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

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correspond to huge increases in the decibel scale, which can be seen in Fig. 6.8. (dBm) −60 −65 −70 −75 −80 −85 Noise Power (100 samples noise average) 0 10 20 30 40 50 Time (s) 60 70 80 90 100 Figure 6.8: The same sequence as in Fig. 6.7 (except that in this case the entire sequence is shown). The small initial steps of Fig. 6.7 become huge steps on the logarithmic scale. The above examples, which used the new detection method, relied on close-to-carrier noise measurement data. However, the mechanism which is utilised requires primarily that the input signal is amplitude modulated, that the detected signal is proportional to the difference between the input power and the multipactor threshold and that the detected signal responds quickly to changes in the multipactor event. The two first conditions are likely to be fulfilled by all detection methods for multipactor, but the last will have to be verified for each method. Results from measurements presented in [14] show that third harmonic detection is faster than close-to-carrier noise detection, which should make it excellent for AM detection. In general, probably any method can be used provided that a suitable AM frequency is chosen and that the instrument used for detection has a sampling frequency that is more than two times larger than the modulation frequency in order to fulfil the Nyquist criterion. 104
6.2.1 Single carrier When using the AM detection method in the single carrier case, the test setup implementation is quite straight forward. In practice, the only difference between a standard multipactor test setup and one that uses the AM detection method is that the signal data must be sent to a computer or some other instrument that can perform a FFT. It is also important that the signal generator is capable of producing an AM signal, but that is a standard feature of most signal generators. The spectrum analyser, which receives the signal, should be set to a sampling rate that is at least two times larger than the AM signal, i.e. if the AM signal has a frequency of 1000 Hz, then a minimum sampling rate of 2000 samples/second should be used. However, in order to study the shape of the modulation signal, it is better to use a sampling frequency more than twenty times greater than the frequency of the AM. The signal data can be stored for future processing, or if a powerful computer is used, real time FFT can be performed, thus allowing the operator to get an immediate indication when a discharge takes place. 6.2.2 Multicarrier AM detection in the multicarrier case [74] is somewhat more difficult and the method has not yet been experimentally confirmed. When performing a multicarrier multipactor test, the phase of each carrier will have to be adjustable in order to enable the test engineer to produce the wanted shape of the signal envelope. The aim is to produce a signal envelope corresponding to the assumed worst case. When the phases have been set to their predefined values, the envelope will be periodic. If the envelope exceeds the multipactor threshold for a time long enough to allow a sufficient number of gap crossings, a discharge will occur, provided that a suitable seed electron is available. When the envelope drops below the threshold again, the electrons will disappear quickly [21] and there will normally be no electrons left from the discharge the next time the envelope exceeds the threshold. If no specific source of seed electrons is available, a multipactor discharge may not occur every envelope period. However, if an efficient electron seeding source is used, there should be an ample amount of electrons available to initiate a discharge each time the envelope exceeds the threshold for a sufficiently long time. The use of AM detection of multipactor requires that the discharge event is continuous or that it occurs regularly. Single or very sporadi- 105
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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
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 and 114: software and the time evolution of
Page 115 and 116: ditional test sample, a resonant ca
Page 117: the FFT. It was concluded that ther
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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