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Plans for Increasing the Magnetic Field and Plasma Temperature in the SSPX Spheromak dnh ICC 2004
Plans for Increasing the Magnetic Field and Plasma Temperature
in the SSPX Spheromak
David N. Hill
Lawrence Livermore National Laboratory dnh ICC 2004
Summary Good confinement (low core transport) is obtained with controlled decay. Peak temperatures of ~250eV observed when magnetic fluctuations are small (< 1%). SSPX discharge database points to importance of increasing the magnetic field and
especially the current amplification factor (A
I
=I
toroidal
/I
gun
) or B
pol
/I
gun
. Very slow formation and double-formation-pulse discharges yield the highest
magnetic fields in SSPX. Magnetic field evolution during formation suggests that higher efficiency (B/I
gun
)
would be achieved with longer formation pulses. We plan to install a new small-radius injector in SSPX which should increase the
helicity input and the resulting spheromak magnetic field We also plan to install a modular capacitor bank for flexible current programming dnh ICC 2004
SSPX cross-section and operating parameters
Typical SSPX parameters
Flux conserver RxH (m)
0.5x0.5
Radius of mag axis (m)
0.31
Minor radius (m)
0.23
Discharge current (kA)
200
Toroidal current (kA)
400
Edge poloidal field (T)
0.2
Pulse length (msec)
3.5
Electron Temperature
(eV)
20-200
Ion Temperature (eV)
?-600
Lundquist number, S
10
5
Fluctuations (kHz)
20
Plasma density (m
-3
)
5x10
19
[1] E. B. Hooper et al Nuclear
Fusion 39 863 (1999)
[2] D. N. Hill et al J. Nuclear
Mat. 313-316 941 (2003)
1m
Gas
Injection dnh ICC 2004
High electron temperatures and low core transport are
observed when fluctuations are low Peak electron temperature now approach 250eV Temperature climbs as fluctuations decrease Focus over last year: Quality of flux surfaces
(n=1 perturbation coil) Density control Isotope effects
0
1
2
3
4
5
0
0.5
1
1.5
2
2.5
3
3.5
4
Edge Magnetic Fluctuations vs Time
dBp/Bp9 (%)
pts_trel (ms)
0
50
100
150
200
0
0.5
1
1.5
2
2.5
3
3.5
4
T
e
vs Time
Te_max (eV)
pts_trel (ms)
T
e
dB/B dnh ICC 2004
Initial current pulse determines the peak field:
best confinement observed during controlled ramp down Short high-current formation pulse sets the
peak field Sustaining edge current maintains overall
stability Asymmetric modes evolve during ramp
down General decrease in overall mode
amplitude Particular modes come and go during
ramp down Highest Te when total amplitude is lowest dnh ICC 2004
Higher magnetic field and lower plasma density allow
operation at higher plasma temperature in SSPX Upgrades directed towards increasing
the magnetic field New gun to test helicity injection
physics Modular bank for current control
and better efficiency Major new diagnostic planned for
energy confinement studies charge exchange analyzer collaboration with Florida A&M
University A new small-radius coaxial injector has been designed: increased voltage and
helicity injection. Plans are being developed to convert the present bank to a multi-pulse
modular design to increase peak current and coupling efficiency.
0
50
100
150
200
250
0
0.5
1
1.5
2
2.5
3
core <T
e
(eV)>
core <n
e
/B
n
2
> (10
20
-m
-3
)
2001 McLean PRL e
=0.03 e
=0.05
2003 data
2004 data dnh ICC 2004
Design parameters for a next-step spheromak are sensitive to
density control and current amplification Target 1 keV spheromak plasma. What are the device requirements? Transport scaling sets device size (minor radius). Beta limit and density scaling determine field and toroidal current. Field generation efficiency (B
p
/I
gun
) sets bank and injector requirements.
T(eV)
B (T)
1000
500
ß~2ßsspx
ß~ßsspx
SSPX-PoP
SSPX-CE
0
0.25
0.50
n=const
ß~2ßsspx
ß~ßsspx
n~B
0.75
Ig (MA)
Ig (MA)
0.36
0.38
0.254
B=k*Igun
Helicity balance dnh ICC 2004
Double-pulse and very slow formation discharges obtain
higher magnetic field per unit current than fast formation Standard fast formation followed by
sustainment yields maximum
B/I
gun
~0.65 T/MA
. Slowstart formation has steadily
growing B with
B/I
gun
= 0.75 T/MA
. Double-pulse also produces highest
fields of
0.78 T/MA What physics is involved in
determining
B/I
gun
ratio? Why is
B
max
~ B
gun
for these regimes?
