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

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cally occurring discharges will be difficult to identify in a FFT plot. By using a source of free electrons in the test setup, e.g. a hot filament or a UV light [14], seed electrons will be abundant and a multipactor event is likely to occur each time the envelope exceeds the threshold for a long enough time. If the envelope is amplitude modulated, the discharge events will vary in strength with the AM frequency. This periodic variation will appear as a peak in the FFT plot of the detected signal at the same frequency. By applying a weak, 1-5% depth, synchronised AM to the input signals, the multipactor threshold can be determined with high accuracy and without risking ambiguous test results. A 1% AM corresponds to less than ±0.1 dB variation in the input signal and will have no significant effect on the measured threshold. The detected signal must then be processed by a computer or a similar tool in order to reveal the periodicity, as in the single carrier case. In Fig. 6.9 an example of a possible test setup is given. In order to achieve a good AM in the multicarrier case, all the signals should be modulated using the same reference signal of modulation. Many signal generators have a signal reference input, thus allowing the user to synchronise several signal generators. To perform a successful multicarrier experiment, the phases have to be stable in relation to each other and thus a common reference signal will have to be used in any case. Another way of achieving synchronised modulation could be to modulate the gain adjustment of the high power amplifier. In Fig. 6.9 it is suggested that the third harmonic should be monitored and that is probably the best choice when studying multicarrier multipactor, since third harmonic detection is fast and sensitive. Closeto-carrier noise detection is a possible alternative, but it may not be as sensitive and thus weak multipactor events may be overlooked. 6.2.3 Main achievements A method of multipactor detection has been devised, which can be used to obtain accurate and unambiguous measurement results for both single and multicarrier multipactor. The method does not aim to replace any of the existing methods of detection, rather it can serve as a complement to the other methods to improve accuracy and confidence in the test results. Close-to-carrier noise and third harmonic detection are two fast and sensitive methods of multipactor detection. Both methods rely on noise generation, which makes them prone to non-multipactor generated noise. 106
Reference signal generator Phase control ∆φ ∆φ M U X Instrument control & Data storage and processing HPA dB Wave form monitor DUT Spect. Analys. Vacuum Chamber Reflected power Power Meter dB LNA Forward power dB Power Meter Third harmonic detection Figure 6.9: Test setup for multicarrier multipactor measurements using RF power modulation. Each input signal is amplitude modulated and by phase locking the signals using a signal reference, the entire signal envelope will be modulated. A computer can be used to control the instruments and collect the output data, which can then be real-time Fourier transformed to reveal any periodicity in the detected signal. Spect. Analys. 107
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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
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 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: 6.2.1 Single carrier When using the
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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