title: Characterizing an image intensifier in an full-field range image system

authors: Payne, Andrew D.

subjects: image intensifier

abstract: We are developing a high precision full-field range imaging system. An integral component in this system is an image intensifier, which is modulated at frequencies up to 100 MHz. The range measurement precision is dictated by the image intensifier performance, in particular, the achievable modulation frequency, modulation depth, and waveform shape. By characterizing the image intensifier response, undesirable effects can be observed and quantified with regards to the consequence on the resulting range measurements, and the optimal operating conditions can be selected to minimize these disturbances. The characterization process utilizes a pulsed laser source to temporally probe the gain of the image intensifier. The laser is pulsed at a repetition rate slightly different to the image intensifier modulation frequency, producing a continuous phase shift between the two signals. A charge coupled device samples the image intensifier output, capturing the response over a complete modulation period. Deficiencies in our measured response are clearly identifiable and simple modifications to the configuration of our electrical driver circuit improve the modulation performance.

text: Characterizing an image intensifier in an full-field range image system We are developing a high precision full-field range imaging system. An integral component in this system is an image intensifier, which is modulated at frequencies up to 100 MHz. The range measurement precision is dictated by the image intensifier performance, in particular, the achievable modulation frequency, modulation depth, and waveform shape. By characterizing the image intensifier response, undesirable effects can be observed and quantified with regards to the consequence on the resulting range measurements, and the optimal operating conditions can be selected to minimize these disturbances. The characterization process utilizes a pulsed laser source to temporally probe the gain of the image intensifier. The laser is pulsed at a repetition rate slightly different to the image intensifier modulation frequency, producing a continuous phase shift between the two signals. A charge coupled device samples the image intensifier output, capturing the response over a complete modulation period. Deficiencies in our measured response are clearly identifiable and simple modifications to the configuration of our electrical driver circuit improve the modulation performance.

year: 2008

title: Current sheet formation in uniformly twisted magnetic flux tubes

authors: Ali, Farhad

abstract: A viable mechanism for coronal heating is conversion of magnetic field energy into heat through magnetic reconnection in current sheets. Two processes capable of generating current sheets in the solar corona are studied here. The corkscrew kink instability of an axisymmetric magnetic flux tube, as a result of uniform twisting, is addressed first. Linear growth rates of the instability due to a helical perturbation are calculated for a variety of tubes. It is shown that plasma pressure and tube radius reduction are stabilising.

The helically symmetric geometry allows us to use an effectively two-dimensional formulation. A magneto-frictional code is developed to simulate relaxation to a second helical equilibrium of the tubes. Lower energy equilibria resulting from the kink are studied. Only one tube, whose twist decays with radius, is shown to result in a true field singularity or a current sheet. The Gold-Hoyle tube relaxes to a smooth kinked equilibrium. Tubes having reversal of a field component result in current concentrations as they kink and relax to a neighboring equilibrium. Plasma pressure reduces the strength of the kink in all cases.

The second process resulting in current sheet formation is the interaction of multiple flux tubes. The same frictional code is used to show that repelling tubes move away from each other as the superposed field relaxes to equilibrium. Twisting them together results in current concentrations. Attracting tubes with no neutral point also give rise to current concentrations when they are twisted together and relax to equilibrium. Current sheets are formed when attracting flux tubes, whose initial combined field have a neutral point, coalesce in the absence of plasma pressure. Introducing plasma pressure prevents current sheet formation when the tubes are not twisted together. In case the tubes are twisted together we again see current sheets, the lack of symmetry permitting plasma to flow out from between approaching flux surfaces, allowing them to come into contact. Interaction of unequal attracting tubes results in curved current sheets.

text: Current sheet formation in uniformly twisted magnetic flux tubes A viable mechanism for coronal heating is conversion of magnetic field energy into heat through magnetic reconnection in current sheets. Two processes capable of generating current sheets in the solar corona are studied here. The corkscrew kink instability of an axisymmetric magnetic flux tube, as a result of uniform twisting, is addressed first. Linear growth rates of the instability due to a helical perturbation are calculated for a variety of tubes. It is shown that plasma pressure and tube radius reduction are stabilising.

The helically symmetric geometry allows us to use an effectively two-dimensional formulation. A magneto-frictional code is developed to simulate relaxation to a second helical equilibrium of the tubes. Lower energy equilibria resulting from the kink are studied. Only one tube, whose twist decays with radius, is shown to result in a true field singularity or a current sheet. The Gold-Hoyle tube relaxes to a smooth kinked equilibrium. Tubes having reversal of a field component result in current concentrations as they kink and relax to a neighboring equilibrium. Plasma pressure reduces the strength of the kink in all cases.

