Fusion An Energy Option For The Future By T.A. El

Report
Highlights on EGYPTOR Progress
By
H. Hegazy
Plasma Physics Dept., NRC,
Atomic Energy Authority
13759 Enshass, Egypt
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Research Using Small Tokamaks
Beijin, China
Cross sectional view of the vacuum chamber
The basic item of EGYPTOR
is its Stainless Steel discharge
vessel consisting of two toroidal
segments insulated from each
other and sealed- off by two
viton O- ring.
The chamber has a rectangular
cross section 25cm by 20cm.
The major radius(R):30 cm
The minor radius(a):10 cm
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Beijin, China
Photograph of EGYPTOR Device
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Toroidal Field System
140 rectangular toroidal field coils (TF) are directly glued onto the
insulated chamber by epoxy resin. The inductance of the TF coil is
approximately 1.4 mH. The TF is created by discharging a 116mF
electrolyte capacitor bank energized up to 270 kJ for a maximum
charging voltage 2.16kV, however the nominal charging voltage is 1.7
kV, then the nominal bank energy is 167.6 kJ.
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Beijin, China
OH System
100 primary Ohmic heating (OH) turns form the cylinder air
solenoid for the OH transformer. The design of the OH
systems consists of two capacitor banks; the ionization
bank
and
heating
bank
as
shown
in
fig.
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Beijin, China
Plasma Investigations and Studying
Obstacles Preventing the Prolongation of
Plasma Discharge and Plasma Current
in EGYPTOR Tokamak
By
H. Hegazy
Plasma Physics Department, NRC, EAEA,
13759 Inshass ,Egypt
and
Yu.V.Gott , M.M.Dremin
Russian Research Center
“Kurchatov Institute”,
123182 Moscow ,Russian Federation
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INTRODUCTION
The main target of this experimental work is
to clarify the possibility to obtain the plasma
discharge and to prolong its duration as much as
possible.
There could exists several reasons as a possible
obstacles preventing obtaining this result:
*. improper operation of power supply system,
*. the high level of stray magnetic fields,
*. the lack of equilibrium,
*. the influence of MHD instabilities
*. The influence of impurities.
So we tried to analyze all this reasons
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Beijin, China
The duration of toroidal field pulse is long
enough (30 ms with half battery), so the time
interval with relatively small (20%) variation
of toroidal field is about 10 ms.
That’s why first of all we checked the
operation of the Ohmic heating power
supply system.
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Beijin, China
1. Operation of the Ohmic Heating
Power Supply System
With existing circuitry it critically depends on normal
operation of Vacuum Interrupter (VI) in the circuit of so
called “slow” battery.
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Beijin, China
Operation of the Ohmic Heating Power
Supply System
Because without VI the
“slow” battery couldn’t
give the loop voltage
necessary to breakdown
discharge,
we were forced to obtain
the discharges with the
help of only ”fast” battery
which could provide for
discharge duration of only
1 ms.
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2. THE COMPENSATION OF STRAY MAGNETIC FIELDS
For the measurements of vertical magnetic field from OH coil the pick up
coil was used [1]. This coil was placed in the plasma chamber center.
without compensation
with compensation
The compensation reduces the stray magnetic field about 4.6 times.
Taking into account the sensitivity of the pick-up coil, then the value of the
stray magnetic field after compensation is
about 1.2x10-3 UOH [kV]
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; UOH is the OH battery voltage
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The measurement of the vertical component of
the toroidal magnetic field with help of the same
pick-up coil is practically impossible because it is
very difficult to place this coil properly
Pick-up coil  Btor
Pick-up coil
Btor

Position of the pick-up coil for stray magnetic field measurement.
The plan of pick-up coils must be parallel to the toroidal magnetic field
Btor. If it is not so in pick-up coil will be generated signal which is
proportional to sin.
Practically the value of this signal is much greater
than signal from stray magnetic field.
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For the measurements of the stray vertical field from toroidal field coils the
four (1-4) additional loops were used. These loops were placed on the
bottom and top sides of vacuum chamber.
The coil 1 was connected with the coil 2 in series
1
2
2
5
1
4
3
1 – 4 – loops, 5 – vacuum chamber
The 1 – 2 coils connections
Difference between signals from loops 1 and 2 (or 3 and 4), gives
after integration the value of vertical magnetic flux
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The vertical component of the TF measured by coils 1+2
Estimation of the value of stray vertical fields from toroidal coil and
Ohmic heating coil gives no more than 5 Gs is deduced.
So we can conclude that the measured values of stray
magnetic fields can’t prevent the discharge breakdown
and limit its duration
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Beijin, China
3. Investigation of the plasma column equilibrium
For these estimations we use the pair of Mirnov probes (outer and inner)
installed in vacuum chamber. The precise evaluation of plasma position in the
chamber envisage the calibrated measurements of poloidal magnetic field and
average vertical magnetic field in accordance with formulae


