Multipactor in Low Pressure Gas and in ... - of Richard Udiljak

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At altitudes where most satellites operate, the pressure is very low and for most purposes it can be approximated as a perfect vacuum. However, during the launch phase of a satellite and during its first days of operation as well as at times when the satellite fires its attitude and altitude control engines, the microwave parts may not be completely vented and thus it is important to understand what happens with the multipactor threshold with increasing pressure. This has been one of the main topics of this thesis. It was found that for materials with a low first cross-over energy, for the lowest order resonance, the threshold increases with increasing pressure until reaching a maximum, after which it starts to decline. The reason for the increase is the friction force experienced by the electrons when colliding with the neutral gas particles. In all other cases, for materials with a high first cross-over energy and in general for the higher order modes, a monotonically decreasing threshold is noted. This behaviour can be explained by the thermalisation of the electrons, which leads to a higher total impact energy, as well as the contribution of electrons from collisional ionisation, which reduces the necessary secondary emission yield and consequently also the required impact velocity. Improved quantitative results require a more detailed investigation of the fraction of the electrons from impact ionisation that actually contribute to the multipactor bunch, but that is left for a future study. Furthermore, in order to make the model useful for all frequencygap size products, an extension of the model to include also the hybrid modes is necessary. Due to the complexity of such a study, it was not included in this first analysis. However, this may be an interesting topic for a future investigation. In addition to designing with respect to the proper thresholds, most hardware will require some type of testing to ensure compliance with the standard. Such tests must be of good quality to avoid ambiguities, which could disqualify a component that is multipactor free. In this thesis a method that gives unambiguous and highly reliable test results was presented. It is a method based on a weak amplitude modulation of the input signal, which becomes very distinct in the multipactor signal, since the generated noise power is a function of the difference between input power and the multipactor threshold. By using this auxiliary method of detection in connection with two other detection methods, reliable test results can be obtained. It is quite satisfying when looking back at the chapter on future work in my licentiate thesis [75] and realizing that the two main topics men- 112
tioned there were multipactor in nonuniform fields and irises, which were then successfully studied during the second part of my PhD work. However, there are still interesting projects to consider, e.g. multipactor in irises where not only the effect of the random walk is considered, but also the effect of the actual nonuniform field in the structure. There are many other important structures in microwave systems, which have not been studied with respect to multipactor discharge. Among these are crossed irises, septum polarisers and ridged waveguides. The 20 gap-crossings rule, which is part of the ESA standard [50], should be re-investigated both theoretically and experimentally to make sure that the rule has a sound theoretical base with good agreement between simulations and experiment. As a continuation of the coaxial study, the axial dimension could be included by considering not only a travelling wave signal but also a standing wave. Since electrons affected by the Miller force will drift towards positions with low field amplitude, it may not be feasible to directly apply the peak amplitude of the standing wave in the coaxial model presented in this thesis. 113
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Thesis for the degree of Doctor of
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Multipactor in Low Pressure Gas and
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Conference contributions by the aut
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viii
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3.2.4 Key findings . . . . . . . .
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xii
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xiv
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addition, it is difficult to make c
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numerical study of the phenomenon i
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to physically damage microwave comp
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odd positive integer (N = 1,3,5...)
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σ se [−] 2.5 2 1.5 1 0.5 W 1 W m
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Voltage [V] 10 4 10 3 10 2 10 0 N=1
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even though it may be sufficient fr
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where φL and φR are the left and
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(cf. Fig. 2.7). When taking the env
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2.1.4 Methods of suppression Many o
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Noise (dBm) Power #1 (dBm) Match #1
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andom spread in emission velocities
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Figure 2.12: Multipactor experiment
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where kn is a factor determining th
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allows for another design margin, w
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Boundary function Method One of the
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carriers, because the NLSQ method r
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multipactor discharge. Using a Mont
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the assumption of a constant ratio
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Voltage (V) 10 3 10 2 10 1 10 −2
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3.1.3 Main results The main result
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lytical model is obviously needed.
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where 〈vt 2 〉 represents the av
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these quantities in the range from
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analytical. Attempts were made to f
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only model, the inclusion of ionisa
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Normalised Voltage 1.1 1.05 1 0.95
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Voltage [V] 10 3 10 0 µ=0 µ=0.75
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Chapter 4 Multipactor in irises A c
Page 75 and 76: h l Figure 4.1: The geometry used i
Page 77 and 78: N e /N 0 [−] 3 2.5 2 1.5 1 Multip
Page 79 and 80: to compensate for electron losses i
Page 81: 4.4 Main results This analysis has
Page 84 and 85: support for the qualitative analyti
Page 86 and 87: where R(t) is the average position,
Page 88 and 89: where d stands for the gap width, V
Page 90 and 91: there are different oscillatory vel
Page 92 and 93: Phase [degrees] 60 50 40 30 20 10
Page 94 and 95: G 100 90 80 70 60 50 40 30 20 10 0
Page 96 and 97: possible for quite large Ro/Ri-valu
Page 98 and 99: upper right region of the figures,
Page 100 and 101: σse,max the zones become wider and
Page 102 and 103: G 50 45 40 35 30 25 20 15 10 5 0.2
Page 104 and 105: creasing ratio Ri/Ro was observed.
Page 107 and 108: Chapter 6 Detection of multipactor
Page 109 and 110: Amplitude of acceleration [Gm/s 2 ]
Page 111 and 112: tipactor events that are short-live
Page 113 and 114: software and the time evolution of
Page 115 and 116: ditional test sample, a resonant ca
Page 117 and 118: the FFT. It was concluded that ther
Page 119 and 120: 6.2.1 Single carrier When using the
Page 121 and 122: Reference signal generator Phase co
Page 123 and 124: Amplitude [A.U.] 2.5 2 1.5 1 0.5 De
Page 125: Chapter 7 Conclusions and outlook T
Page 129 and 130: References [1] W. Henneberg, R. Ort
Page 131 and 132: [27] A. Ruiz et al, “Ti, V, and C
Page 133 and 134: [51] D. Wolk, D. Schmitt, and T. Sc
Page 135 and 136: [75] R. Udiljak, Multipactor in low
Page 137: Included papers A-F 123
Page 141: Paper B R. Udiljak, D. Anderson, M.
Page 145: Paper D R. Udiljak, D. Anderson, M.
Page 149: Paper F V. E. Semenov, N. Zharova,
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Multipactor in Low Pressure Gas and in ... - of Richard Udiljak

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