# Presentation: 065486

## Steady-State Operation of Fusion Plasmas

**Steady-State Operation of Fusion Plasmas**
**- the Stellarator Project Wendelstein 7-X **

**Thomas Klinger****Max-Planck Institute for Plasma Physics**
**on behalf of the **
**enterprise Wendelstein 7-X**

**PPPL Colloquium**

## Introductory comments

**Tokamak steady-state operation is a subject of current research**
** ********N****~4 required - achieved only with advanced tokamak scenarios (ITB‘s)**
** ****Control of pressure profiles, current profiles and instabilities required**
** ****Several 100MW of CD required – efficiency only 20-40% must go up**
**Stellarators are steady-state „by nature“**
** ****The control issue is much less a problem – only weak CD needed**
** ****Plasma performance not yet satisfactory (esp. ********E ****and ********)**
** ****Divertor solution and control of impurities required**
**Several technologies must be developed in general**
** ****Superconducting magnets (also HTSCs)**
** ****Steady-state H&CD solutions **
** ****Steady-state capable in-vessel components**
** ****The right materials in general**

**Introductory comments**

## Introductory comments

**Introductory comments**

**J. Pamela, SOFT 2008****www.soft2008.ipp.mpg.de**

## Outline of the talk

**Outline of the talk**

**Principles**
**Island divertor**
**Plasma currents**
**Performance**
**Construction status**

## Steady-state – a stellarator feature

- In stellaratorssteady stateoperation istaken for granted.

## Tokamak and Stellarator

** ****current in coils and plasma **
** ****current-carrying plasma**
** ****self-organized equilibrium**

** ****current in the coils only**
** ****very small plasma current**
** ****field-defined equilibrium**

- A very schematic comparison ...

## The stellarator family

***shut down***
***operating***
***under construction***
***planned***

## Stellarator symmetries 

**quasi-helical**

- by courtesy of J. Sanchez

***|B|***

## A missing link 

** ****complete the physics understanding of quasi-symmetry**
** ****development of compact stellarators**
** ****a physics link to advanced tokamak operation**** **
** ****let‘s advertise a revision of previous decisions**

## LHD and W7-X

- Rax = 3.4 – 4.1 m, a ≤ 0.65 mVpl = 30m3
- B ≤ 2.9 T, (0) ≥ 0.35, (a) ≤ 1.5
- high shear, 10 field periods, l = 2

**W7-X **
- Rax = 5.5 m, a ≤ 0.53 m
- Vpl = 30m3
- B ≤ 3.0 T, (0) ≥ 0.88, (a) ≤ 0.97
- low shear, 5 field periods

**LHD and W7-X**

## MHD equilibrium - comparison

** ****stable high-pressure equilibrium**
** ****minimized stochastic layer formation**

**W7-X (PIES-Code)**

## W7 & LHD magnetic configuration

**W7 & LHD magnetic configuration**

**W7-line: ****	****avoid islands by low shear****LHD-heliotron: high shear, small islands**

## The optimized stellarator W7-X 

**The optimized stellarator W7-X **

** ****feasible modular coils**
** ****good, nested magnetic surfaces**
** ****good finite-********** equilibria**
** ****good MHD stability**
** ****small neoclassical transport**
** ****small bootstrap current**
** ****good confinement of fast particles**

**7 optimization criteria**

**development tasks**

**optimum *****nT************E***** ****and high *********** ***** discharges**
**steady state operation**
**plasma-wall interaction**
**island divertor operation**
**turbulent transport**

## Island divertor – the idea

**ASDEX upgrade**

**Wendelstein 7-AS**

**Wendelstein 7-X**

**poloidal divertor**

**island divertor**

## The W7-AS island divertor

- one of five magnetic field periods

## Operation without divertor 

## Operation with divertor – HDH 

**high density**     ***n******e*** = 2.51020 m-3
-                             ***n******e******/n******eGW*** = 2.5

**high pressure**   ******* = 3.4%*
- 	             *******N **~ 9.3*

**absorption**      ***P******abs*** = 2.5 MW

**confinement**   ***H******ISS95*** = 1.4

**MHD stability**

**stationarity** ***T******flattop*** ~ 100 *********E***

## HDH - an advanced operation regime

**HDH-regime - a true bifurcation with hysteresis, profile differences, ...**
**	********  ****confinement of energy increases**
**	******** **** ****confinement of impurities deteriorates**

**the high-density high-confinement (HDH) mode****was discovered in 2002 on W7-AS**

**[McCormick PRL 2002 No 015001]**

## High-beta experiments in HDH

**MHD modes dissappear ****at high** 

** ****inwards shift of ********=1/2 surface**
** ****formation of magnetic well**

-   3%

**[Weller 2003]****[Werner 2003]**

- (m,n)=(2,1)

## W7-AS Performance /w island divertor

** ****Based on resonant magentic islands @ edge**
** ****A viable divertor solution for stellarators**
** ****Controlled heat and particle deposition**
** ****Density control even at high plasma densities ~ 2****·****10****20****m****-3**** **
** ****Establishment of High-density H-mode (HDH)**
** ****Establishes at n~1.5 ... 2.5 ********10****20**** m****-3**
** ****Quiescent and MHD stable ****	******** magnetic well formation**
** ****Different from ELM-free H-mode**
** ****Improved energy confinement****	**
** ****Impurity screening****	****	******** high edge densities**

