Multipactor prevention

1. CLADDING METALLIC SURFACE WITH THIN FILM NANO-DIMENSIONAL CARBON
FOR DIMINISHING OF SECONDARY ELECTRON EMISSION YIELD
I.A.Kossyi, A.M.Anpilov, E.M.Barkhudarov, G.S.Luk’yanchikov, A.S.Tsybul’skii
Prokhorov General Physics Institute o RAS, 119991, Vavilov Street 38, Moscow, Russia
E-mail: kossyi@fpl.gpi.ru
During the past 20-30 years, multipactor phenomena has mainly been studied due to the adverse effect it can
have on microwave systems operating in a vacuum environment. It can disturb the operation of high power microwave
generators [1], and electron accelerators [2], but, above all, it can cause severe system degradation and failure of
satellites, which are difficult or impossible to repair after launch [3]. Satellites operate under vacuum conditions and the
most common means of communication with the Earth is microwave transmission. Many microwave components are
hollow metallic structures that guide the electromagnetic power. A free electron inside the device will experience a force
due to the electric field and since there is no gas or other material stopping the electron can knock out other electrons and
under certain circumstances this procedure is repeated continuously until the electron density is large enough to counter-
act the effect of the applied electric field and a steady state is achieved. This is the physics of a multipactor discharge
excitation in the microwave apparatus on the board of satellite applicable for communication purposes.
Development of means dedicated to the suppression of multipactor excitation is one among the most urgent
problems of modern microwave techniques. Feasible solution of this problem is processing of metallic components
surface by such a means that it leads to the significant diminishing of multipactor excitation probability without any
visible change radio-physical properties of units.
In this report results of developed in the GPI techniques for such a kind of metallic components treatment are
presented. Method is based on the coating of surface of metal by thin film composed of nano-dimensional carbon.
Original principles of nano-structural carbon production have been elaborated. Setup in which nano-dimensional carbon
has been produced is presented on the Fig.1.
Fig.1. Scheme of setup dedicated to the nano-dimensional carbon production. 1-cuvette made of Plexiglas; 2-ethyl spirit;
3-multispark discharger; 4-gas (Ar); 5-power supply; 6-Rogovskii coil; 7-voltage divider; 8-spectrograph; 9-optical
waveguide; 10-insulator; 11-electrode; 12-bubbles.
The basis of system forms the multispark pulse discharge excited in the ethyl spirit by means of developed and
investigated in the GPI tailor made discharger [4]. In the interelectrode gaps of multielectrode discharger argon has been
introduced as it is shown on the Fig.1. Power supply parameters were: U ≤ 20 kV, storage capacity C=1-2 10-8 F. Current
pulse duration was equal to τ = 3 – 5 μs. Repetition rate was f=50 Hz. Experiments were carried out using 95% ethyl
spirit. The discharge proceeded in bubbles of argon filled with spirit saturated vapor. The electrical discharge parameters
in distilled water and spirit were practically similar. The electron density ne = 2-3 1017 cm-3 was determined from the

2. profiles of hydrogen spectral lines broadened due to the Stark effect in plasma electric field. After processing by the
discharge, spirit became dark-colored, and at the exposition τ ≥ 5 min it becomes almost black. The sedimentation did
not occur during nearly month observation.
Spectra of combination scattering were detected using the spectrometer I-1000 operating in the mode of micro-
sampler research having the resolution of 5 nm-1. Exciting radiation wavelengths were λ = 488 nm and 647 nm. The
spectra of the black film on a glass substrate and the evaporated drop on a silicon substrate are identical and correspond
to the spectra of disordered carbon in the form of graphite nano-particles.
The combination scattering spectra of solutions are shown in the Fig. 2. Here (1) is for the spectrum of initial
spirit. There are many impurities in it which give a wide luminescence band in the spectral area 1000 – 3500 cm-1.
Presence of the impurities and carbon nano-particles in the solution leads to the occurrence of a strong luminescence
band (2). Spectra (1) and (2) were registered at exciting radiation wavelength of λ = 488 nm.
As the exciting wavelength was changed to λ = 647,1 nm, the luminescence band disappears (3). The research
results allow concluding that carbon nano-particles of 1-10 nm size are present in a dark solution and contain atoms with
sp2 and sp3 links.
The X-ray structure analysis indicates the presence of different carbon structures with particles dimensions of
about several nanometers. We identified the chemical composition of the powder obtained by evaporating the treated
spirit: C – 79,05 %, O – 19,57 %, other are Si, K, Ti, Cr, Fe.
Fig. 2. Spectra of Raman scattering
Nano-dimensional carbon produced by above described means has been applied for coating of metallic (Cu)
plates by thin films having acceptable adhesion bond with the substrate. Film deposition follows two methods. First one
consists in layer (drop) of spirit containing carbon nano-particles suspension deposition on the substrate surface (see
Fig.3). After evaporation of spirit on the substrate surface thin film composed of nano- and micro- carbon particles has
been remained. Typical photograph of thin nano-carbon film obtained by evaporation method is shown on the Fig. 4.
Fig. 3. Scheme of carbon thin film deposition through the evaporation of colloidal solution. 1-substrate; 2-nano-
dimensional carbon; 3-ethyl spirit.

