beijin, china2nd rcm - iaea - crp on joint research using small tokamaks 1 highlights on egyptor...

Beijin, China2nd RCM - IAEA - CRP On Joint Research Using Small Tokamaks

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Highlights on EGYPTOR Progress

By

H. Hegazy Plasma Physics Dept., NRC,

Atomic Energy Authority13759 Enshass, Egypt


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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 cmThe minor radius(a):10 cm

Cross sectional view of the vacuum chamber


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Photograph of EGYPTOR Device


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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.

Toroidal Field System


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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.

OH System


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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, Egyptand

Yu.V.Gott, M.M.DreminRussian Research Center

“Kurchatov Institute”, 123182 Moscow, Russian Federation


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INTRODUCTIONThe 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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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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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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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 compensationThe 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] ; 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

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.

Pick-up coil

Btor


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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 – 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

2

34

5

1

1

2


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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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3. Investigation of the plasma column equilibriumFor 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

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.

2

2

2

2

2

2

2

2

112

11

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2

1ln

2 b

aB

b

aR

bB

R

bB

J

cb

b

a

a

b

b

a

R

b

b


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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)

1 2

3

4

Tokamak axis


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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 differenceis not more than 0.125displacement 0,06b, The signals from Mirnov’s coils;

i.e. 0.5 cm. coil1 (Ch. 3) , coil2 (Ch. 4), plasma current (Ch.1), and loop voltage (Ch. 2 )


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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 instabilitiesAre 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] = 210-5Jp[A]/a[cm]

Taking in mind that R = 30 cm for q we obtain formulae

q = 1.7103 Bt[T]a2[cm]/Ip[A]For Bt = 0.4 T (corresponding to Utor = 1 kV), Jp = 5 kA , a = 7 cm

q =1.71030.4 49 / 5103 = 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 = 1.6510-9ln/Te3/2 Ohmm , Te in keV,

Z = N(Z)ZH.

Knowing the plasma resistance from plasma current Jp

and loop voltage U taken in the moment of maximum plasma current

UL = LpJp/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 = RS/l where S is the plasma cross section a2 a is the plasma minor radius which usually can be taken as limiter radius, l = 2R 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] = 1.0110-7/Te3/2[keV]

and

Te[keV] = 1.5410-5 (Jp/Ua2)2/3

For plasma current Jp = 5 kA and loop voltage 25 V

and assuming a = 7 cm we obtain

Te = 1.5410-5(5103/254910-4)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

C1= 412.5 mF, C2= 5 mF, C3= 2200 mF

CASTOR Tokamak CDX-U Spherical Tokamak


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Modification of the Toroidal Current Generation Scheme in EGYPTOR

Tokamak

By

H. Hegazy

Plasma Physics Dept., NRC, Cairo, EGYPT

And

K. Dyabilin

Institute for High Temperatures

Moscow, Russia


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Toroidal Current Organization

E*R

20-30 V

4-5 V

fast Slow - stationary


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Plasma column

“slow”Lower voltage

circuit

“fast”High voltage

circuit

Previous version

• Problems with interference

• Does not work


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Now

“fast”

Plasma column

“slow”

It works.Due to the increased ratio “M/L “ the efficiency of the induced loop voltage generation also increased substantially.


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“fast ”circuit “slow”

circuit

chamber

Scheme on the Tokamak


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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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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 Publications1- 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 Press4- H . Hegazy, and K. Dyabilin “ Modification of the Toroidal Current Generation Scheme in EGYPTOR Tokamak”, in press


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Expected activities for the Next yearExperimental 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

Study of Surface waves propagation along a Toroidal Plasma

Column.


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beijin, china2nd rcm - iaea - crp on joint research using small tokamaks 1 highlights on egyptor...

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