the
plasma-wall interaction have been made to produce more stable plasmas at full
power and to widen the operative magnetic configurations map.
attainable density, seems to be limited by thermal collapse
41st Conference on Plasma Physics, Berlin, Germany, 2nd-6th July 2014
1135- 9420