the a-effect and current drive using electron bernstein...
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i:i-:iir' a b d u s s a l a meducational, scientificand cultural
organization
the
international centre for theoretical physics
international atomicenergy agency
SMR 1331/25
AUTUMN COLLEGE ON PLASMA PHYSICS
8 October - 2 November 2001
The a-effect and Current Drive Using Electron Bernstein Waves in the RFP
Cary Forest
Dept. of Physics, University of Wisconsin, U.S.A.
These are preliminary lecture notes, intended only for distribution to participants.strada costiera, 11 - 34014 trieste italy- tel. +39 040 22401 I I fax +39 040 224163 - [email protected] - www.ictp.trieste.it
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Outline
The a effect in the RFP-components of dynamo physicsRF heating and current in an RFP- Impact of current profile modification on performance- Magnetic geometry and dielectric properties
Electron Bernstein Waves- Propagation, absorption and current drive- Coupling
Emission measurements- Black body levels of emission observed
Coupling measurements- Agreement with waveguide coupling theory
Plans
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The a effect in the Reversed Field Pinch
The a-effect predicted from mean-field electrodynamicsplays an important role in the RFP- Predicts generation of current in direction of pre-existing
magnetic field from fluctuations
Global kink-tearing modes provide fluctuating magneticand velocity fields (driven by gradients in currentdensity)
a effect generate toroidal magnetic flux from poloidalmagnetic fieldNot a dynamo since free energy source is gradients inmagnetic field energy- Referred to as "RFP dynamo-effect"
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The standard RFP current profile is sustained by atoroidal electric field and MHD fluctuation driven currents
toroidal electric field drives toroidal current in core
peaked current profile is unstable to tearing modesConducting Shell
Surrounding Plasma
\
BT Small & Reversed at Edge
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Tearing modes drive poloidal current in the edge andsuppress current in core
broad spectrum of tearingmode are driven by gradientsin X = JI I /B
core resonant m=0m=1 tearing modes modes
q(r) typical island width
magnetic stochasticity
current profile driven by MHDtowards a marginally stablecurrent profile (flat X)Island overlap causes Measured n.spectrumstochastic magnetic transport 1.5in core
0.5
0
i I I I I
Bin)B
mi
i ! I ! I
MST
I l l l i1 3 5 7 9 11 13
n
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An cc-effect is needed to sustain the RFP equilibrium(Cowling's theorem applied to the RFP)
Assume axisymmetry and a simple Ohm's law (J = crE). Now considercurrent at reversal surface (surface where B^ = 0)
but
Je
dt
1 II
1<
oB4
w,
a1
o,
2nr
dr
thus no steady-state field reversal is compatable with simple Ohm's law.
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An a-effect is needed to sustain the RFP equilibrium(continued)
Now consider an Ohm's law of the form (J = <TE +Now,
= -E027rrat
a>BA
OrNow, steady state is possible if
or(Recall Taylor state V x B = AB)
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Toroidal flux for RFP equilibrium is periodicallyregenerated in discrete sawteeth events
Discrete Dynamo Events
10 20 30Time (msec)
40 50
Reversed field equilibrium can be sustained for manyresistive diffusion timesToroidal flux follows current
Theory of relaxation is beyond scope of this talk, but itinvolves
8
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Velocity and magnetic field fluctuations can be measuredto estimate motional EMF from tearing modes
-1.5 -1.0 -0.5 0 +0.5 +1.0 +1.5Time relative to flux jump" (msec)
-1.0 -0.5 0 +0.5 +1.0Time relative to flux jump (msec)
+1.5
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Mean-field EMF from tearing modes regenerate toroidalflux
1.0 -0.5 0.0 +0.5 +1.0Time relative to flux jump (msec)
+1.5
10
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Hypothesis:
The RFP's poor confinement is a result of themisalignment of inductive CD profile and tearing modestable current profiles.
By non-inductively sustaining a current profile which islinearly stable to all tearing modes, the confinement inthe RFP will improve.
