ADVANCES IN LOWER HYBRID CURRENT DRIVE …...LH2 4-way split with toroidal bi-junction 25 G.M....

ADVANCES IN LOWER HYBRID CURRENT

DRIVE TECHNOLOGY ON ALCATOR C-MOD G.M. WALLACE,1 S. SHIRAIWA,1 J. HILLAIRET,2 M. PREYNAS,2 W. BECK,1 J.A.

CASEY,3 J. DOODY,1 I.C. FAUST,1 D.K. JOHNSON,1 A.D. KANOJIA,1 C. LAU,1 R.

LECCACORVI,1 P. MACGIBBON,1 O. MENEGHINI,1 R.R. PARKER,1 D.R. TERRY,1 R.

VIEIRA,1 J.R. WILSON,4 AND L. ZHOU1

1MIT PLASMA SCIENCE AND FUSION CENTER, CAMBRIDGE, MA 02139, USA 2CEA, IRFM, 13108 SAINT PAUL LEZ DURANCE, FRANCE 3ROCKFIELD RESEARCH, LAS VEGAS, NV 89135, USA 4PRINCETON PLASMA PHYSICS LABORATORY, PRINCETON, NJ 08543, USA

E-MAIL CONTACT OF MAIN AUTHOR: [email protected]

IAEA FEC 2012

San Diego, CA, USA

October 9, 2012

Improvements in LHCD technology allow

for longer pulses and higher power

Advanced transmitter protection system allows for long pulses (tpulse >> τR) without jeopardizing klystrons

Moveable integrated LH protection limiter reduces reflection coefficients, but at the cost of power handling

Design of new off mid-plane LH launcher will ~double available power and increase single-pass absorption

This work was supported by US Department of Energy collaborative agreements DE-FC02-99ER54512 and DE-AC02-09CH11466 and SBIR grant award DE-FG02-07ER84762.

G.M. Wallace, IAEA FEC, San Diego, CA USA

2

LH waves launched by slow wave

structure at plasma edge

ω ~ 3 – 5 x ωLH

ωLH = ωpi(1+ ωpe2/ ωce

2)-1/2

LH launcher couples electrostatic slow mode

Waves launched preferentially in counter-current direction

3

Current Drive

Counter-current

Waveguide array

LH waves transfer energy and parallel

momentum to fast electrons to drive current

LH waves Landau damp on

electrons at v|| ~ 3vte

Asymmetry in f(v||) results in

net current

[Fisch, Rev. Mod. Phys., 1987]

v||

f(v

||) Fast electrons

carrying current

4


LHCD system on C-Mod investigates

LH physics with ITER-like parameters

ne = 0.5-5x1020 m-3 (ITER = 0.5-1x1020 m-3)

BT = 3 – 8 T (ITER = 5 T)

Upper, lower, or double null diverted plasma configuration (ITER = lower null)

n|| = 1.5 – 3 co- or counter-current (ITER ~ 2)

fLHCD = 4.6 GHz (ITER = 5 GHz)

4 rows of 16 waveguides

PSource = 2.5 MW

X-mode

Reflectometer horns

Langmuir

probes

5


LH power provided by CPI/Varian

klystrons operating at 4.6 GHz

Vb = 50 kV

Ib ~ 12 A

PRF = 250 kW

Coolant flow rate 70 gpm

Minimum coolant pressure 16 psi

Grouped in “carts” of 4 klystrons

Carts connected in parallel to Thales HVPS

6


Transmitter Protection System (TPS)

upgraded to allow for longer pulses

Modeling by klystron manufacturer

Coolant should not boil during 5.0 s RF pulse at 250 kW

Coolant will boil after 1.2 s with no RF output

Boiling could result in damage to the klystron collector

Need to monitor coolant outlet temperature to prevent boiling

Administrative limit on pulse length of 0.5 s to avoid harming klystrons

7

TPS Control Hardware

Development of TPS funded

by USDoE SBIR grant award

DE-FG02-07ER84762.


Collector Over Temperature System

(COTS) models coolant temp in real time

COTS models average outlet water temperature based on integrating heat equation:

𝑑𝑇𝑜𝑢𝑡

𝑑𝑡=

𝐼𝑏𝑉𝑏 − 𝑃𝑅𝐹 − 𝑄𝐶𝐻2𝑂(𝑇𝑜𝑢𝑡 − 𝑇𝑖𝑛)

𝐶0

Integrated at 1 kHz

Beam power, RF power, and coolant flow are directly measured

Collector heat capacity, C0, determined empirically by time constant of outlet temp

Max coolant temperature proportional to Tout – Tin based on CPI model data

8

Klystron

VbIb

PRF

QCH2OTin QCH2O

Tout

TPS shuts off HVPS if a fault is

detected on any klystron

Independent TPS hardware for

each cart

Fault conditions:

COTS temperature exceeds

threshold

Electron beam remains on for

integrated time of > 400 ms with

no RF output power

VSWR at klystron output window

too high

Optical arc detector indicates arc

at klystron window

Klystron body current


9

FAULT!

