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

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Noise (dBm) −60 −70 −80 −90 0 10 20 30 40 50 Time (s) 60 70 80 90 100 Input Power (dBm) Return Loss (dB) 46 44 42 0 10 20 30 40 50 Time (s) 60 70 80 90 100 −8 −10 −12 −14 −16 0 10 20 30 40 50 Time (s) 60 70 80 90 100 Figure 6.3: Multipactor monitored by studying close-to-carrier noise as well as the return loss. At t ≈ 50 s the component starts to experience multipactor discharge and becomes increasingly mismatched. Excellent agreement with the second detection methods, close-tocarrier noise, can be seen. 98
software and the time evolution of the electron density can be studied. The probe does not have to be located inside or close to the area where the discharge will take place. Even though it is advantageous for higher sensitivity to have the probe pointing at the critical gap it is also reliable when located outside the device under test and thus it can be used for waveguides and coaxial transmission lines. It is not completely clear from the paper, however, how the electrons escape from a completely confined device like a typical waveguide or coaxial cable and one will have to assume that there must be some small opening somewhere in the system, where electrons can leak out. Furthermore, the method can be used to quantitatively measure the amount of generated electrons. however, this requires some kind of calibration of each test setup and that may prove to be problematic. When used in this way, the method can no longer be viewed as a global method, which can detect a multipactor event anywhere in the system, instead it has become a local method. Among the main advantages of the method is the low cost involved, since no expensive microwave instruments are needed. Residual mass A very slow global method of detection is to detect the gas molecules, which are outgassed from the device walls due to the electron bombardment during a multipactor event. The gas molecules consist of residuals of water, air and contaminants, and using a mass spectrometer, the different molecules can be identified. It has been noted [31] that a detectable increase in the water spectrum can be seen during a multipactor discharge. The major drawback of this method of detection is its inability to detect fast multipactor transients (not enough molecules are released from the walls) and thus it is not a suitable method for multicarrier multipactor studies. Another disadvantage is that there is a certain delay between onset of the discharge and indication in the instrumentation. However, it can be useful as a diagnostic tool together with one or two of the other described methods. 6.1.2 Local methods In cases where it is not sufficient to only confirm the existence of a discharge in the system, but also to determine the exact position, local methods of detection will have to be used. The two most common local 99
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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
Page 62 and 63: 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
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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: tipactor events that are short-live
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 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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