b

 b
B
1


B
1





 

 a2 
 a 2 

b  a 2 b 1  a 2  cb 
R
R




1  2   B 1  2 


 2 ln  1  2  
b 2R  b
a 2  b  4 J 
2
 b 
 b 



where  is horizontal displacement of plasma column,
a is minor plasma radius,
R is major plasma radius,
b is minor radius on which the Mirnov probes are
placed,
J is the plasma current,
B+ and B - are the aziumuthal magnetic field
measured by outer and inner Mirnov
probes accordingly,
B - averaged transverse magnetic field measured by loops 1-2 or 3-4.
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3
1
2
Tokamak
axis
4
Mirnov’s probes locations.
Because Mirnov probes and these loops were not calibrated.
If the center of plasma current coincides with the center of chamber
i.e. at equal distances from both probes) these signals must be
equal (in cylindrical approximation).
In torus these signals will differ due the toroidicity in ratio
(R + b)/(R – b)
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if we adjust the signals from
these two Mirnov probes
in accordance with their
toroidicity
and subtract these signals
we will obtain the signal
proportional to displacement
of plasma current.
one can see from these signals
that their shapes are similar
and repeat practically the shape
of plasma current signal.
The relative difference
is not more than 0.125
displacement 0,06b,
i.e.  0.5 cm.
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The signals from Mirnov’s coils;
coil1 (Ch. 3) , coil2 (Ch. 4),
plasma current (Ch.1), and loop voltage (Ch. 2 )
2nd RCM - IAEA - CRP On Joint
Research Using Small Tokamaks
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So we can conclude that the plasma equilibrium in these
discharges is good enough and in any case couldn’t be
the reason for their short duration or small plasma
current value.
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4. MHD instabilities
Are MHD instabilities responsible for short
duration and small amplitude of plasma current ?
Conditions for this development are characterized by the
q parameter which is determined as
q  (Bt/B)(a/R)
where Bt is toroidal magnetic field,
B is the azimuthal field of plasma current
B[Gs] = 210-5Jp[A]/a[cm]
Taking in mind that R = 30 cm for q we obtain formulae
q = 1.7103 Bt[T]a2[cm]/Ip[A]
For Bt = 0.4 T (corresponding to Utor = 1 kV), Jp = 5 kA , a = 7 cm
q =1.71030.4  49 / 5103 = 6.7
This value is large enough because most dangerous MHD modes
have q values of 2 and 3. So
MHD instabilities most likely couldn’t be responsible for short
duration and small amplitude of plasma current.
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5. Estimations of plasma electron temperature Te
They were based on the dependence of plasma
resistivity  on Te expressed by formulas
H = 1.6510-9ln/Te3/2 Ohmm , Te in keV,
Z = N(Z)ZH.
Knowing the plasma resistance from plasma current Jp
and loop voltage U taken in the moment of
maximum plasma current
UL = LpJp/t
Lp is the inductance of plasma column equals to zero due to Ip/е = 0
R = U/Jp
one can estimate the plasma resistivity
 = RS/l
where S is the plasma cross section a2
a is the plasma minor radius which usually can
be taken as limiter radius,
l = 2R is the length of plasma axis
( R =0.3 m is the plasma major radius).
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The value of Coulomb logarithm ln is weakly dependent on plasma density
and can be taken as 17. Parameter N is weakly dependent on effective
charge of plasma ions Z and in assumption that the main impurity is carbon
(Z  5) can be taken as 0.72.
With these parameters we obtain the formulae
 = U[V]a2[m]/0.6Jp[A] = 1.0110-7/Te3/2[keV]
and
Te[keV] = 1.5410-5 (Jp/Ua2)2/3
For plasma current Jp = 5 kA and loop voltage 25 V
and assuming a = 7 cm we obtain
Te = 1.5410-5(5103/254910-4)2/3  20 eV
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This value is very close to so called “radiation limit”
which was observed in first Tokamaks and is associated
with high level of impurities.
So as an obvious way to improve the plasma
performance in EGYPTOR tokamak
we consider the decreasing of the level of plasma
impurities using cleaning discharge system
( Dc Glow discharge, as the first step
&
50 Hz Taylor discharge as the second step if it is still
necessary)
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Glow Discharge in EGYPTOR
Smooth operation of 600V, 0.6 A DC Glow Discharge is in operation and
special study of the impurity contents using Emission Spectroscopy will be
part of the aim of the next year.
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IN CONCLUSION
For simultaneously operation of both batteries in EGYPTOR, the tokamak
power supply system must be modified. For instance, the system used in
many other Tokamaks such as in CASTOR OR CDX-U Tokamak
CASTOR Tokamak
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C1= 412.5 mF, C2= 5 mF, C3= 2200 mF
