1. GOLEM operation based on some results from CASTOR
J. Stockel, and the CASTOR team, Association EURATOM / IPP.CR, Czech Republic
Several historical picture
Tokamak CASTOR/GOLEM
Basic tokamak diagnostics
Power supplies
Basic plasma performance
Marianská, January 2009

2. CASTOR tokamak Major radius 40 cm Minor radius 8,5 cm
Toroidal magnetic field < 1,5 T
Plasma current kA
Pulse length < 50 ms
Features:
Small tokamak
Small ripple (24 TF coils)
Routine operation (100 shots/day)
Flexible (a good plasma already 1 day after opening)

3. Hardware required to have tokamak plasma?
A tokamak (CASTOR/GOLEM)
Vacuum and gas handling system
Power supplies
Basic diagnostics
Control and Data acquisition system

4. Preparation of the GOLEM vessel for operation
Pumping of the vessel down to 10-6mBar (10-4 Pa) by
turbo-molecular pumps is suficient
Baking of the vessel to
250o C (CASTOR) every day before shots
Glow discharge cleaning
Vessel is filled by Hydrogen gas (~ 1 Pa)
Electrode (anode) is inserted into the vessel an biased to ~ +500 V
Glow discharge is burning at ~0.5 – 1 Amp
Inner wall is bombarded by H ions
Molecules are desorbed from the inner surface

5. Basic diagnostics to operate GOLEM
Toroidal magnetic field
Plasma current
Loop voltage
Position of plasma column in the vessel
Plasma density
Plasma radiation in visible range (H_alpha, impurity lines, soft x-ray, …..)

6. Basic power supplies of CASTOR/Golem - schematically
Capacitor bank
1 MJ, 2 kV
TF COILS
Toroidal magnetic field
Grid
Rectifier
Capacitor bank
0.4 MJ, 0.5 kV
Primary winding
of transformer
Breakdown and plasma current
Grid
Rectifier

7. Loop voltage d/dt – magnetic flux Uloop = - d/dt
The loop voltage is measured by a loop located in the proximity
of the plasma column
The toroidal electric field E tor = Uloop/2R accelerates charged
particles in the toroidal direction (and drives plasma current)

8. Toroidal magnetic field
Signal of the loop has to be integrated
analogue integrator
numerical integration

9. I halpha ~ no*ne ~ dne/dt
dne/dt = Sion * no*ne
Plasma is fully
ionized
I halpha ~ no*ne ~ dne/dt
~ density of electons
Maximum of I halpha
is observed when
no= ne
~ density of neutrals
Time [ms]

10. Plasma current
Plasma current is measured by means of the Rogowski coil
(the solenoid of a toroidal shape surrounding the plasma column)

11. I_vessel = I_plasma – Uloop/Rvessel
Start-up phase of a discharge on the CASTOR/GOLEM
Loop voltage [V]
Uloop
Toroidal current [kA]
I_plasma+ I_vessel
Plasma density
ne [1019 m-3] 2 4 6
Time [ms]
I_vessel = I_plasma – Uloop/Rvessel

12. Maximum plasma current - Safety factor q
Stability of plasma requires the safety factor at the edge
q(a)MIN > 3
Dimensions of a tokamak and the value of the toroidal magnetic field determine the maximum plasma current which can be driven in a tokamak:
CASTOR a = m, R = 0.40 m, Bt = 1.0 T => IpMax ~ MA
JET a = m, R = 2.96 m, Bt = 3.45 T => IpMax ~ MA
GOLEM Reality ~ 15 kA max

13. Safety factor

14. Temporal evolution of a discharge on the CASTOR tokamak
Vessel is filled
by working gas
Toroidal
mag. field Bt
Loop voltage
Uloop
Plasma current Ip
Plasma density ne
-20 20 40
Time [ms]

15. Plasma radiation
Photomultipliers equipped by interference filters or scintillators
puffs of H2
Plasma density
ne [1019 m-3]
Gas puffing
H_alpha [a.u.]
Recycling
H_alpha [a.u.]
Additional gas (Hydrogen) is
injected into the plasma to
keep plasma density at
a “constant” value.
A piezoelectric valve is used
Impurities
CIII line [a.u.]
Hard x-ray
E>40 keV [a.u.]
Time [ms]

16. Ohmic heating in tokamaks
Plasma – secondary winding of the transformer (a single loop)
with a finite conductivity
Ohmic power dissipated in the plasma column
Electron temperature can be estimated from plasma resistivity
(Spitzer formula)

17. Resistivity of plasma column and ohmic power
R = Uloop/Iplasma
Poh = Uloop*Iplasma

18. Global confinement of energy (1)
Balance equation for energy
E Energy confinement time !!
globally characterizes losses of energy
Total kinetic energy in the torus
W = 3/2 (ne*Te + ni*Ti) * V = 3 ne Te
Heating
Energy losses
E = W/ P

19. e = (ne*Te*V)/ POH Electron temperature and confinement time
Te ~ R-3/2
e = (ne*Te*V)/ POH

20. Conclusions
GOLEM is the oldest tokamak in the word
It will be operational soon (hopefully)
Its education potential is huge!
Its scientific potential is not definitely exhausted
We are looking forward to meeting you in
the control room of GOLEM