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

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methods are charge detection using a probe and optical monitoring and these two methods will be described below. Charge probe A special case of the method of electron monitoring, which was described in a previous subsection, is the use of a probe that monitors only a certain position within the microwave device. A very common approach used when detecting electrons inside a waveguide is to flush mount an SMA connector and apply a positive potential to the centre pin. The negatively charged electrons are attracted to the pin and causes a small, but detectable, current to flow, which can serve as an indication of the electron density. The method is easy to implement and therefore it has been quite common in many test setups. Unfortunately, it is also quite slow due to the circuit used to amplify the weak current [14] and thus it is not feasible for measuring fast multipactor events. In addition, it requires modification of the component, which makes it useless when testing flight hard ware. In such a case the EDDM is a better choice. Optical detection Optical detection is possible since the electrons that make up the multipactor discharge can excite or ionise either the remaining gas molecules within the device or the molecules in the device wall. It can be divided into two groups, viz. photon detection via optical probe and photon detection via photographs or video camera [72,73]. Both methods are common, but the former seems to be more frequently used. The main advantage with these methods is that they can be used to pin-point the location of the discharge inside the device. A major disadvantage is that they may be impossible to use for studying real parts, as the devices may not have any suitable openings. This is especially true for the methods based on photographic techniques. 6.2 Detection using RF Power Modulation During verification of a test setup for multipactor at Saab Ericsson Space, Göteborg, Sweden, an odd, spike-like phenomenon was found in the noise generated by a discharge in a coaxial test sample. An ad- 100
ditional test sample, a resonant cavity, was manufactured and the same type of spikes were noted again (cf. Fig. 6.4). (dBm) −55 −60 −65 −70 −75 −80 −85 −90 Noise Power (92.8 − 93.0 s) 92.82 92.84 92.86 92.88 92.9 Time (s) 92.92 92.94 92.96 92.98 93 Figure 6.4: Periodic spikes, which appeared during a multipactor experiment. The main periodicity is 100 Hz and emanates from the power supply of the high power amplifier. A fast Fourier transform revealed that the spikes were periodic and it was noted that the same periodicity could be found also in the input signal after it had been amplified by the TWTA (travelling wave tube amplifier). Due to interference from the main power supply, the signal was amplitude modulated with a main modulation frequency of 100 Hz and with harmonics at multiples of this frequency. The interference was very weak and would in most cases be disregarded. In order to see if the periodicity was present only in conjunction with multipactor events, a large number of test runs were performed. The results were consistent - the periodic noise only appeared when a discharge was detected. Figs. 6.5 and 6.6 show one of the test runs where the noise is non-periodic before onset of multipactor but periodic directly afterwards. The AM (amplitude modulation) that was present in the input signal was very weak and not deliberately added. A stronger AM was added to the signal before the high power amplifier, resulting in a more distinct modulation. This made it possible to study if there was any correlation between the modulation strength and the corresponding peak in 101
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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
Page 64 and 65: analytical. Attempts were made to f
Page 66 and 67: only model, the inclusion of ionisa
Page 68 and 69: Normalised Voltage 1.1 1.05 1 0.95
Page 70 and 71: Voltage [V] 10 3 10 0 µ=0 µ=0.75
Page 73 and 74: 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: software and the time evolution of
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 and 126: Chapter 7 Conclusions and outlook T
Page 127 and 128: tioned there were multipactor in no
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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