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An approach of electrodes phenomena in MXP arcs
ARC Studies MXP – Progress report R. D’Inca – March, 19th 2008 An approach of electrodes phenomena in MXP arcs
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Contents Experiment: objectives and technical description
Technical facts Presentation of results Arc modeling Next steps
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Objectives Characterization of arcs in ICRF feeding lines:
References: [MXP1] – Experiment description – 1/12/07 – R. D’Inca Edge plasma – RF system interactions – Presentation – Ringberg Meeting – Nov 2007 – R. D’Inca Characterization of arcs in ICRF feeding lines: in the time domain: impact of forward power, reflected power and line voltage in the frequency domain: nature and evolution of frequencies in the range 5-25Mhz For different configurations and conditions: vacuum arcs, gas discharges and multipactor effect of geometry, material and magnetic field => Improvement of the SHAD system specifications
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Experiment’s overview
Reference: [MXP1] – Experiment description – 1/12/07 – R. D’Inca CAMAC DC current Current probe Couplers Tuner #2 Tuner #1 Vacuum vessel PAL camera Video G Voltage probes 5λ/4 Resonator Decoupler Helium FAS SHAD Filter
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Typical experiment day
Technical facts Operations: More than 1200 tests, 50 used for in-depth analysis Emphasis on vacuum (1e-5 Pa) and low pressure arcs (0.1 Pa) Detection of arcs through reflected power, SHAD in observation mode Typical experiment day
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Technical facts Technical issue: arc positioning
Position of the arcs could not be controlled nor precisely determined: severe issue. Arc near the front end Arc at the back end However traces observed near the second high voltage node Trace of moving arc Single impacts
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Results Arcs that occurred during this campaign can be classified in three families: #1: vacuum arcs with SHAD detection #2: low pressure arcs with SHAD detection #3: arcs without SHAD detection (with or w/o gas) Each family present distinct physical features, apparently independent from the level of injected power.
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Results Family 1: vacuum arcs with SHAD detection FAS Test #347
Power Voltage Spectrogram Voltage Filtered Voltage Pressure FAS Test #347 Injected power: 290kW Pressure: 1.6e-7 mBar Length: 200ms Configuration: 2b DC-Current FAS CAMAC
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Results Family 1: vacuum arcs with SHAD detection Power:
reflected power increases in ~ 10μs at a moderate level before generator shutdown Voltage Spectrogram Voltage Filtered Voltage Pressure FAS Test #347 Injected power: 290kW Pressure: 1.6e-7 mBar Length: 200ms Configuration: 2b DC-Current FAS CAMAC
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Results Family 1: vacuum arcs with SHAD detection Voltage: FAS
Power Voltage Spectrogram Voltage Filtered Voltage Pressure FAS Voltage: voltage decreases but does not immediately reach the zero level apparition of noise between 1 and 15MHz with characteristic frequencies during 500μs. Test #347 Injected power: 290kW Pressure: 1.6e-7 mBar Length: 200ms Configuration: 2b DC-Current FAS CAMAC
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Results Family 1: vacuum arcs with SHAD detection Pressure:
Power Voltage Spectrogram Voltage Pressure: apparition of oscillations: injection of particles inside the resonator. Filtered Voltage Pressure FAS Test #347 Injected power: 290kW Pressure: 1.6e-7 mBar Length: 200ms Configuration: 2b DC-Current FAS CAMAC
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Results Family 1: vacuum arcs with SHAD detection DC-current: FAS
Power Voltage Spectrogram Voltage Filtered Voltage Pressure FAS DC-current: apparition of a DC-current with a trend towards negative value: inner electrode receives electrons. Test #347 Injected power: 290kW Pressure: 1.6e-7 mBar Length: 200ms Configuration: 2b DC-Current FAS CAMAC
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Results Family 2: low pressure arcs with SHAD detection FAS Test #431
Power Voltage Spectrogram Voltage Filtered Voltage Pressure FAS Test #431 Injected power: 200kW Pressure: 1.0e-3 mBar Length: 200ms Configuration: 2c DC-Current FAS CAMAC
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Results Family 2: low pressure arcs with SHAD detection Power:
reflected power increases in ~ 2μs at moderate level before generator shutdown. Voltage Spectrogram Voltage Filtered Voltage Pressure FAS Test #431 Injected power: 200kW Pressure: 1.0e-3 mBar Length: 200ms Configuration: 2c DC-Current FAS CAMAC
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Results Family 2: low pressure arcs with SHAD detection Voltage: FAS
Power Voltage Spectrogram Voltage Filtered Voltage Pressure FAS Voltage: Voltage decreases quickly but remains for some time above zero. noise appears during the decrease but with a lower intensity than for the family #1. Although the drop is quick, there is no spurious modulation of the generator frequency Test #431 Injected power: 200kW Pressure: 1.0e-3 mBar Length: 200ms Configuration: 2c DC-Current FAS CAMAC
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Results Family 2: low pressure arcs with SHAD detection DC-current:
Power Voltage Spectrogram Voltage Filtered Voltage Pressure FAS Test #431 Injected power: 200kW Pressure: 1.0e-3 mBar Length: 200ms Configuration: 2c DC-current: a trend to negative value of current, with some positive peaks: the inner conductor captures electrons DC-Current FAS CAMAC