Produce a non-inductively driven Taylor-State
11
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MHD simulations indicate partial current drive in the edgeregion of the RFP can reduce magnetic fluctuations
B2
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
-0.5
n e
with J( r)-control
StandardRFP
ad hocforce
profile
i I i I i
0. .2 .4 .6 .8 1.0rM
Nonlinear, 3D resistiveMHD computation
log
'~\2D
-2
-4
-6
-8
-10
; fir f;V' \\*/'. O>
,£: cc
: ?o
: IF• s s
Standard \RFP j
( \ ;
with \r '.Mr)-control •
-20 -10 0n
10
12
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Current drive can heal flux surfaces
6p
StochasticField
Standard RFP RFP with Jdj-Control6p
Closed FluxSurfaces
0.00.0
v t
i •
II/ ; 1
)t^l
• f . \?::tK S-.
)•£'?'':".'"(•
>-4% \ i •
0.5 ij
13
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PPCD: inductive electric field programming resultsreduction of magnetic fluctuations (for 10 ms)
Pulsed PoloidalCurrent Drive
4
E 2
i o
-4
C/5
0)
05
10
05
| 7
co 5
? 310
PPCD
1
" SXR
- density
t i l l
Ee(a) E|,(a) ^ ~
^ applied inductive JE,;>(a) ^ electric field
m=0 fluctuations
m=1 fluctuations^ (rms; ^ = 6 - 1 5 ) * ^ ^
8 10 12 14 16time (ms)
18 20
14
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Electron energy transport is improved during PPCD:excellent target plasmas for RF current drive
Electron "Temperature
00.0 0.1 0.2 0.3 0.4 0.
p(m)
100Qtransport coefficients
100-
o.o d ' i 6 . 2 as A o-4f 0.5r ( m ) q=0 q=0
• %e t ' r o P s
PPCD
-T;E=10 ms, ne
P<1MW
1 0 during
= 0.5-1.0 x 1013 cm3
15
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PPCD shows good fast electron confinement: runawayelectrons are present
80
60
40
20
Pulse height analysis of PPCD plasmas show 100 keV electrons
RF generated tail of the electron distribution function can be confined!
Fokker-Planck Modeling shows diffusion coefficient of 5m2/s, measured Zeff
matches data well, even with Diffusion=0m2/s, Zeff -3 needed
10~2msi5 20time (ms)
io-3
10-4
10-5
_
\
\
\\
-
-
-
' I '
/
,/••"
1 1 1
PPCD flux1
"».
I 1
1
at
re-
time
I
L <
ofMSTFit1 | . 1
II ,
—̂̂^̂ <><J
> • —
7sxr_./sxr
<
-—:
1 •
diff5"diff5
<>
* —
<
1 ,
_
-
z2.datz4.5.dat
-
-
<
:
> _
20 40 60Photon Energy (KeV)
80 100
16
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MST plasmas are over-dense
High dielectric constant
> 5ce
Conventionalelectromagnetic waves inthe electron cyclotronrange of frequencies donot propagateLH wave accessibility islimited
40
N 3 0 L
CD
CD
20 L
1 0 L
0
lp = 300 kAmp, Ne = 10 13 em "3
0.0 0.1 0.2 0.3 0.4 0.5
r(m)
17
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What type of waves might propagate for these parametersin the ECRF
Qi = &
Core plasmain low fielddevices forG>~2coce
0 Usual ECregime for to~2coce
18
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The EBW propagates when the plasma is overdense
Hot plasma wave
Wavelength of order pe
Propagates nearlyperpendicular to magnetic fieldPrimarily electrostatic
8s
Typical EBW Dispersion Curves
Maxwellian *
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Hot plasma dispersion can be evaluated numerically
Assuming a maxwellian distribution function f(v) =the hot plasma dielectric tensor is
oo / %InAn -in (In - I'n) An
K = 1 + Xe~x ]Tn——oo
in (In - I'n) An (?fln + 2A (/„ - I'n)) An
I -%N±InBn/Y -iN±(In-I'n)Bn/Y
K* K* \ / v ft {i) f~*\ /-v f\ 1 r
\A/hpm \ — -L — ±^ V — Pe \S — ^^ Q^ — ^ ^ J
VV1 IV-̂ I \^ /\ C^O —
the functions2Y'2' " w 2 f '
/ m \\27rfcT/
tATj. (/„o(l—ny)^" iVii/32
2. NL
3/2
- / ;J n B n
_ k
cxp 2kT ,
) s n /y
( i )
Bn — -
1 - nY
/ n = /n(A) is the modified bessel function, and Z(£) is the plasmadispersion function.
