Comparison of innovative concepts to help in the decision of the UST_2  fusion experiment.  RFP and Spheromak. (Part 2)    



Abstract :  Basic features are analysed for several fusion concepts, particularly the ones that have implications in the engineering cost and difficulty. Stellarators and tokamaks excluded. Cost of coils and maintenance, Beta,  possibility of advanced fuels, current drive, energy confinement times, particle handling and reactor studies are analysed. The study is divided in 3 Parts, two devices per page. The comparison of the basic features and recent experimental results should help in the decision of a new fusion device, UST_2. If there is not a device  more convincing than a stellarator, UST_2 may be a stellarator.



* RFP  and  Spheromak  (Devices analysed in Part 2  )


See Part 1 and 3 for :

*  Dipole and FRC

Spherical tokamak and IEC

 Stellarator and tokamak not studied here.



   One and a half years ago the analysis of different devices was omitted due to lack of knowledge [100].  The objective is to clarify the alternatives to design a small device for fusion research in engineering. In [100] the result was to build UST_1 stellarator which finally was designed and built as an optimised modular stellarator with external resistive coils on a circular toroid, plaster frame for grooves and copper vacuum vessel. UST_1 has obtained good magnetic surfaces and some initial plasmas.


   Important additional knowledge has been acquired from then, many spreadsheets are of immediate use, a flexible and powerful JAVA code has been developed to simulate, calculate and optimise configurations of coils, many creative and risky techniques, successful and unsuccessful, have been experimented. Building a new stellarator similar to UST_1 would be a matter of one or two intensive months of work and about 300€ of cost. However such a possibility lacks of sense.  


   Some new key engineering features should be included in the new design. Additionally the physics design should be improved, however it might depend on the help of experts in plasma physics.


   UST_2 does not need to be a stellarator. Any other fusion device with better perspectives as a reactor could be chosen.


   In principle  UST_2 is not intended to be constructed without some funding and/or interest from foundations, university, company, sponsor, etc.


  The next compilation will be expanded in the future if new or more accurate information is found. 

  Maybe a table will be completed when all the devices had the same basic data. It will help to have a global vision.






   No-one can be an expert in all fusion devices so some inaccuracies surely will appear in the next comparisons of devices. If you know about one of the devices as a fusion reactor and would like to correct some mistakes, add recent data or an opinion, please send further experimental data supported by reasons. Scaling laws, based even in very reduced experimental results only to have an order of magnitud, are extremely important in concepts barely studied.









Reversed Field Pinch



References [40] to [49] plus general ones. 


Devices  : [20] : RFX (Italy), MST (USA) , TPE-RX ( Japan) 

Extrap-T2 (Sweden) , ZT-40M RFP , HBTX-1C , TITAN reactor (study year 1993) [5]

The  experimental and theoretical information for RFP is acceptable, the best after tokamaks and stellarators.  


Advantages : Low external toroidal magnetic field is necessary so resistive and/or economical superconductor coils, low stress. and medium complexity of coil structure. High “engineering beta”,

Not major disruptions observed up to now.


Disadvantages :  Need of important current drive (more than tokamaks), little space for RF antennas due to close conducting shell (stability difficulties if no shell).


Stability : Kink modes if not close conducting shell. Solutions are fast control loops but complexity and reliability for reactors is a concern (similar to advanced tokamaks)



Alpha heating regime : Alphas may increase magnetic fluctuations and lower Tau E. Other instabilities my be excited by alphas [1]



Energy confinement and transport:

MST: [41] :  Pulse lenght flat top =  30ms, induction V loop, Ip<=0.5MA,  Tau E using PPCD poloidal current drive reached 5ms [41] near to tokamak values, IPB98, a very important improvement. However advance tokamaks with profile tailoring are more than two times superior : AUG,DIII–D, TFTR [5].

Te max =~  800eV , Ti=~300eV(perhaps heating power limited)


One method to create the plasma is  self-generated dynamo but it generates tearing modes resulting magnetic stochasticity that enhances energy and particle transport [41.

Traditionally  magnetic turbulence was a key issue.



Beta :  Experimental 15% max Beta.  More current drive is necessary at lower Beta.      "Engineering Beta" is very high.



Advanced fuels :  Not specially suited for advanced fuels but  more than a tokamak (lower B so less cyclotron radiation)



Self-heated and CW/ pulsed :  Self-heated , CW or pulsed.  CW needs important current drive. Pulsed regime have some power handling issues [5].



Current drive  : YES.  Oscillating Field Current Drive (OFCD) has been proved but the influence on magnetic turbulence in reactor regime is not clear.

Present RF current drive will cause excessive recalculating power [5] [41] or more power than produced will be necessary.

One method to create the plasma is self-generated dynamo but it generates tearing modes and the resulting magnetic stochasticity enhances energy and particle transport [41] . "Pulsed poloidal current drive" (PPCD) needs to be insvestigated. The length of pulse for reactor regime is not known.

Profile shaping is important to reduce magnetic turbulence.


Particle handling : Wall loading is excessive in pulse mode (>10MW/m2).  Divertor and blanket have less space than in tokamaks so more engineering difficulties.


Maintenance :  Difficult, but slightly less than in a tokamak due to thinner neutron shield if resistive coils. Coil failure is an easier issue due to much lower size.


Example of reactor : TITAN reactor. Compact , major radius 4m, minor 0.6m, only approx. half than RFX(2MA , Te= 0.5 to 2keV at ~1e20m-3, Beta 10%), seems too compact. Neutron wall load may be impossible to withstand (15-30 years life) in less than 50 years (late IFMIF results). Light. Design Beta=23%. Low plasma temperature near the divertor is stated. RFP size scaling is fulfilled  High start-up power (500MW). Plasma current 18MA. No severe coil issues.  Based on Beta=23% ,[1]  ne ~1e20 m-3 (achieved 3e19 m-3),  s > 25 (achieved 4 in LSX). nm =  1e20m-3 , Tt= 25KeV , Be = 1T , Rs=2.5m , fw =10kHz, fr=4.7kHz, vde =1.5e5m/s  ,  vs = 13e5m/s , vde/vs~0.13 (achieved 4 in present exp.) [23]


Critical issues :   - Efficient and reliable current drive without relevant negative influence in confinement.















 Reasons and insights to chose a suitable

device for UST_2


     A decision will be taken some time after the end of the study of the devices.








[1] "Comments on Innovative Confinement Concepts
and Burning Plasma Science" E. B. Hooper

[2] "Fusion research : Experiments " T. J. Dolan , Pergamon Press.

[3] "Fusion Energy Science Opportunities in Emerging Concepts" Los Alamos National Laboratory Report -- LA-UR-99-5052


[5] "MFE Concept Integration and Performance Measures" Magnetic Fusion Concept Working Group ,  M.C. Zarnstorff, D. Gates, E.B. Hooper, et al.




[40]  "Introduction and synopsys of the TITAN reversed-field-pinch fusion-reactor study", Farrokh Najmabadi et al.  Fusion Engineering and Design 23, 1993.

[41] "Tokamak-like confinement at high beta and low field in the reversed field pinch", J S Sarff1, J K Anderson1, T M Biewer1, D L Brower2, et al.









"First approaches for the design and construction of an ultra small torus to experiment with fusion technologies" Vicente M. Queral. See "List of all R&D"













Date of publication 28-02-2007. Continuous addition of data