3. Fig. 4. Photograph of thin nanocarbon film deposited by evaporation method.
The second method of coating bases on the electrophoresis of spirit containing suspension of nano-dimensional
carbon. Scheme of this method is shown on the Fig. 5.
Fig. 5. Scheme of carbon thin film deposition through the electrophoresis of colloidal solution. 1-glass jar (150 ml); 2-
colloidal solution; 3-positive electrode (Cu plate); 4-negative electrode.
Here cuprum plates are used as electrodes immersed in a treated spirit (colloidal solution) containing carbon nano-
particles. A potential difference ~ 200 V is applied to the electrodes. Current recording under the electrophoresis is ~ 1-2
mA. Nano-particles of carbon having negative charge deposit on the positive electrode (substrate). Typical photograph
of film deposited by such a means is shown on the Fig. 6.
For measurement of secondary electron emission yield σ of investigated samples special setup has been
assembled. Measured secondary emission characteristics of non-treated sample and coated samples are given on the Fig.
7.
As it follows from Fig. minimal secondary electron emission yield has Cu plate coated by soot (carbon-black)
layer (σmax < 1). Such type of coating has been applied in early experiments [5] with application of rubber burning.
Unfortunately this type of film has too low adhesion bond with the substrate. σmax was markedly reduced (from ~ 2,5 on
the non-treated sample to the ~ 1,15 after coating) on the sample with the film obtained by evaporation method. As to the
samples coated with the help of electrophoresis, their σmax value is close to the inherent to the non-treated samples (~ 2).

4. However it should be noted that for all coated by nano-carbon samples value of a first cross-over point ε1
considerably increases (ε1 is energy of electron at which secondary electron emission yield equals 1).
Fig. 6. Photograph of thin nano-carbon film deposited by electrophoresis method.
σmax and ε1 values measured in described experiments are accumulated in the Table1. Just value of a first cross-
over point assigns the threshold value of microwave power density necessary for excitation of most urgent for a current
microwave techniques so called “nonresonant” or “polyphase” multipactor [6]. According to [5] the necessary conditions
for this type discharge excitation is electron oscillatory energy Wosc attainment of a value higher than the value of a first
cross-over point:
Π thr
[Wosc ]thr ≅ 6 ⋅ 1017 ≥ 0,9ε 1 , (1)
ω2
where Π thr (W/cm2) is threshold value of microwave energy density, ω (s-1) is cyclic microwave frequency and ε1 is
the first cross-over point in (eV). Π thr values of investigated samples are also accumulated in Table 1.
It is necessary to call attention to the fact that the second requirement for polyphase multipactor excitation is
necessity for σmax of irradiated by microwave metallic surface to excess value over 2 [5].
Reasoning from such a requirements we can conclude that excitation of polyphase multipactor is eliminated for
the sample with the film made by means of precipitation of nano-dimensional carbon from treated by electric discharge
ethyl spirit. As to the films made with help of electrophoresis they have to have substantially higher threshold value
compared to the non-treated sample.

5. Fig. 7. Dependence of secondary electron emission yield on the impacting electron energy. B-non-treated Cu; C-short-
time electrophoresis; D-coating through the evaporated spirit drops, containing nano-carbon particles; E-coating by soot;
F- long-time electrophoresis.
Table 1
Sample ε1 (eV) σmax Πthr (W/cm2)
Non-treated Cu plate 12 2,25 3,6 103
Nano-carbon deposition 120 1,13 3,6 104
from spirit emulsion
Short-time electrophoresis 50 2,25 1,5 104
Long-time electrophoresis 45 2,00 1,35 104
Soot layer <1
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