Studies of high voltage breakdown phenomena on ICRF (ion cyclotron range of frequencies) antennas [Elektronische Ressource] / Volodymyr Bobkov

Sujets

• Physik
• Astronomie

Informations

• Publié par: technische_universitat_munchen
• Publié le: 01 janvier 2003
• Nombre de lectures: 12
• Langue: English
• Poids de l'ouvrage: 4 Mo

Extrait

Technische Universitat Munchen
Fakultat fur Physik
Studies of high voltage breakdown phenomena on
ICRF (Ion Cyclotron Range of Frequencies) antennas
Volodymyr Bobkov
Vollst andiger Abdruck der von der Fakult at fur Physik
der Technischen Universit at Munc hen
zur Erlangung des akademischen Grades eines
Doktors der Naturwissenschaften (Dr. rer. nat.)
genehmigten Dissertation.
Vorsitzender: Univ.-Prof. Dr. A. J. Buras
Prufer der Dissertation: 1. Hon.-Prof. Dr. R. Wilhelm
2. Univ.-Prof. Dr. K. Krischer
Die Dissertation wurde am 13.02.2003 bei der
Technischen Universit at Munc hen eingereicht und
durch die Fakult at fur Physik am 05.05.2003 angenommen.Abstract
Coupling of ICRF (Ion Cyclotron Range of Frequencies) power to the plasma is one of the
standard methods to heat plasmas in toroidal devices with magnetic con nemen t. However
voltage limits on the ICRF antenna used to launch the waves sometimes lead to a limitation of
the power. These limits are related to a variety of high voltage breakdown phenomena in the
presence of plasma that depend, in particular, on spatial charge e ects and particle uxes to
the electrodes.
An ICRF probe has been developed to study the high voltage phenomena. The open end of
a coaxial line models the high voltage region of the antenna. The voltage limits were studied in
well de ned conditions in a test facility without magnetic eld and in the real conditions of the
peripheral plasma of the ASDEX Upgrade divertor tokamak.
The ICRF probe was installed in the test facility and conditioned in vacuum by high power
pulses to reliable operation with 60 kV, 200 ms or 80 kV, 20 ms pulses. During the conditioning,
vacuum arcs occur mainly at the probe head. The arcs appear often when dark eld emission
15 3currents are measured. The presence of a plasma density of 10 m (delivered by a high
aperture ion source) does not a ect the voltage stand-o of the probe unless the pressure of
working gas is increased beyond a critical level: a semi-self-sustained glow discharge is ignited
at a pressure of 0.15 Pa for He and 0.03 Pa for air. These pressures are about one order of
magnitude lower than the pressures required for ignition of a self-sustained glow discharge at 80
kV. Cathode spots on the surface of the inner conductor are formed in the semi-self-sustained
discharge and often lead to the formation of the arc discharge.
When the ICRF probe is installed in ASDEX Upgrade and is well conditioned (to the
maximal voltages achieved in the test facility), high voltage breakdown on the probe often
correlates with activity of edge localized modes (ELMs). Thewn characteristics are
similar to that of the cathode spots formation in the semi-self-sustained discharge glow discharge.
The maximal RF voltage on the ICRF probe increases from shot to shot, i.e. an additional
conditioning e ect is observed during plasma operation. The voltage limit of the probe can be
increased by application of a positive DC bias to the inner conductor while at the same time
the recti ed current associated with the collection of ions across magnetic eld is suppressed.
It was found that the appearance of ELMs and other intermittent events in the scrape-o -la yer
(SOL) plasma in the region of the probe head lead to a local dissipation of a high fraction of
RF power.
The role of ELMs as RF breakdown trigger is con rmed by observations during operation
of the full-size AUG ICRF antenna. A reliable arc detection system is required for the ICRF
antennas (not every breakdown triggered by ELMs is easy to detect), otherwise the overall
performance of the antennas degrades due to appearance of quasi-stationary arc discharges.
The antennas operates more reliably when the antenna conductors are conditioned with plasma.
Measures to improve the antenna voltage stand-o in the presence of plasma are suggested:
an optically closed Faraday screen; glow discharge conditioning; a form of antenna conductors
to minimize ion collection across the magnetic eld and minimize asymmetry of electrodes along
the eld; neutral density reduction inside the antenna. Further work should be focused on the
choice of the antenna materials, parasitic absorption of the RF power and the antenna-plasma
interaction for di eren t DC boundary conditions of the antenna circuit.Contents
1 Introduction 1
