Remote operation of the GOLEM tokamak

Gent University, December 14, 2009

Jan StockelInstitute of Plasma Physics, Prague

stockel@ipp.cas.cz

Vojta Svoboda Faculty of Nuclear Sciences and Physical Engineering

Czech Technical University, Prague

svoboda@br.fjfi.cvut.cz

Introduction of the GOLEM tokamak• Basic hardware• Basic plasma diagnostics

Remote operation of GOLEM• Basic plasma performance

Unique feature of the GOLEM tokamak

It is the only tokamak in the world, which can be operated remotely via Internet!

Tokamak - basic principle

Tokamak is composed of

three basic components

• Large transformer

• Plasma ring as secondary winding

• Coils for confinement of plasma ring by magnetic field (toroidal solenoid)

Electric current I generated in the plasma ring by the transformer

• delivers the ohmic power Pohmic = I2Rplasma to plasma (heating)

• generates the poloidal magnetic field in the plasma ring Bpoloidal ~I/2a

REMEMBER! Because of the transformer, tokamak is a pulsed device

The GOLEM tokamak is located at the Faculty of Nuclear Physics and Physical Engeneering (Czech Technical University in Prague) located in the proximity of jewish cemetery.

GOLEM tokamak in Prague

Major radius 0.4 m

Minor radius of the vessel

0.1 m

Minor radius of the limiter

0.085 m

Maximum B-toroidal 0.3 Tesla

Maximum plasma current

8 kAmp

Discharge duration ~ 5 ms

Iron core of the transformer

Toroidal magnetic field coils (28)

Primary winding of the transformer (12 turns)

Diagnostic ports (18)

Front view of GOLEM (schematically)

Engineering scheme of the GOLEM tokamak

Gas handling system

Vacuum pumping

Generation of toroidal magnetic field

Generation of plasma current

Power supplies of GOLEM (schematically)

For generation of toroidal magnetic field

For breakdown of working gas and plasma current drive

Capacitor bank Toroidal Field CoilsGrid Rectifier

Triggeredswitch

Capacitor bank Primary winding of transformerGrid Rectifier

Triggeredswitch

At the moment, we measure:• Toroidal magnetic field (by a pick-up coil)

• Plasma current (by a Rogowski coil)

• Plasma radiation (a photodiode)• Loop voltage

Basic diagnostics of GOLEM

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, which accelerates charged particles in the toroidal direction (and drives plasma current) can be easily derived as:

E tor = Uloop/2R

1. Tokamak is pumped down to to the pressure <1 mPa

2. Tokamak vessel is filled by a working gas, typically the pressure of

hydrogen is ~ 30 - 80 mPa

3. Some free electrons are generated in the vessel by means of an a small

electron gun or by a cosmic background (pre-ionization)

4. Power supplies are activated by charging two capacitor banks:

* generation of the current in the toroidal magnetic field coils (0.6-1.2 kV)

* generation of the current in primary winding of the transformer (0.6-1.2 kV)

Sequence of events before a tokamak discharge

1. A trigger pulse is applied to start the data acquisition system

Experimental data a collected

2. A trigger pulse is applied to discharge the capacitor bank UBt to toroidal field coils

Toroidal magnetic field is generated inside the vessel

3. Wait until a reasonable level of the toroidal magnetic field is reached. Typical time delay is 2 - 4 ms

4. A trigger pulse is applied to discharge the capacitor bank Uoh to primary winding of the transformer

Time-dependent current in the primary winding generates the toroidal electric field inside the vessel

Start-up of a tokamak discharge

Sequence of events during a discharge

Toroidal magnetic field

Pressure of Hydrogen

Loop voltage

Uloop is high enough – Breakdown

50 mPa

Time

Delay

Trigger Bt

Trigger Uoh

Electron are accelerated in toroidal direction and ionize the working gas

Fully ionized plasma fills the vessel (in 0.1-10 ms – depending on the size of tokamak)

Density of charged particles increases exponentially in time

Free electron(s) appear in the vessel

Now, we can check the reality