Peak field vs.. peak
current
0
0.1
0.2
0.3
0.4
0
100
200
300
400
500
bp09_stnd
bp09_vsf
Peak Edge Poloidal Field (T)
Peak current (kA)
Double-pulse buildup
Slow
start dnh ICC 2004
I-V Trajectories show time-dependent efficiency of field generation Data compares three reference
discharge scenarios Standard fast formation
followed by sustainment Very slow formation using
sustainment bank only Double formation pulse on
top of sustainment pulse Standard formation pulse is
clearly too short to achieve
maximum field. Double pulse seems to recover
trajectory of the first pulse. After formation pulse, all
sustainment bank shots on
same trajectory.
0
200
12560-stnd
discharge
400
Injector current (kA)
Edge Poloidal field (T)
600
0.1
0.2
0.3
0.4
0.0
11372-double pulse
7683-slow
start
decay -
- buildup
Best Double-pulse dnh ICC 2004
Field is still building when formation bank current peaks
and begins decaying: higher B/I if pulse were extended. Running longer formation pulses should
produce higher fields Assume constant efficiency during
pulse Assume fixed energy decay time
(no heating during pulse)
Gun voltage (V)
B
pol
090p09
B
pol
270p09
Density (10
20
m
-3
)
co2c1
Midplane H Gun current (kA)
B
pol
090p17
B
pol
270p17
tau_B (msec) tau_pulse
(msec)
fract of
peak E
fract of
peak B
increase
0.5
0.05
0.10
0.31
0.5
0.1
0.18
0.43
1.00
0.5
0.15
0.26
0.51
1.20
0.5
0.2
0.33
0.57
1.35
0.5
0.25
0.39
0.63
1.47
0.5
0.3
0.45
0.67
1.58
0.5
0.4
0.55
0.74
1.74
0.5
0.5
0.63
0.80
1.87
1
0.05
0.05
0.22
1
0.1
0.10
0.31
1.00
1
0.15
0.14
0.37
1.21
1
0.2
0.18
0.43
1.38
1
0.25
0.22
0.47
1.52
1
0.3
0.26
0.51
1.65
1
0.4
0.33
0.57
1.86
1
0.5
0.39
0.63
2.03 dnh ICC 2004
Slowstart formation steadily builds helicity content
Poor correlation between field and
voltage fluctuations suggests fine-
scale turbulence. Injector current is maintained just
above threshold ( g
sph
).
No
large initial formation pulse. Helicity content increases with or
without coherent n=1 oscillations. Buildup limited by pulse length. High source voltage with large
fluctuations. dnh ICC 2004
Multiple pulses yield higher fields and larger confined volumes:
NIMROD and experiment
0.0
0.2
0.4
0.6
0.8
1.0
2
4
6
8
x10
-1
R
Z
(m)
(m)
Decay
nim3.034a
t=0.165s
0
1
2
3
4
5
1.0
1.2
1.4
1.6
x10
-1
-1
0
1
2
3
4
5
6
t
E_m
n=
n=
driven crowbar decay
2nd pulse
nim3.034a
Surface of section Magnetic energy vs. t Surface of section Higher order (asymmetric) field components
decay first. Multiple pulses build the field in a
stepwise manner. Double-pulse experiment successfully
builds the field: NIMROD also suggests pulse length may play a role. Density control remains an issue: n
e

I
p
or ? dnh ICC 2004
A multi-pulse modular capacitor bank may allow buildup
to higher fields and temperatures Assume same efficiency for each pulse Assume that resistive decay time
remains constant.
Gun current (kA)
Gun voltage (V)
B
pol
090p09 (T)
Density (10
20
m
-3
)
co2c1
Input Energy (kJ)
Field Energy
(kJ) dnh ICC 2004
The modular bank would provide programmable control of
the current waveform and improve energy coupling Physics Design is complete for a
10-module bank upgrade. Estimated about $350k total cost. Proposed to test a single module
late in FY04, do full conversion in FY05. Anticipation of flat budgets at present
level drives us to explore alternatives Add only another formation type
pulse Use students for preliminary design
starting this summer Procure hardware late in FY05 Would extend present multi-pulse
buildup to three pulses
0
0.2
0.4
0.6
0.8
1
0
2
4
6
8
10
I
sspx
(MA)
time (ms)
dt=40
µs
dt=728
µs
200
400
Modification of bank will provide three
important conditions: Better control of operation near
threshold
(Operation near th