Investigation of Radio Frequency Investigation of Radio Frequency Breakdown in Fusion ExperimentsBreakdown in Fusion Experiments

T.P. Graves, S.J. Wukitch, I.H. Hutchinson MIT – Plasma Science and Fusion Center

APS-DPP October 2003Albuquerque, NM

Outline• Multipactor Basics

– Simple multipactor description– Multipactor discharge properties– Other types of multipactor discharges

• RF Breakdown Experiment– Motivation behind experiment– How multipactor discharges apply– Experimental Results– How this could apply to Fusion/Space systems

• Future Plans

AbstractAn electron multipactor discharge is a resonant vacuum discharge which occurs in RF systems in the MHz to tens of GHz frequency range. This discharge can develop in systems such as microwave devices, RF satellite payloads, or accelerator structures. The RF Breakdown Experiment for Alcator-CMOD, designed to test high-voltage, RF arcing, appears to be limited by coaxial multipactoring within the vacuum system. The underlying physics and properties of multipactor discharges along with the experimental results of the RF Breakdown Experiment will be presented.

Multipactor Basics• A multipactor discharge is a resonant condition for

electrons • Radio Frequency effect – MHz to 10’s GHz frequencies• Observed in:

– Accelerators– Microwave devices and resonators– RF satellite payloads

• Vacuum conditions required • Electron multiplication from secondary electrons

– Need SEC > 1 and sufficient impact energy– Copper SEC δ = 1.3, Energy = 600eV (peak), 200eV (min)

• (Handbook of Chemistry and Physics, 72nd Edition)

• System geometry and E field structure

R.A. Kishek, Phys. Plasmas 5, 2120 (1998)

Parallel Plate Multipactor

E1

0

e-

SecondaryElectronsSecondaryElectrons

SecondaryElectrons

E2E3

E field wave

T(0 to π) T(π to 2π ) T(2π to 3π)

Emin

Emax

Multipactor Properties• Parallel Plate (PP)

Geometry– Lorentz equation can be

solved for multipactorcondition:

• Due to simple geometry, PP situation well studied

• More complicated due to secondary emission energy range

( )24 dfemV ⋅⋅

=π

Susceptibility Curve

R.A. Kishek, Phys. Plasmas 5, 2120 (1998)

Other Multipactor Characteristics• Advantages to Multipactor Discharges

– Surface conditioning– Electron source (Farnsworth Oscillator)– Plasma display technology

• Disadvantages– Detunes microwave circuits, dissipating all excess

power• Typically impossible to push thru discharge

– Introduces large noise signals in space applications– Surface heating – Vacuum window breakage– Induce vacuum breakdown (arcing)

Preventing MultipactoringThree General Approaches

• Surface conditioning – Increases the necessary energy for secondary

emission• Low SEC Coatings

– Less secondary electrons, yet tend to wear off with time

• Change geometry– Make geometry such that multipactoring is

impossible

Other Multipactor RegimesSingle Surface Dielectric Multipactor

Coaxial Transmission Line Multipactor

+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +

Erfsin(ωt)Edc

Vo

rtEE o

rf)sin( ⋅⋅

=ω

Inner diameter

Outer diameter

Coaxial Multipactoring• Not well explored

– Space and Fusion applications have vacuum coaxial transmission lines

– Coaxial lines typically filled with insulator like SF6 –preventing multipactoring

• Result of non-linear behavior of electrons– Single or double surface multipactoring– Still need large enough impact energy for secondary

emission• Other Non-linear Multipactor Regimes Possible

– Other non-linear, non-uniform field geometries– Influence of external magnetic fields in particle orbits

R. Woo, J. Appl. Phys. 39, 1528 (1968)

RF Breakdown Experiment• Motivation behind experiment

– Alcator C-MOD utilizes high power (MW) RF systems for ICRH (~80MHz)

– Empirically determined E-field breakdown limit, E=15kV/cm, on C-MOD

– Similar limits seen on experiments such as JET and NSTX• RF Breakdown Testbed was built to explore E-field

limit– High Q (~1500) resonator to build up high power (extremely

sensitive to small changes)– Transition to strip line at high voltage point to locate arcing– Problems arose when circulating power levels were limited at

surprisingly low voltage (~100V)

RF Breakdown Setup

~

Source4kW Pulsed

DC1

Double StubTuner

DC2

Vacuum System

•Base pressure 10-7 torr, He

•Tested with and without Ground plane (suspected series arcs, redesign)

Stub #2

Stub #1

Directional Couplers

Directional Couplers

AmplifierVacuumBreak

Vacuum Break

RF Short

Vacuum Chamber

Initial Power Limit Data

8-15-03, No Ground Plane PresentBase Pressure = 1e-5 torr (prebake)

• Limit detected by rise in both reflected power to source (mismatch) and ion gauges (discharge cleaning surfaces)

• Seems to line up with parallel plate d=1.25 cm multipactor

• Problem – no 1.25 cm gaps in system!

• Possible strange regime in square vacuum chamber?

• Possible Coaxial Multipactoring?

• Next step – Achieve better vacuum (10-7) to increase necessary impact energy for secondary emission

Why the limit?• Baked system – 10-7 torr – much less adsorbed gases

– Result: Small increase in limit voltage, but limit now completely independent of pressure

• Put ground plane back into system – Changed voltage pattern completely inside box– Result: Very small variation with/without ground plane

• Replaced quartz window with SS conflat – checking for possible dielectric multipactoring– Result: No variation with and without window

• Voltage in vacuum box not constant with frequency • Seems that multipactor is not focused in the box

150 MHz

80 MHz

72.25 in Frequencies78.197580.1188115.25151.0437192.8562239.125150 MHz

80 MHz

CST Fieldscode,T.Graves, P. Koert

Coaxial Multipactor Regimes

J. Irby, T. Graves

E=8eV

E=23eVE=22eV

E=8eV

Each figure below corresponds to the experimentally determined voltage @ that

frequency

Current Results Summary

Application to Fusion Systems• Use of vacuum coaxial transmission lines• J-port has 2’ sections of 4 inch vacuum coaxial

lines (as well as poor operation)• Situation in plasma non-vacuum operation –

transmission of power – Finite VSWR– Can the voltage get low enough and the phasing be just

right to initiate a multipactor in the proper region?• ITER plans to have multi-wavelength long

vacuum coaxial transmission lines for ICRH system

Conclusions• Multipactor discharges are resonant electron discharges

which effect many different RF systems• Typical scaling is V ~ (f d)2 and can occur in many

different geometries including coaxial transmission line• The RF Breakdown Experiment seems to be limited by

what appears to be a coaxial multipactor – and it is this might relate to the much higher voltage limit on C-MOD of 15kV/cm

• New information is needed to address the problem of coaxial multipactoring

Future Plans• Is power limit definitely a multipactor effect?• What is the electron impact energy? What mode

(harmonic) is the multipactor? Where in the line is it occurring (high voltage point?)

• Move short outside vacuum region and determine location of multipactor

• Diagnostic inside coax to investigate multipactor electron potential and energy

• What happens when there is a net power out? Can you get multipactoring with a non-infinite VSWR - Attach load to resonator to look at VSWR scenario