T.H. Stix Waves in Plasmas, AIP (1992) >o
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Hot plasma dispersion (continued)
Solve wave equation
which in matrix form is
N X (N X E) + K . E = 0,
Kxy
^yx
\ Kzy
KXz
Kyz
Kzz -Nl j
Ex\Ey = 0,
Functionally, this is specified as
- N? K.xy
^yx
K zy
Kxz +Kyz
Kzz - Nl
(1)
(2)
(3)
Solve for ATj_, given X, Y, (3 and Nn. Dispersion found from ReD(N±r) = 0, whereiVj_jr is the real part of N; damping found from
(4)
21
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Ray Tracing in general toroidal geometry
Once root is found, it can be followed with Hamiltonian ray tracing equations to solvefor the ray trajectories of the waves:
dz _ c dD/dNz dNz _ c dD/dz m
dr _ c dD/dNr m dNr _ c dD/dr .~ u dD/duj ' df ~
d<f> c dD/dm dm c dD/d<fi
tudD/du' dt ~The spatial derivatives are computed from equilibrium quantites and chain rule, ie.
3D _ dDdX 8DdY dDd(3
"57 ~ dX 3r dY dr ~d(3~drdD _ dDdX dDdY dDd/3
~dz ~ dX dz dY dz ~d/3~dz
= o
Toroidal equilibrium is used for calculation of magnetic field.
22
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Numerical study of EBW propagation and absorption hasbeen carried out using GENRAY and CQL3D
Full hot, non-relativistic dispersion solver
Absorption is complete (nearly infinite optical thickness)
(a) Poloidal View
Forest, Chattopadhyay, Harvey and Smirnov,Phys. Plasmas 7, 1352 (2000) 0.0 0.1 0.2 0.3. 0.4
minor radius (m)0.5
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N|, up-shift and direction of CD can be controlled byi i d i l h i ipoioidai launch position
0.2 0.3minor radius (m)
0.4 0.5
24
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RFP physics is similar to ST physics(ECH characteristics of Pegasus are similar to MST)
(a) poloidal view
0.2 0.4 0.6 0.8minor radius (r/a)
1.0
/Vdriven current densitytotal = 39.5 kApower = 100 kWne(Q)=1019rrf3
Te(0)=1 keV
\
0.2 0.4 0.6minor radius (r/a)
0.8
Forest and Ono, Bulk Am. Phys. Soc. 35, 2057 (1990)Forest, Chattopadhyay, Harvey and Smirnov, Phys. Plasmas 7, 1352 (2000)
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Current drive is computed from solution of Fokker-PlanckEquation (CQL3D)
Hot plasma dispersion is non- 1relativistic
CQL3D uses non-relativisticpolarizations but fully relativisticFokker-Plank treatment
Wave diffusion determined fromelectric field (from ray tracingcode) o
1
contours of quasi-linear diffusion
U||/Uo,
1
26
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History of Coupling
Early theory- Mode conversion theory Stix (Phys. Rev. Lett, 1966)
• Couple to the EBW using an X mode launched from the high field side- Hot plasma waves (Bernstein)
Early experiments (-1970)- Observation of the EBW (Leuterer, Stenzel, Armstrong)- Non-linear effects and heating (Porkolab)
OXB mode conversion identified (1972)- Pioneering work by Preinhalter and Kopecky, 1972- Later work by Batchelor and Goldfinger