## Plasma currents - overview

|  | RFP | Tokamak | W7-X | LHD | NCSX |
| --- | --- | --- | --- | --- | --- |
| diamagnetic current | O (1) | O (1) | O (1) | O (1) | O (1) |
| Pfirsch-Schlüter current | 0 | O (1) |  0 | O (1) | O(1) |
| bootstrap current | 0 |  O (1) |  0 | O (-1)...O (1) | O (1) |
| induced current |  O (1) | O (1)  0 | 0 | 0 | 0 |
| plasma dynamo | O (1)  0 | 0 | 0 | 0 | 0 |
| current drive | 0  O (1) | 0  O (1) | 0 | 0 | 0 |

**Plasma currents - overview**

**all currents are normalized to their respective frame**

**By courtesy of F. Wagner**

**It is possible to reduce systematically the currents in the plasma.**

## W7-AS with induced current

- current driven instability
- tearing-mode typeonset at iota=1/2

- magnetic fluctuationsandsoft-X radiation

**W7-AS with induced current**

**[Zarnstorff 2003]**

## Non-inductive equilibrium currents

**L = 2 stellarator**

**W7-X**

**current lines**

- j

- j

- j

- B

**equilibrium current**
*J=J**dia**+J**PS**+J**BS*

## Electron-cyclotron current drive – Ex:

** ****1d transport modelling coupled with ray-tracing code**
** ****Off-axis X2-heating in standard configuration**
** ****Current drive required for control of edge iota**
** ****Current drive efficiency *****P*****=10MW** **<**********>=2.7% *****I******CD*****=-88kA**

## ECR heating of dense plasmas

** ****O2-launch angle 12°**
** ****Heating up to 2.1********10****20****m****-3**
** ****Single pass absorption drops to 50%**
** ****Double pass with retro-reflectors**

## Current drive in X2 and O2

## Global energy confinement

**10 MW ECRH**

**10 MW NBI (H****+****)**

**ITER**

**Numerical simulations:**
**much improved confinement**

**Global energy confinement**

## Wendelstein 7-X key parameters

**key parameters**
**major radius: ****	****5.5 m**
**minor radius: ****	****0.53 m**
**plasma volume****	****30 m****3**
**non-planar coils:****	****50**
**planar coils:****	****20**
**number of  ports:****	****253**
**rot. transform:****	****5/6 - 5/4**
**induction on axis:****	****< 3T**
**stored energy:****	****600 MJ**
**heating power****	****15 - 30 MW**
**pulse length: ****	****30 min**
**energy turn ar.:****	****18 GJ**
**machine height:****	****4.5 m**
**machine diameter:****	****16 m**
**machine mass:****	****725 t**
**cold mass:****	****425 t**

## Wendelstein 7-X main components

**the volume in the cryostat is very constrained ******

## Steady-state ECRH heating

** ****power gyrotrons**
** ****10 ******** 1MW 140GHz **
** ****pulse length 1800s**
** ****quasi-optical duct**
** ****world record shot**
** ****integrated cw design**
** ****series production?**

## Design of In-vessel components

****** ****target elements CFC sealed on cooled CuCrZr**
****** ****baffle elements graphite clamped on CuCrZr**
****** ****cryopumps and sweep coils**
****** ****about 250.000 parts w 130.000 being non-standard **
****** ****about 4km in-vessel water pipe lines**
****** ****start of operation with inertially cooled test divertor**

**test divertor**

**wall panel cooling **

**divertor module**

## Manufacturing in-vessel comp‘s

**Target modules****(Plansee)**

**Manufacturing in-vessel comp‘s**

**Heat shield****(IPP)**

**Cooling pipes****(IPP and n.n.)**

**Wall pannels****(MAN DWE)**

**Cryo pumps****(IPP)**

## Superconducting coils

**CAD drawing non-planar coil**

**Status coil delivery**
**20 planar coils**
**50 non-planar coils**
**100M€ contract ******** consortium**
**100% manufactured**
**70% successfully cold tested**

## Components and assembly

**plasma vessel modules**

**thermal insulation**

**assembly**

**central ring modules**

**outer vessel with domes**

**assembly****progress **

**Components and assembly**

## The outer vessel

**The outer vessel**

**More than 1000 openings ~ thermal insulation laborious**

## Coil assembly

**The 20 coils of first two modules assembled ******** – supports are welded**

## Torus hall - machine base

**The five assembly stands are ready and fill step-wise (now 3/5)**

**Torus hall - machine base**

## Magnet module

**The completed first magnet module went into the next assembly stand**

## Schedule

**Schedule**

** ****completion date 2014**
** ****68 weeks buffer time**
** ****29 milestones**
** ****tight schedule control **

## The start of exploitation

** ****inertially cooled divertor and only partial cooling of in-vessel comp‘s**
** ****shut-down (15 months) for completion and hardening**
** ****2nd operation phase to approach 30min @ 10MW**
** ****3rd operation phase with 10MW ECRH, 20MW NBI and 10MW ICRH**

**The start of exploitation**

## The Wendelstein 7-X stellarator

** „****Fully“ optimized magnetic field configuration with simultaneously ...**
** ****low equilibrium currents**
** ****good magnetohydrodynamic equilibria**
** ****good magnetohydrodynamic stability**
** ****good neoclassical confinement**
** ****good fast-particle confinement**** **
** ****First superconducting stellarator with modular magnetic field coils**
** ****steady-state island divertor for full power load **
** ****steady-state 10MW 140GHz ECRH with quasi-optical wave guide**
** ****steady-state diagnostic and CoDaC system**
**A promising new high-temperature plasma device**

**The Wendelstein 7-X stellarator**

## Fusion power - „think big“

## The tokamak principle

**The tokamak principle**

**Tokamak ****(1951 Sacharov und Tamm)**
**тороидальная камера в магнитных катушках****„toroidal chamber in magnet coils“**** **

## The stellarator principle

**Stellarator ****(1951 Spitzer)*****Stella***** = the star**

***j******tor************ 0***