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 ) T(2π to 3π) e - Secondary Electrons E 1 E 2 E 3 Secondary Electrons E max 0 E field wave E min
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, 72 nd 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
Coaxial Multipactoring Position (cm) Outer diameter Inner diameter E rf = E o sin( ω t) r 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 C- MOD 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 Retarding Potential Analyzers/Current Probes ~ DC1 DC2 DC3 Double Stub Tuner Measured Quantities Forward/Reflected Power Electron Current vs. Grid Voltage Pressure 4 TLine Vacuum System max voltage Standing Wave Pattern Adjustable Shorted Stub 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
Entrance Grid Collector Supressor Grid e - Experimental Results 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 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 V max V min RF voltage Breakthru/ Partial Multipactor Multipactor 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. R.A. Kishek, Y.Y. Lau, et. al. Phys. Plasmas 5 (5), May 1998 2. R. Kishek, Y.Y. Lau., D. Chernin. Phys. Plasmas 4 (3), March 1997 3. Woo, Richard. J. Appl. Phys. 39 (3), 1968