Experimental Results of the Coaxial
Multipactor Experiment
T.P. Graves, B. LaBombard, S.J. Wukitch, I.H. Hutchinson
PSFC-MIT
Summary
A multipactor discharge is a resonant condition for electrons in an
alternating electric field. This discharge can be disruptive to RF
circuits, cavities, and resonators. Coaxial multipactoring occurs in
the non-linear field of a coaxial transmission line and very little
has been done experimentally to investigate this phenomena. The
Coaxial Multipactor Experiment (CoMET) investigates this
discharge with goals of measuring the electron distribution,
current, and absorbed power from the RF field. An array of 12
retarding potential analyzers measure the multipacting electrons
with a set of 2 grids and a collector. In order to fully suppress
secondary emission from the collector, at least –30 volts is
required. Preliminary results depict a unique electron energy
distribution, but further investigation is needed to extract the
features of this distribution.
Outline
• Multipactor Basics
– Simple multipactor description
– Multipactor discharge properties
– Coaxial multipactor description
• CoMET (Coaxial Multipactor Experiment)
–
–
–
–
Motivation behind experiment
CoMET Experimental Setup
Experimental Results
How this could apply to Fusion/Space systems
• Conclusions and Future Plans
Overview and Motivation
T(0 to π)
T(π to 2π )
e-
E1
T(2π to 3π)
Secondary
Electrons
E2
E3
Secondary
Electrons
Emax
0
E field wave
Emin
Multipactor Basics
• A multipactor discharge is a resonant condition for
electrons in an alternating E field (ref. 1)
• 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 secondary electron coefficient (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 determine the electron
motion
•Scaling law -> V α (freq . dist)2 for parallel plate and coaxial
geometries
• Typically, once the
multipactor is fully
developed, it is
impossible to push thru
to higher voltage
• Multipactor current
detunes the circuit,
dropping the Q factor
Position (cm)
Coaxial Multipactoring
Eo ⋅ sin(ω ⋅ t )
Erf =
r
Outer diameter
Inner diameter
Time (sec)
•Not well explored – Most RF transmission lines not
in vacuum conditions, only space and fusion groups
•Result of non-linear behavior of electrons
•Single or double surface multipactoring, with
sufficient energy for secondary emission
•Simulations show possibility of discharge moving in
traveling and mixed wave case (finite VSWR); Also
magnetic field complications
CoMET
• Coaxial Multipactor 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 CMOD
– Similar limits seen on experiments such as JET and NSTX
• Built High Q resonator to build up high power in order to study
high voltage breakdown
• Found much lower voltage limit due to coaxial multipactoring in
vacuum region
• CoMET Main Goals: Experimentally determine energy, current
density, energy and spatial distributions of coaxial electron
multipactor discharges for a range of pressures, frequencies, and
wave structures
Experimental Setup
RF
Source
I
~
DC1
DC2
Retarding Potential
Analyzers/Current
Probes
DC3
4” TLine
Double Stub
Tuner
Vacuum System
Measured Quantities
max voltage
Adjustable
Shorted Stub
•Forward/Reflected Power
•Electron Current vs. Grid
Voltage
•Pressure
Standing Wave Pattern
0 volts
Adjustable
Shorted Stub
Tuning Network
DC3
Vacuum Chamber
DC2
24 Channel High Voltage Bias
and Multipactor Current
Amplifier Crate
RPA Array inside vacuum
Collector
Supressor Grid
Entrance Grid
Experimental Results
e-
Suppress
Voltage
Secondary
Suppressed
• Multipactor begins at
precise voltage, current
increases with input
power
• No addition of
circulating power once
multipactor is established
• Rf circuit is detuned,
proportional to
multipactor current
• Amount of power
absorbed by the
discharge is measured by
directional coupler
difference
Spatial Distribution of Discharge
• No bias voltage on
collector, -30 V
supression voltage on
grid
• Figure depicts small
azimuthal variation
for multipactor
absorption ranging
from 0 to 15 W
RPA I-V Characteristic
Electron Distribution Functions
• Due to drift in electronic signal,
baseline signal not zero
• I-V characteristics can be taken
relative to baseline
• Fit baseline with quadratic
polynomial and subtract to give
real characteristic data
• Fit corrected characteristic and
take derivative to extract electron
distribution functions
• For circulating power of 178 W
(133 V) and 5 W of multipactor
absorbed power, Electron
distribution functions quadratic
shape, with energy (eV) up to
peak RF voltage (135 V)
Application to Fusion Systems
• Use of vacuum coaxial transmission lines
• Alcator C-MOD ICRH system has sections of 4 inch
vacuum coaxial lines (as well as poor operation on J
antenna)
• ITER plans to have very long, low voltage coaxial
transmission lines for ICRH system
• 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?
• Compare time constant for voltage on transmission line
and time constant for multipactor – How long does voltage
stay within the multipactor limits?
• Multipactor time ~ 100-200
cycles (ref. 2)
• RF voltage time ~ 10Q/ω
Breakthru/
Partial
Multipactor
– Q=1000, t=1600 cycles
– Q=100, t=160 cycles
• If RF voltage moves thru
susceptibility region faster
than time to steady state
multipactor, breakthru can
occur
• In this case, a partial
multipactor would occur,
creating many free electrons
which could seed a high
voltage arc
Vmax
Multipactor
Vmin
RF voltage
time (Ref. 2)
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
• CoMET has been successful in creating a measurable multipactor
discharge
• Up to 10 Watts of power can be absorbed in the multipactor
discharge before reflection coefficient = 0.5
• Azimuthal distribution of multipacting electrons mostly even
around coaxial line
• Non-monoenergetic, broad energy distribution spread up to peak
RF voltage – Multiple harmonics in electron motion?
Future Plans
• Refine RPA electronics for low current measurement to
eliminate zero current signal
• Determine spatial variation of multipactor energy distribution
• Determine if discharge is single surface or two surface (ID or
OD or both)
• Pressure dependence on electron distribution
• Verify with parallel plate geometry exp.
• Frequency dependence, Finite VSWR (loaded system), different
conductor materials, and DC offset experiments
• Understanding coaxial multipactoring good for both Alcator and
science community
– Better prevention techniques quenching partial multipactoring that can
seed high voltage arcing
– Technology benefit – Electron or radiation source
References
1.
2.
3.
R.A. Kishek, Y.Y. Lau, et. al. Phys. Plasmas 5 (5), May 1998
R. Kishek, Y.Y. Lau., D. Chernin. Phys. Plasmas 4 (3), March 1997
Woo, Richard. J. Appl. Phys. 39 (3), 1968