Fishtail divertor – Swing strike point for active control of

heat load on divertor plantX.D. Zhang1, Y Zhang1, J.P. Qian1, J.X. Zhen1, B. Li1, G.N. Luo1, Q. Yu2, S.J. Du1, L. Wang1, B.J. Xiao1, D.M. Yao1, Y.Y. Huang1

1Institute of Plasma Physics, Chinese Academy of Sciences, Hefei 230031, P.R.China2Max-planck Institute of Plasma Physics, D85748, Garching, Germany

A S I P P

*X.D. Zhang, et al, IAEA Second Technical Meeting on Divertor Concepts,13 – 16 November 2017, Suzhou China, P-3

Tel: 86-551-5591894 Fax: 86-551-5591310 E-mail: xdzhang@ipp.ac.cn

A new divertor concept, the Fishtail divertor (FTD), is proposed and investigated on

EAST.

FTD can quickly move the strike point along the radial and poloidal direction like

the swing of fishtail by additional alternating magnetic field.

The maximum moving distance of the strike point is active controlled by the

alternating field amplitude.

The wetted area of the heat flux is widened, so that the averaged heat load is reduced.

PF coils are needed and set behind divertor plates near the strike point, the

alternating magnetic field effect on the location of X points is very small .

Introduction

SN Divertor configuration

in Tokamak like fishtail

Long-pulse hybrid scenario in JT-60U

Long pulse H-mode operation with upper W divertor

• Input heating power~ 5MW，heat flux of ~3 MW/m2

on the divertor , temperature increase ~500℃

• Plasma is uncontrollable when the temperature on the

divertor plate is above 600℃

Periodic change of divertor configuration: USN-DN-

LSN.

• Moving strike points to Control heat load on target and

reduce the temperature of divertor for steady-state operation.

• Reduce impurities and material outgassing.

Development of divertor concepts

Some mew divertor concepts are developed to reduce the heat load unit area of divertor target.

These new concepts are all the static magnetic field

To broaden the distance between adjacent magnetic surfaces and increase heat flux channel widths in the

region from X point to target.

nonuniform distribution of the heat flux – the heat flux also need to be analyze and verified in broadening

magnetic field.

Strike point sweeping was firstly carried out on JET experimental

campaigns of 2006 year

Sweeping strategy was implemented within the Shape Controller

system, so the strike point sweeping is limited, only 4Hz.

The X point also produce larger displacement and changing of the gaps.

The Improvement effect is not significant. No cooling behind the plate

X-point

strike pointsFTD coil FTD coil

This divertor concept is like the strike point swing on JET,

but that is needed a special PF coil – FTD coil, which only

move the strike point.

FTD independent operation, does not affect the plasma

control system.

Strike point moving distance is controllable by controlling

the FTD coil current, so can active control of thermal

deposition area.

Because of the swing broadening, the heat flux deposition of

FTD is more uniform and is not to analyze the heat flux

distribution.

Reciprocating motion of the strike points like fishtail swing

Heat load has already become a crucial issuefor steady-state operation at high heating powerand requires a solution.

Divertor plate temperature gradually increased

due to long pulse operation

Impurities and outgassing from the divertor plate

affects the steady-state operation of tokamak.

Simulations of FTD Heat Load on Divertor

Possible advantages of FTD:

Good uniformity

Very small effect on the plasma

Good results for high and narrow heat flux

ELMy mitigation by means of fast swing

Neutron shielding of FTD coil

FTD can effectively reduce the heat load on the target and especially is more effectively for higher and

narrower heat flux.

May be used in the future reactor - must be controllable and safe:

1. Far away from the plasma, hidden behind the shield.

2. long leg divertor, special divertor chamber, combine with the

radiation divertor technology.

FTD Can Be Used in DEMO or CFETR ?

long leg

Neutron

shielding

FTD coil FTD coil

Target

Swing

Swing distance 10cm

Heat flux

Width1cm

10MW/m2

Heat flux

width 2cm

10MW/m2

C: no swing 1057 ˚C 1872 ˚C

C: 10 Hz 525 532

C: 20 Hz 520 525

C: 40Hz 516 520

Mo: no swing 1235 1994

Mo: 10 Hz 578 586

Mo: 20 Hz 572 578

Mo: 40Hz 567 570

W: no swing 1155 1878

W: 10 Hz 550 557

W: 20 Hz 544 550

W: 40Hz 540 544

Mo target

C target

FTD can make full use of the cooling capacity of the target plate and reduce the thermal stress of the material

Fishtail divertor – FTD and configuration

Design of FTD coil on EAST

1870

1090

Solution 1： 4 sections will be installed and brazing

Coil clamps

• TU1 conductor, OD30/ID15

• Ceramic sleeves ID33，OD46

Solution 2： 1 complete coil will be installed

• TU1 condutor+MgO layer +316L conduit• Shrinkage after assembly• Clamps are employed to hold the coil firmly

26m1086.529 S24 m10766.1 sA

1.74m1l

4109.9R H101.21 -5L

228.0|| Z

AC current operation

• Low frequency mode: 10-100Hz , 5kA

• High frequency mode: 3kHz, 4.2kA

Parameters estimation

Two solutions for the coil manufacture

and installation

Long-pulse H mode operation with input power of 20-30MWwill take a biger challenge to the EAST divertor

T. Eich,et al., NF 53(2013) 093031

High heat load on the divertor target plate is

one of the major problems to be solved for fusion

reactors

The recent results of multi-machines show that

the heat flux channel widths is extremely narrow in a

reactor and about 1/5 of the initial estimate value.

In DEMO the heat flux widths is more narrow than

in ITER.

On EAST, the heat load has already become a crucial issue for steady-state operation at high heating

power, so a special operation mode is required to reduce the target temperature

Heat Flux on Divertor Target in Tokamak Divertor in Steady State Operation

WITER

~350 MJ

W

PFC of ITER,

10MW/m2 for SS

20MW/m2 for 10s

EAST (2012) > 400s EAST (2016) H-mode > 60s

EAST (2017) H-mode > 100s

Divertor in long-pulse operation on EAST

As the heat flux increases, the target temperature increase is very small

if the striking point swing.

Heat deposition zone can be expanded by 100 times for ~mm heat flux.

p

p

F

Fs

I

d

d

Ikd

cmdkAI sF 10,5

Simulation of FTD heat load on EAST divertor

Heating flux 10MW/m2, spreading distance ~10cm.

Temperature on the divertor plate is reduced about 2~3

factor

No significant decrease when sweeping frequency is

above 10Hz

No swing Swing with 10Hz

EAST

ITER-like

Simulation of FTD heat load on ITER-like divetor

Heating flux 20MW/m2, width ~1 mm, and

spreading distance ~10cm

Temperature is reduced almost half by using FTD

sweeping frequency of 10Hz