# Presentation: 199624

## Aspect Ratio Optimization of 

- Burning Plasma Tokamaks

- Pietro Barabaschi
- ITER International Team Garching
- Prepared for IEA Workshop on Optimization of High- Steady-State Tokamaks
- February 14-15, 2005
- General Atomics

## Introduction

**The ITER EDA.. developed****	**
**	****	****-needed design solutions, **
** ****	****	****-****enabling technologies, **
** ****	****	****-****and knowledge base**** **
**BUT, the Tokamak is a complex system and for its optimisation it requires a d****etailed understanding of all interplays ****of ****design drivers (and the devil is in the details!)**** **
**We do not have the basis and criteria to design (or even more to optimise!)  a fusion reactor today, most notably we are missing:**
**Plasma burn demonstration**
_**Adequate **__**understanding of Plasma **_
**Practical viability of fusion**
- Beta (device optimisation)
- _Reliability, Availability, Maintainability (R.A.M.)_
- Materials
- _Divertor power exhaust_
- Higher performance structures/SC
**Until we understand and develop all these points the cost optimisation of a reactor may not be realistic**

## Tokamak Design: Machine parameters

**For a SC Tokamak, given:**
**	****-Desired Plasma “performance”: Q, burn time, # of shots**
**	****-Plasma Boundary conditions: q, n****gwmax****, ********Nmax****, ********, ********, H****H**
**	****-Physics Criteria: ********, n****gw****, P****LH****, Beta**
**	****-Engineering Criteria: Stress, loads, SC criteria, times and solutions for maintenance, Access to Plasma (diags, H&CD), Nuclear criteria, Design solutions...**
**Only Aspect ratio (or Peak Field in magnet) is left ‘free’**
**However, allowable ******** and ******** are function of R/a for divertor space, plasma shape and position control. Access to plasma is function of ripple requirements and R/a**
***NB:In the case of Steady State tokamaks also the safety factor may be an optimisation parameters.***
**A System code is normally used to study options combining physics rules with engineering design knowledge. It can **_**only be used if  a more detailed design of a particular type of machine has been done and the experience / knowledge has been implemented**_

## Example : Nb3Sn SC Design Criteria

**Temperature Margin**** from the max predicted T at any point to the local current sharing T. **
**Tmarg>1K with FP plasma  **
**Stability**** (Heat transfer to Helium)**
**Hot Spot Temperature<150K**
**1 and 2 determine the amount of SC strand, 3 determines the amount of additional copper and overall conductor size**
**Main significant system interactions with: Magnet (local cost, System size and cost, VV (stress due to Fast Discharge)**

-  Strand	1 < Cu:nonCu < 1.5
- 	RRR 100
- 	Tcom= 18K
- 	Bc=28T
- 	Jc (12T,4.2K,-0.25%) = 650 A/mm2

## Plasma Performance Criteria

_**BUT....**_
_**All 3 main scaling relationships have little real physics basis!! Optimisation have big limitations and uncertainties!**_
_** **__**It is likely that there’s an interplay between H, shaping, q, nGW, ...**_

**q95**

## Typical results from the System Code Study

**The main machine parameters and the cost change with increasing aspect ratio in the following way:**
**The toroidal Field, the Magnetic Energy increase with Aspect Ratio**
**The minor radius and the Plasma Current decrease with Aspect Ratio**
**However, the cost of the machine stays ~constant over most of the Aspect Ratio range investigated**

## Machines with different R/a – Elevation View

## Machines with different R/a – TF Magnet section

**Machines with different R/a – ****TF Magnet section**

## System Analysis: Design Drivers

**Radial Build: ****	************ 10cm****	****R ******** 18cm****	****C ******** 60kIUA**
**Shielding (heating, damage, reweldability)**
**CS Magnet**
**TF Magnet**
**Assembly and tolerances**
**Elongation:****	********95 ******** ****0.1****	****R ******** ****17cm****	****C ******** 80kIUA**
**BUT ...(Stability, VDE’s, SN-DN control, Divertor space, flexibility)**
**Triangularity:****	********95 ******** 0.1****	****R ******** 10cm****	****C ******** 100kIUA**
**BUT ...(SN-DN control, Divertor space, Sawtooth R, Magnet loads)**
**Safety Factor:****	****q****95 ******** 0.1****	****R ******** 5cm****	****C ******** 50kIUA**
**BUT ...(HH degradation, Disruptions loads, Magnet loads)**
**Confinement:****	****H ******** 0.1****	****R ******** 12cm ****	****C ******** 130kIUA**
**(Note: ********C/********R not constant)****	**

## A Power Reactor

**Where is a Power Plant quite different from an experiment?**
**Additional problems**
**In physics : **
- Beta(density, peaking),
- Steady State
**In engineering :**
- Remote Maintenance
- Current drive efficiency
- Reliability
- Availability
- Power exhaust
- Materials

**With some simplifications **
**In physics :**
- Experimental Flexibility
**In engineering:**
- Disruptions/VDEs?
- Diagnostics
- Heating methods
- Fatigue

## Aspect Ratio related issues

**Relation between shape and density „limit“ is far too simplified. Effects of shaping must be included.**
**Beta limit is NOT an invariant of aspect ratio**
**limit = f(A)**
**At low A the relative distance between plasma and wall is less (RWM)**
**Achievable shaping is NOT an invariant of aspect ratio**
**Natural elongation increases at low A**
**Available space for divertor at given d increases at low A**
**Relative (and absolute) distance between plasma and PF magnet reduces at low A. This impact shape control (in particular in SN) as well as plasma vertical controllability.**
**In steady state tokamak q95 is also a „free“ optimisation  parameter (HH=f(q95) ?)**
**All these effects are crucial for the „optimisation“ of steady state tokamak and when included point to reduction of value of optimal A**

## Important (but often forgotten) Engineering Issues

**Access to plasma for H&CD (and maintenance of internals) . In particular tangential for NBI (ans shinethrough issues). Check carefully at High A!!**
**Space available for divertor**
**Thermal and EM loads on Blanket a function of A (PF almost constant but TF not!)**
**Cost and replacement reqs of internals is a function of complexity (thermal and mechanical loads)**
**TF discharge parameters is important for VV stresses (and cost) at high A.**
**Space for water cooling and pipe extraction**
**Again all the above issues, when included, tend to benefit low**** A**
**(note: for Cu devices with limited pulse duration – e.g. FIRE – High A benefits largely from reduction of required pulse length with reduced a)**

## Conclusions

**We cannot optimise a s****teady state tokamak today. **
**First we must understand the underlying PHYSICS and gain experience in construction, operation, and MAINTENANCE of a large super-conductive tokamak.**
**Warning: Simple scaling optimisation of steady state tokamak typically points to relatively high A (e.g. 3.5 - 4) due to low current requirements and high I****BS**** fraction. This may be well be an illusion. Real engineering and more accurate physics basis may well reverse this conclusion.**