R E S E A R C H C E N T E R Helimak

The HelimakExperiment

R E S E A R C H C E N T E R Helimak

The Helimak is a major internationalexperiment for the study and control of

plasma turbulence

• Devised by Stan Luckhardt at UCSD• Managed and operated by the University of Texas• Engineered and fabricated by the Institute of

Plasma Physics of the Chinese Academy of Sciences with the Shanghai Boiler Works, PRC

• Installed at the University of Texas Fusion Research Center

R E S E A R C H C E N T E R Helimak

� Physics: Plasmas are further from thermodynamicequilibrium than any other laboratory system; plasmas are aunique challenge to our understanding of non-equilibriumsystems

� Fusion Power: Transport -- the processes by whichplasmas lose heat and relax to thermodynamic equilibrium --is the least-understood aspect of device physics and design.

R E S E A R C H C E N T E R Helimak

Even the best-confined plasmas -- the best cases in tokamaksand stellarators -- lose heat and energy orders of magnitudefaster than predicted by the most sophisticated fundamentalcalculations -- neoclassical theory.

Just as fluids far from equilibrium develop instabilities,convective cells, and turbulent eddies to enhance transport,plasmas are unstable and exhibit turbulence.

R E S E A R C H C E N T E R Helimak

� Design an experiment with instabilities and turbulencerepresentative of that found in fusion plasmas

� Make the geometry as simple as possible to facilitatecomparison with theory and computer simulations

� Choose experimental parameters for which completemeasurements of the turbulence characteristics are possible

R E S E A R C H C E N T E R Helimak

� The magnetic field lines are helical

� The plasma is symmetric around the rotation of thehelix and approximately uniform along the length of thehelix; the plasma varies only with the radius of the helix

� A plasma density gradient together with the curvedmagnetic field lines suffices to drive plasma instabilitiesand turbulence of the drift-wave class that is universal infusion

R E S E A R C H C E N T E R Helimak

� Design� Construction� Assembly� Operation� Results

R E S E A R C H C E N T E R Helimak

Design ConceptThe genesis of the Helimak was the observation that electron cyclotron

heating applied to start a tokamak discharge produced a very good plasma. Thecombination of toroidal and vertical magnetic field produced helical field lines.Although a simple configuration with toroidal symmetry should lack anequilibrium and be violently unstable, the field lines terminated on the top andbottom of the vacuum vessel. Since the vessel was conducting, currents couldflow from top to bottom, stabilizing the plasma.

This is a Helimak. In cylindrical coordinates (r,θ,z), a strong Bθ with aweak Bz create helical field lines. The field line terminations at the ends areconnected electrically. The fields are created in a toroidal vacuum vessel with aset of 16 toroidal field coils (equivalent to a current along the axis of the cyclinder)and three vertical field coils in a modified Helmholz arrangement. The verticalfield is constant over the plasma volume, whereas the toroidal (azimuthal) fieldvaries as 1/r from the axis of the cylinder.

Vacuum Vessel

VF Coil

TF Coil

HelimakHelimak

The Helical Magnetic Field LinesThe Helical Magnetic Field Lines

R E S E A R C H C E N T E R Helimak

Engineering Parameters

<R> = 1.1 m0.6 m < R < 1.6 m

h = 2 mBT = 0.1 TBv ≤ 0.01 TPulse ≥ 60 sPlasma Source: 8 kW ECH

@ 2.45 GHz

R E S E A R C H C E N T E R Helimak

Plasma Parametersn ≤ 1011 cm-3

Te = 10 eVConnection length:20 m < L < 500 mcs = 3 x 104 m/sVdrift = 100 m/sVdiamagnetic = 103 m/stp (parallel loss) > 1 ms2 kHz < ndrift-wave < 20 kHz

Flow: 0 < Er < 103 V/m

R E S E A R C H C E N T E R Helimak

Design Features The Most Simple Realistic Geometry

> Cylindrical slab -- no toroidal effects, easy comparison totheory and codes

> One-dimensional (radial) equilibrium> Magnetic shear> Accurate model of tokamak SOL; long field lines good

approximation to infinite field lines of confinementdevices

Thorough Diagnostics> Langmuir probes and probe arrays may be used

everywhere to measure all parameters> Spectroscopic measurements of flow

R E S E A R C H C E N T E R Helimak

Special Feature

Access to field lines at end platesallows control of flow shear

> Control of Er(r) -- E X B flow> Control of j||(r)

R E S E A R C H C E N T E R Helimak

From the conceptual design, the Institute ofPlasma Physics of the Chinese Academy of Sciences(ASIPP) in Hefei completed the engineering design,built the magnet coils, and supervised the fabrication ofthe vacuum vessel and mechanical components at theShanghai Boiler Works (SBW). The components wereassembled, the vacuum vessel tested, and the partspacked for shipment at the SBW.

TF CoilsWinding

Test

VF Winding

Oven for curing coils

VF Winding

TF test atfull current

Vacuum Vessel Caseat SBW

December 2001

Assembling TF Coils on to the Vacuum Vessel

Setting the assemblyon its stand

Completing the test assembly

Trial AssemblyComplete

Shanghai Boiler WorksMarch 2002

Vacuum Leak Test

R E S E A R C H C E N T E R Helimak

The apparatus arrived at theUniversity of Texas in October2002. Over the next fourmonths, it was assembled andcommissioned.

The first truckwith the TF coilsand base plate

The second truck

The lifting jig beinglowered through the roofhatch into theunderground laboratory

Unloading thevacuum vessel

The VF coils,a tight fit

Threading the TF coilson to the

vacuum vessel

Mating the halves

The final lift

Completing the coil assembly

Huang HeKeith Carter

Ken Gentle

The Crew

R E S E A R C H C E N T E R Helimak

3 February 2003First Plasma

Glow Cleaning Discharge in ArgonView looking up from bottom

Outside Port Inner wallTop Port

Top Port with Electrode

R E S E A R C H C E N T E R Helimak

R E S E A R C H C E N T E R Helimak

• The first objective will be to characterize theplasmas produced in the device: densities,temperatures, profiles• The second objective will be to characterize theturbulence -- frequencies, wavelengths, dependenceon plasma parameters -- for comparison with relevantcodes, e.g. those of Bruce Scott at Max-Planck-Institut für Plasmaphysik, Garching• The third objective will be to impose controlledradial electric fields to determine the effects of flowshear on the turbulence

R E S E A R C H C E N T E R Helimak

R E S E A R C H C E N T E R Helimak