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Results Family 3: arcs without SHAD detection FAS Test #1435
Power Voltage Spectrogram Voltage Filtered Voltage Pressure FAS Test #1435 Injected power: 400kW Pressure: 3.0e-7 mBar Length: 200ms Configuration: 2c DC-Current FAS CAMAC
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Results Family 3: vacuum arcs without SHAD detection Power:
reflected power increases in ~ 2μs at high level before generator shutdown. Voltage Spectrogram Voltage Filtered Voltage Pressure FAS Test #1435 Injected power: 400kW Pressure: 3e-7 mBar Length: 200ms Configuration: 2c DC-Current FAS CAMAC
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Results Family 3: vacuum arcs without SHAD detection Voltage: FAS
Power Voltage Spectrogram Voltage Filtered Voltage Pressure FAS Voltage: voltage collapses at zero level. no physical noise observed (only measurement noise) The generator frequency broadens, probably meaning a reaction to the strong change in load impedance Test #1435 Injected power: 400kW Pressure: 3e-7 mBar Length: 200ms Configuration: 2c DC-Current FAS CAMAC
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Results Family 3: vacuum arcs without SHAD detection Pressure:
Power Voltage Spectrogram Voltage Pressure: for the vacuum arcs, a short oscillation of pressure at the very beginning of the arc Filtered Voltage Pressure FAS Test #1435 Injected power: 400kW Pressure: 3e-7 mBar Length: 200ms Configuration: 2c DC-Current FAS CAMAC
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Results Family 3: vacuum arcs without SHAD detection DC-current: FAS
Power Voltage Spectrogram Voltage Filtered Voltage Pressure FAS Test #1435 Injected power: 400kW Pressure: 3e-7 mBar Length: 200ms Configuration: 2c DC-current: strongly positive (with sometimes some oscillations at the very beginning): the inner conductor mainly emits electrons. DC-Current FAS CAMAC
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Arc modeling The main questions:
why some arcs do not produce detectable noise and how to explain the features of the noise spectra? how to get a coherent model explaining the different behavior of arcs in time domain ? Difficulties of the problem: We do not control the basic parameters of the arc: current and location (i.e. gap and material) we have to rely on results of more fundamental experiments published in the open literature (DC arc and RF breakdowns in linear accelerators)
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Voltage noise in a 4 rods high current vacuum arc
What is arc noise? The burning noise voltage is mainly studied on DC arcs. cathode spots: largely based on field-electron-emission mechanism coupled to self-vaporization anode spots: avalanche ionization due to electron bombardment Cathode Plasma Electron beam Anode ESD ESD/ISD Cathode Plasma FEE Joule heating Ion impact Ionization / Charge exchange Cathode spot Anode spot Voltage noise in a 4 rods high current vacuum arc References: Development of high-current vacuum arc in a rod electrode system – D.F. Alferov – High temperature, Vol. 39 (2001) Pulsed electrical discharge in vacuum – G.A Mesyats
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What is arc noise: cathode phenomena
Source of noise: lifecycle of cathode spots Observation of voltage fluctuations due to ignition, movement and extinction of cathode spots. The time scale is in the range 1-100ns Basic phenomena at stake: formation of ecton, Brownian motion. Problems: it creates a brown noise, with a low intensity. This noise is only the manifestation of elementary phenomena. It is mainly observed for low current arcs Brown noise of an arc => This case seems less suited to explain our observations References: Cathode spots of electric arcs – B. Juettner – J. Phys. D: Appl. Phys. 34 (2001) Ecton or electron avalanche from metal – G.A. Mesyats – Physics – Uspekhi 38 (1995) Time and material dependance of the voltage noise generated by cathodic vacuum arcs – J. Rosen & A. Anders – J. Phys. D: Appl. Phys. 38 (2005)
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What is arc noise: anode phenomena
Formation of anode spots Intense high frequency voltage fluctuations are associated with apparition of anode spot at high current. The presence and intensity of noise is linked to the nature of anode spots: footpoints, spots, intense arc. the profile of current variation modifies the burning voltage and then, the noise. the presence of an Axial Magnetic Field modify or even can suppress the noise Voltage noise during high current transient Plasma sheath Origin of the noise could be rooted in the interaction between the plasma jet and the anode surface: it would explain the high intensity of the noise signal. Plasma column Anode Magnetic constriction Modification of conditions on surface => This case could explain our observations References: A Review of Anode Phenomena in vacuum Arcs – H.G. Miller – Contrib. Plasma Phys. 29 (1989) 3, Transition from Constricted to Diffuse Vacuum Arc Modes furing high AC Current Interruption – Z. Zalucki – Trans. Plasma Phys. Vol. 27 – 1999 Noise arc voltage and dynamic constriction of high-current vacuum arcs – Y. Wang – J. Phys. D: Appl. Phys. 24 (1991)
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Application to RF breakdown
