1. 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

2. Contents Experiment: objectives and technical description
Technical facts
Presentation of results
Arc modeling
Next steps

3. 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

4. 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

5. 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

6. 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

7. 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.

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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

16. 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

17. 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

18. 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

19. 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

20. 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

21. 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

22. 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)

23. 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

24. 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)

25. 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)

26. 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

27. 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

28. 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

29. 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

30. 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

31. 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

32. 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

33. 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.