Tunneling through XB (Sugai)Demonstration of OXB on Wendelstein 7-AS- Heating, emission (Laqua, et al 1996, 1998)
Recognized as possible heating source for STs- Forest (1990), Ram and Bers (1998)
Ongoing research on STs and RFPs
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X-mode incident on upper-hybrid resonance from highfield or high density side smoothly mode converts into
EBW: classic mode-conversion problem
.1 R =
"Fast"X-Mode
increasingdensity or B
T.H. Stix, Phys. Rev. Lett. 15 878 (1965)
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Launching the EBW with high field side X mode for insidelaunch 60 GHz ECH on Dlll-D
Top View
4
3
i.00.0
density
0.2 0.4 0.6 0.8 1.0rho
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EBW may play a role in understanding the "Heat-Pinchresults on Dlll-D
CD
CD un
EO> en
0>
oQ_
C\l
power deposition
EBW
/ \ first pass/ V X-mode
0.0 0.2 0.4 0.6rho
0.8 1.0
C.B. Forest, R.W. Harvey, A.P. Smirnov, Nucl.Fusion 41 619 (2001)
111
0.2 0.4 0.6
rho1.0
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Constructive interference between right and left handcutoffs can result in efficient mode conversion
Plasma cutoffs at
/L = 2
/ft — 2
/ce +
/c2e +
4 /2pe
1 - AT,
1/2 \
2 | ~h /ce
/
4 / 2
1-AT,
1/2
ce
The cold plasma resonance at fuh = yjff%, + .couples the x-mode and EBW when hotplasma effects are considered.
Cutoff-resonance-cutoff triplet can give 100percent mode conversion:
Cmax —
1 (u>peL
a \n
uh1 + a1 - 1
ai ~~
Ram and Schulz, Phys. Plasmas 7 p. 00(2001)
1.0
w 0.8
E 0.603
Hc
NX
0
EBW
slow x-mode
cold plasmadispersionat 3.6 GHz
fast x -mode
0.47 0.48 0.49 . . 0.50r (m)
0.51 0.52
acuum= 9 cm
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Full-wave calculations validate mode conversion processand predict surface impedance for coupling calculation
M
0
+
wo4
047
1: 1 1 i | I 1
juAAV1
ft I
i , * )
IBI =f =Te =
i i t
i i | i t _
0.08 T :
3.6 GHz "50 eV
-
i i i . , "
0.48 0.49 0.50minor radius (m)
0.51
Simulations using GLOSI code, M. Carter ORNL
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Impedance match depends upon density scale length andangle of incidence perpendicular launch
0,0 0.2 0,5 0.8 1.0transmission forX mode incident
1,-0.25 cm L= =0.5 cm
Shallowest gradient
r » 3 mm
r >> 0.05 mm
Transmission depends on perpendicular wavenumber (n )̂ ofincident waveCalculations done for a plane wave incident from vacuum
33
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Phased array of waveguides can be used to match toEBW
E l
E l
tBo
yA
Nf
Matching Ey at x = O_|_and Bz in waveguide opening determines powercoupled to plasma. For one waveguide
1 - P A koa= A =+r sin2 (/ dnvY(n<u) ^
~2T'ly
(i)
where Y(ny) =x=0
is determined from plasma physics. Has beengeneralized to multiple waveguides.
[M. Brambilla, Nucl. Fusion 16 47 (1976)] 34
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Reflected power depends upon phase betweenwaveguide, consistent with freespace picture
80
70
50
40
30
20
10
0
CDOO
§o
2CD
ba.