CDX-U Spherical Tokamak
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Research Using Small Tokamaks
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Modification of the Toroidal Current
Generation Scheme in EGYPTOR
Tokamak
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By
H. Hegazy
Plasma Physics Dept., NRC, Cairo, EGYPT
And
K. Dyabilin
Institute for High Temperatures
Moscow, Russia
2nd RCM - IAEA - CRP On Joint
Research Using Small Tokamaks
Beijin, China
Toroidal Current Organization
E*R
20-30 V
4-5 V
fast
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Slow - stationary
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Research Using Small Tokamaks
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Previous version
Plasma
column
“slow”
Lower voltage
circuit
“ fast”
High voltage
circuit
• Problems with interference
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• Does not work
2nd RCM - IAEA - CRP On Joint
Research Using Small Tokamaks
Beijin, China
Now
Plasma
column
“slow”
“fast”
It works.
Due to the increased ratio “M/L “ the efficiency of
the induced loop voltage generation also increased
substantially.
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Scheme on the Tokamak
chamber
“fast ”
circuit
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“slow”
circuit
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T.F. = 400 V
OHF = 4 KV
OHS = 400 V
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Features of the new scheme
Positive
• Very cheap, no needs for expansive vacuum
interrupters, powerful diodes, …
• Very effective.
• At the stationary phase amplitude of the loop
voltage can be up to 10 V.
Negative
• Separation of both circuits is not absolute (mutual
flux influence),but orders of magnitude lower than
in previous version.
• One need to induce an additional compensation coil
in the same way as it was done previously.
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Numerical Estimations and Expectations
Part of the activity was devoted
few numerical estimations and
expectations of the possible
Tokamak plasma parameters.
This is obtained by creating a
one dimensional and time
dependent code. The primary
current , radial profile of the
electric field, ion and electron
temperatures were yielded by
solving set of coupled nonlinear
equations.
It was shown that expected
parameters are:
*. plasma current= 4-10 kA
*. Time duration 5-7 ms
*. Plasma density= 5x1012 cm-3
*. Electron Temp.= 100-200 eV
*. Ion Temp.= 15 eV
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Primary Current and Toroidal Induced Electric Field eqs
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Ion/electron Energy Balance eqs
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Plots of Primary/Plasma Currents,
Central temperatures, Scenario of Battery and Plasma Loop Voltage
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Beijin, China
Plots of the temporal behavior of the inductance voltage
and toroidal magnetic field
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Radial/Temporal Behavior of the Electric Field
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Electron Temperature
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Ion Temperature
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Output Publications
H. Hegazy, and F. Zacek
“Calibration of Power Systems and Measurements of
Discharge Currents Generated for Different Coils in The
EGYPTOR Tokamak”,
J. of Fusion Energy V. 25 (1-2),73-86, (2006)
2- H. Hegazy, and F. Zacek
“Absolute Measurements of the Magnetic Field Generated by
different Coils in the Center of EGYPTOR Tokamak”,
J. of Fusion Energy V. 25 (1-2),115-120, (2006)
3- H. Hegazy, Yu. V. Gott, and M. M. Dremin
“Plasma Investigations and Studying Obstacles Preventing the
Prolongation of Plasma Discharge and Plasma Current in
EGYPTOR Tokamak, in Press
4- H . Hegazy, and K. Dyabilin
“ Modification of the Toroidal Current Generation Scheme in
EGYPTOR Tokamak”, in press
1-
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Beijin, China
Expected activities for the Next year
Experimental Activities:
*.Improvement of Plasma discharge and Current Ramp up
*. Wall Conditioning of EGYPTOR vessel.
*. Study of impurities emitted during the cleaning discharge By
Emission Spectroscopy.
*.
Measurements of Electron Temperature in EGYPTOR Tokamak
using Langmuir probe .
*.
Development of Control System for EGYPTOR based on Data
Acquisition
Theoretical Activities:
*. Study the effect of External Electric Field on Drift of the Plasma
Across the Magnetic Field in Tokamak
*.
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Study of Surface waves propagation along a Toroidal Plasma
Column.
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Research Using Small Tokamaks
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