1.1 Fusion research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 Fusion of energetic particles . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.3 Tokamak concept and plasma heating . . . . . . . . . . . . . . . . . . . . . 5
1.4 Heating of plasma with ICRF . . . . . . . . . . . . . . . . . . . . . . . . . 8
1.5 Power limitations of ICRF antennas . . . . . . . . . . . . . . . . . . . . . . 11
1.6 Outline of the thesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2 Phenomenology of RF breakdown 14
2.1 Main parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
2.2 Power transmission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
2.3 Breakdown development on the ICRF antenna . . . . . . . . . . . . . . . . 18
2.4 Gas discharge phenomenology . . . . . . . . . . . . . . . . . . . . . . . . . 20
2.4.1 DC discharges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
2.4.2 RF disc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
2.4.3 RF discharges responsible for voltage limitation . . . . . . . . . . . 26
2.5 RF vacuum arc ignition . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
2.5.1 Field emission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
2.5.2 Conditioning by high voltage . . . . . . . . . . . . . . . . . . . . . 28
2.5.3 Spark stage of RF vacuum breakdown . . . . . . . . . . . . . . . . 29
2.6 Charge particles in electrode gap in vacuum . . . . . . . . . . . . . . . . . 30
2.6.1 Particle motion in vacuum at high RF voltage . . . . . . . . . . . . 31
2.6.2 P ux focusing on the microscale . . . . . . . . . . . . . . . . 33
2.6.3 Thermal desorption and skin-e ect . . . . . . . . . . . . . . . . . . 35
2.6.4 Particle stimulated desorption . . . . . . . . . . . . . . . . . . . . . 37
2.6.5 Secondary emission processes . . . . . . . . . . . . . . . . . . . . . 37
2.6.6 Multipactor in vacuum . . . . . . . . . . . . . . . . . . . . . . . . . 38
2.6.7 Mean free-pass and cross-sections of ionization processes . . . . . . 38
2.7 Self-sustained RF glow discharge . . . . . . . . . . . . . . . . . . . . . . . 39
2.7.1 Role of inductively coupled discharge . . . . . . . . . . . . . . . . . 39
2.7.2 Capacitively coupled discharge . . . . . . . . . . . . . . . . . . . . . 39
2.7.3 Multipactor plasma discharge (multipactor a ected by gas) . . . . . 41
2.7.4 Pressure hysteresis for RF discharge existence . . . . . . . . . . . . 42
2.7.5 RF gas discharge conditioning . . . . . . . . . . . . . . . . . . . . . 43
2.8 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
iii3 Plasma in the electrode gap 45
3.1 Approach to a DC sheath . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
3.2 A RF sheath: frequency ranges . . . . . . . . . . . . . . . . . . . . . . . . 48
3.2.1 Comparing ! with ! . . . . . . . . . . . . . . . . . . . . . . . . . 480 pe
3.2.2 ! with ! . . . . . . . . . . . . . . . . . . . . . . . . . 500 pi
3.3 Plasma screening properties . . . . . . . . . . . . . . . . . . . . . . . . . . 51
3.3.1 Electrical eld for the thin sheath (s<d) . . . . . . . . . . . . . . 52
3.3.2 Basic dynamics of the thin high-voltage RF sheath (s<d) . . . . . 55
3.3.3 Basic of the thickoltage RF (s>d) . . . . 62
3.3.4 Surface electrical eld and a transition to sd . . . . . . . . . . . 65
3.3.5 Role of ponderomotive force for density reduction . . . . . . . . . . 66
3.4 In uence of a magnetic eld . . . . . . . . . . . . . . . . . . . . . . . . . . 67
3.4.1 Con nemen t of particles in the electrode gap . . . . . . . . . . . . . 67
3.4.2 Charging of the plasma in the magnetic eld . . . . . . . . . . . . . 68
3.4.3 Multipactor conditions . . . . . . . . . . . . . . . . . . . . . . . . . 69
3.4.4 E ect on the e ectiv e interelectrode distance . . . . . . . . . . . . . 70
3.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
4 Experimental approach 72
4.1 Concept of the experiment . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
4.1.1 RF and DC power generators . . . . . . . . . . . . . . . . . . . . . 75
4.2 Experimental device . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
4.3 Setup of the experiment . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
4.3.1 Setup in the test facility . . . . . . . . . . . . . . . . . . . . . . . . 82
4.3.2 Setup in ASDEX Upgrade . . . . . . . . . . . . . . . . . . . . . . . 83
4.4 Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
4.4.1 RF measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
4.4.2 DCts . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
5 Results and discussion 88
5.1 Test facility results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
5.1.1 Operation at high voltage in vacuum . . . . . . . . . . . . . . . . . 88
5.1.2 Vacuum arc ignition . . . . . . . . . . . . . . . . . . . . . . . . . . 89
5.1.3 De nition of DC current direc