3.4 GHz, pair of7.21 X 3.45 cmwaveguides
Vacuum case
Plasma case
-180 -135 -90 -45 0 45 90 135 180
Phase angle between waveguides (deg)35
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Ordinary-Extraordinary-Bernstein Mode is also possible
Coupled Power Fraction
X-mode O mode
K0.00 0.25 0.50 0.75 1.00
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3.7-8 GHz radiometer has been constructed andabsolutely calibrated
16 Channel Radiometer schematic
16channeHir*l....l6)Central freq, - 4 * (n-l )*0.25 GI fz3 dB bandwidth - 125 MHz
Antenna DC block Amplifier Attenuator AnipIifierAttenuator
Antenna Specification
Bandwith: 3.8-8.4 GHzPolarization: LinearDirectivity: 13-17 dBLocation: 15 deg poloidal from horizontal mid planeAntenna looks at either O or X mode polarization.An II = 0.5
ilandpass E lec to r Video OutpiFitter Amplifier
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Emission correlates with MHD activity
300
Q- 200 :
100 L
CO
E
oT"™
8
6
4
2
- o . TO
- 0 . 2 0 •"
-0.30 r
-0.40 r
0
300
> 200(D
l ioo
> 200
2 ioo
0
M, 200
100
10 20 30 40 50time (ms)
Ch7=5.5 GHz
Gh11=6.5GH;
0 10 20 30 40 50time (ms) 33
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Black-body levels of electron cyclotronemission are observed
250
-. 200 :
£ 150^
SI 00500
• 1st
-
; x *• i i
harmonic
4
ill'. 1 . . .
i ' 2nd1|
li-armfonfc1 ' ' \
$ x mode
f o mode
-
, s * - m *. x * * :j i . . • • i • • • i i
0.2
5 6f (GHz)
8
250200
> 150
^ 1 0 0
500
^~~i*4s1 st harmonic
1 K i \
: , X,
2nd harmonic :* Thomson$ x mode| o mode
-
1* :
* 4 « **:-• i i i i i •
0,3 0.4minor radius (m)
0.5
flux surfaces
harmonicloverlap• i • • 1 1 1 1 • 1 1 1 1 1 • 1 1 1 • 1 1 1 1 1 1 1 1 1 1 1 1 1 • • 1 1 1 • • 1 1 1 1 1 • 1 1 • 1 1 1 1 1
5.5 GHz ray
0.00 0.10 0.20 0.30 0.40minor radius (m)
0.50
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Comparison with coupling theory is underway butcomplicated by wall reflections
Maximum XB modeconversion efficiency
(
Horn antennna
v = -a uh
1 + a2 -
a =COpe
-ce
Applying balance of incomingradiation
T T -*-xbbbl-(l-Txb)R
„, _ (1-R)C „_ I1-RC ' hb
Reflectivity depends uponangle of incidence
^0.4
S 0.2
mode conversionlayer
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Active coupling studies are underway using singlewaveguide and double waveguide array
Reflectometry SchematicRF- source
Bidirectional coupler(22dBcoup,28dBclr)
Isolator
dB IFor Power
ower splitter Power Splitter
Amp Video Det.
Hybrid CouplerCP 9GP
Lim. amp\V
Mixer'
TWT(10W)24GHz
RefPov^r
Video Det.
Low PassFilter
LowPassFilter
T TCos(f) Sin(f)
LevelingLoop
Video Det.
41
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PPCD plasma presents low reflection
1.0
0.6
0.21.0
0.6
0.2
^ 2 . 0
51.0
o8
9 4
z * o 1
Forward Power
Reflected Power
Edge ElectronCyclotron Frequency
Edge Density"
10 20Time (ms)
30
LL
o
200
100
01
0
-1
-2
1.5
1.0
C 0.5
0.8
ffl 0.4
0.0
PlasmaCurrent
Line Average Density -
Reflection Coefficient -
10 20
Time (ms)30
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Reflection varies with frequency in accord with theory
3.2 3.4 3.6 3.8Frequency (GHz)
o
4.0
0.8
0.7
0.6
0.5
0.3
0.2
0.1
0 ,
PPCD like conditionssingle waveguide
A A AA A
A phase shift
O Power reflection coeff.o
° o o o o o
80
60
40
620
2.5 3 3.5f (GHz)
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Plans for EBW
Antenna development and edge conditioning to furtheroptimize couplingDemonstrate heating- TWT: 3.6 GHz, 5 ms, <200 kW- Measure power deposition (HXR)
Wave physics- Does power get to cyclotron layer in this over-dense plasma?- Measure EBW in plasma (probes or CO2 scattering experiment)- Test off-mid-plane launch CD scenario
• N|| up-shift and hence, Doppler shifted resonance location, isdifferent for above and below mid-plane launch (power depositionor emission region differs)
44