Problem: electrodes invert their role each half period, i.e. every 10ns on MXP. So, anode and cathodes have at first glance no particular meaning for RF. But: in coaxial lines, electrodes are asymmetric and plasma sheaths modify the distribution of electrons A cathode is an average emitter of electrons An anode is an average receptor of electrons The role of each electrode is visible on the DC current of the inner conductor. Consequently: The DC current is an important indication of the nature of the arc. If the inner conductor is a cathode arc: positive DC-current. The cathode can provides the electrons required to sustain the existence of spots (largely independent of gap and of the characteristics of the plasma) If the inner conductor is an anode arc: negative DC-current: there is a strong interaction between plasma jet and anode surface phenomena: the plasma plays a major role. References: An origin of RF breakdown in vacuum – M.D. Karetnikov – Particle accelerators, 1997, vol. 57 Studies of high voltage breakdown phenomena on ICRF antennas – V. Bobkov – IPP 4/282
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Role of arc location in the RF line
A major parameter to understand the nature of an arc is its current. In the MXP experiment, the evolution of current, voltage, forward and reflected power depend on the location of the arc. This evolution is difficult to apprehend because four processes are transiently competing: the local increase of current inside the arc due to the breakdown, the global decrease of current in the RF line due to mismatching of the resonator, the global readjustment of current in the RF line due to mismatching with the second stub tuner, instabilities coming from the generator reacting to a brutal change of the load impedance. However, we can evaluate some basic effects of the location: Voltage Arc on high voltage node: brutal decrease of voltage, increase of current, but at the beginning, the current starts from zero: less current available for the arc. More favorable to cathode processes Current ARC Voltage Arc next to high voltage node: less brutal decrease of voltage, increase of current; some current is available from the start: more favorable to anode processes Current ARC
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Interpretation of the results
A simple work model based on the following assumptions: 1st hypothesis: the position of the arc is the key mechanism 2nd hypothesis: anode spots trigger voltage noise 3rd hypothesis: presence of gas can enhances the development of anode spots
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Interpretation of the results
1st hypothesis: the position of the arc is the key mechanism 2nd hypothesis: anode spots trigger voltage noise 3rd hypothesis: presence of gas can enhances the development of anode spots Test 347 Shutdown of generator on Pref threshold Family 1: arcs occur next to the high voltage node: the reflected power increases slower and at a moderately high level. More current is available and trigger anode spots (negative current). Their apparition is linked with the development of voltage noise. Then, depending on current conditions, the anode arc switches on a “quiescent” mode. Relatively slow increase of reflected power Apparition of anode spots: noise and negative DC current Fast ejection of particles: oscillation of pressure Arc near high voltage node
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Interpretation of the results
1st hypothesis: the position of the arc is the key mechanism 2nd hypothesis: anode spots trigger voltage noise 3rd hypothesis: presence of gas can enhances the development of anode spots Test 431 Shutdown of generator on Pref threshold Family 2: arcs occur on the high voltage node: high and fast reflected power. But the presence of gas enhance the interactions between the plasma jet and the anode surface: anode spots can develop (negative DC current) and produce noise. High increase of reflected power Apparition of anode spots: noise and negative DC current Arc at high voltage node
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Interpretation of the results
1st hypothesis: the position of the arc is the key mechanism 2nd hypothesis: anode spots trigger voltage noise 3rd hypothesis: presence of gas can enhances the development of anode spots Test 1435 Shutdown of generator on Pref threshold Family 3: arcs occur on the high voltage node: high and fast reflected power. Either anode spots do not develop because of insufficient current. Or, conversely, the increase in current is so high that the arc immediately switches on a “quiescent” mode. High increase of reflected power No noise: either only cathode spots or intense anode spots Arc at high voltage node
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Impact on operations Can arcs without noise occur on AUG?
Probably not: the MXP resonator has a very high Q in comparison with the ICRF lines of AUG. There is however a risk when the coupling efficiency falls during AUG shots To check this phenomenon: observation of arc during conditions (vacuum mode of antennas: high Q). observation of arc during AUG operations If the “3rd family” arcs should happen on AUG, we would have to rely on the cathode spots noise, which are far less intense
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Next steps This is a first work model – it has to be validated and improved Validation of the model: Evaluation of arc current profile Effect of arc position and gap width Effect of magnetic field: it affects the interactions between anode and plasma jet. Confirmation of no perturbing role from the generator (no HF oscillations) Evaluation of the cathode noise: decrease of measurement noise Possibility to record the light intensity (to observe the possible fluctuations of the plasma) Enhancement of the model: Analysis of the spectrogram and determination of the origin of the frequencies (plasma instabilities and coupling with anode wall) Cases of multipactor and gas discharge To achieve these points, it is necessary to control the location of occurrence of arcs.
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