COTS calculation agrees closely

with downstream coolant temp

Downstream coolant temp measurement lags actual temperature

Model outlet temperature increase agrees with downstream measurement

Best fit for time constant with C0 = 35 kJ/K

Time constant is slower than predicted by CPI simulations (C0 ~10 kJ/K)

10


TPS upgrade allows for longer LH

pulses into plasma

5.0 s pulses into dummy load

1.0 s pulses into plasma

Typical C-Mod Ip flattop lasts

for 1.0 s

Long (tpulse = 1.0 s >> τR) LH

pulses allow other

parameters (ne, Te) to reach

new equilibrium after current

profile changes


11

Moveable integrated LH protection

limiter installed in fall of 2011 12

Fixed position

limiter

Installed ‘04

Removed Fall ‘11

Reinstalled Spring ‘12

Moveable, integrated

limiter

Installed Fall ‘11

Removed Spring ‘12

Moveable integrated limiter allows for

wider range of operating positions

Short connection length between LH protection limiters steep density gradient

Can only operate LH launcher in narrow range of radial positions (0 < RLH – Rlim < 0.5 mm) with fixed limiter

Reflection coefficients very high for RLH – Rlim > 0.5 mm

Launcher exposed to damaging plasma heat flux for 0 > RLH – Rlim

Integrated limiter moves with LH launcher at a fixed value of RLH – Rlim = 0.25 mm

13


Moveable integrated limiter reduces reflection

coefficients as compared to fixed limiter

Moveable

integrated limiter

reduces Γ2 from

~30% to ~20%

Small number of

discharges with high

reflections due to

insufficient density

(Γ2>40%) remain


14

Power handling with integrated

limiter significantly reduced

Arcing became problematic after first few run days, even at low power

Max launcher retraction 1.5 cm with integrated limiters vs. 3 cm with fixed limiters

Buildup of boron coating on launcher surfaces

Melting damage due to insufficient protection from plasma

Electrical isolation of LH limiter tiles


15

BZN

BZN

BZN

Shots w/ arcing o

Shots w/o arcing *

Inspection during manned access

shows damage to launcher 16

Arc tracks

extend

5cm into

guides

Boron film

Launcher refurbished following

removal of integrated limiter

Boron film removed

Alumina vacuum windows grit-blasted to remove contamination

Arc tracks buffed out from waveguide walls

Drips ground off of waveguide septa


17

Before repair After repair

Power handling recovered with

reinstallation of fixed limiter

398 discharges with

LH since reinstallation

of fixed limiter

47 discharges with

arcing in the launcher

No lasting decrease

in performance

observed after arc

“clusters”


18

Shots w/ arcing o

Shots w/o arcing *

Independent, moveable protection

limiter would provide ideal solution

Moveable limiter did allow for lower reflection

coefficients and concept should not be abandoned

Ability to retract LH launcher behind local

protection limiters is necessary to shield launcher

from boronization and disruptions when LH is not in

use

Independent, moveable limiter could be grounded

to vacuum vessel wall, eliminating buildup of charge

on limiter tiles intersection fast electrons from LH


19

Additional LH launcher will ~double

available LH power on C-Mod

Currently use 1 launcher powered by 10 klystrons

Columns 1-6 and 11-16 fed pair-wise by split klystrons

Columns 7-10 fed by individual klystrons

Direct feed klystrons generally operated at 50% power

Plan to add 2nd launcher with each powered by 8 klystrons (16 klystrons total)

All columns of both launchers fed pair-wise by split klystrons operating at full power

Available LH power will increase to ~ 2 MW with additional launcher


20

Poloidal upshift of n|| along rays launched above

mid-plane results in stronger single-pass absorption

n|| upshifts along ray path when waves are launched above mid-plane

Landau damping becomes strong when vph ~ 3vth

Deformation of distribution function at 3vth results in stronger absorption of waves at higher phase vph


21

Off mid-plane “LH3” launcher designed

to increase driven current at high ne

LH2 + LH3 drive up to 200 kA when considered individually

LH2 + LH3 drive up to 300 kA when phase space synergy is considered

22

With Synergy

Without SynergyRay P

ow

er

100%

0%

Envelope of LCFS from all 2010-2011

discharges defines shape of launcher 23

Analytic equilibrium approximates

envelope and existing limiter well

Analytic equilibrium with R, a, κ, and δ [Hakkarainen et al, Phys. Fluids B 2 (7), 1990.]:

𝑅 = 𝑅0 + 𝑟 cos 𝜃

𝑧 = 𝑟 sin 𝜃 𝑟 =

𝑎{1 −𝜅−1

2cos 2𝜃 − 1 +

𝛿

4cos 3𝜃 − cos 𝜃 }

R=0.67m

a=0.241m

κ=1.4

δ=0.45

24

LH3 design based on combination of

LH2 4-way split with toroidal bi-junction 25


Toroidal 90° bi-junction

Poloidal 4-way splitter

8-way splitter parameters optimized based

on ALOHA plasma scattering matrix

Optimization parameters:

Bi-junction length

Poloidal differential phase

Distance from bi-junction to 4-way splitter

Input phasing


26

ALOHA

64x64

COMSOL

9x9 Input

Set parameters:

Bi-junction phasing (90°)

Operating n|| range (2-3)

Plasma edge density = 2-8xncutoff

Vacuum gap distance (1 mm)

9x9

9x9

…

…

RF design of 8-way splitter evaluated

with COMSOL FEA simulation software 27


S11 = -18.5 dB

Sn1 = -8.97 to

-9.35 dB

∆φ = -86.95° to 87.95°

Construction of first 8-way splitter

test setup in progress


28

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ADVANCES IN LOWER HYBRID CURRENT DRIVE …...LH2 4-way split with toroidal bi-junction 25 G.M....

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