The second process resulting in current sheet formation is the interaction of multiple flux tubes. The same frictional code is used to show that repelling tubes move away from each other as the superposed field relaxes to equilibrium. Twisting them together results in current concentrations. Attracting tubes with no neutral point also give rise to current concentrations when they are twisted together and relax to equilibrium. Current sheets are formed when attracting flux tubes, whose initial combined field have a neutral point, coalesce in the absence of plasma pressure. Introducing plasma pressure prevents current sheet formation when the tubes are not twisted together. In case the tubes are twisted together we again see current sheets, the lack of symmetry permitting plasma to flow out from between approaching flux surfaces, allowing them to come into contact. Interaction of unequal attracting tubes results in curved current sheets.

year: 2001

title: He whakaemi kōrero mō Ngā Ruahine

authors: Manuirirangi, Hōri

abstract: Ko te whāinga o tēnei tuhingaroa, hei whakaemi kōrero mō Ngā Ruahine. I whāia e au te āhua o te tipu o te rākau hei hanga i aku kōhinga kōrero mō Ngā Ruahine. Ko te ūpoko tuatahi, ko ngā pakiaka, arā te tīmatatanga o Ngā Ruahine. I konei ka tirohia ngā kōrero mō ngā waka tawhito mete whakapapa hoki o ngā tūpuna tāngatawhenua o Ngā Ruahine. Whai muri i tēnā, he wāhanga e pā ana ki te haeretanga mai o Aotea waka, ā, ka tirohia hoki tētehi kōrero tawhito mō te wehenga o ngā iwi o Aotea.
Mai i ngā pakiaka ka piki ake ki te tinana o te rākau. Koinei te wāhanga e pā ana ki ngā korero whenua o Ngā Ruahine. Kei te ūpoko tuarua nei, ka āta tirohia ngā kōrero mō ngā tongi o te rohe o Ngā Ruahine, me ngā take kōrero e whakawhanaunga ana i te tangata ki te whenua. Kei te ūpoko tuarua anō hoki ētehi whakaaturanga whenua mō te rohe o Ngā Ruahine, hei āwhina i te kaipānui.  
Mai i te tinana, ka aro atu ki ngā peka o te rākau i raro i te ūpoko tuatoru. Koinei te wāhanga nui o tēnei tuhingaroa, nā te tokomaha o ngā toronga o ngā peka nei. I roto i te ūpoko tuatoru, ka tirohia ētehi kōrero mō ia hapū o Ngā Ruahine. Mai i ngā kōrero mō ngā rohe whenua o ia hapū, ki ngā kōrero mō ngā marae me ngā pā tawhito o ērā hapū, ā, tae atu hoki ki ngā tūpuna rongonui me ā rātou mahi whakaari.  
Hei te mutunga, ka herea katoatia ngā kōrero o te tuhinga nei i raro i te ūpoko tuawhā, kia kite ai , he aha ngā hua e pūawai ai i tēnei tuhinga. Koinei te whakamutunga o ngā kōrero, arā ko te whakakapīnga o ngā kōrero.  
Ko tēnei tuhingaroa, he whakaemi kōrero mai i ngā kaumātua o Ngā Ruahine, ā, he mea tango mai hoki ētehi kōrero i ngā pukapuka, ko ēnei kōrero kua whakatakotoria i roto i tēnei tuhingaroa hei whakawhāriki iho ki te aroaro o te tini. Ko te tūmanako, he tuhinga rangahau tēnei hei āwhina i ngā rangatahi o Ngā Ruahine, kia rongo ai rātou i ngā kōrero kāore i te tino rangona i ēnei rā, kia mau hoki ngā kōrero a ngā kaumātua o Ngā Ruahine, kei ngaro. Erangi, he whakamōhio tēnei, he tīmatanga anahe tēnei tuhingaroa mō te kaupapa nei. Ko te tūmanako, he hua anō ka puta i tēnei rangahau, hei whāngai i ngā manu hiakai o Ngā Ruahine.

text: He whakaemi kōrero mō Ngā Ruahine Ko te whāinga o tēnei tuhingaroa, hei whakaemi kōrero mō Ngā Ruahine. I whāia e au te āhua o te tipu o te rākau hei hanga i aku kōhinga kōrero mō Ngā Ruahine. Ko te ūpoko tuatahi, ko ngā pakiaka, arā te tīmatatanga o Ngā Ruahine. I konei ka tirohia ngā kōrero mō ngā waka tawhito mete whakapapa hoki o ngā tūpuna tāngatawhenua o Ngā Ruahine. Whai muri i tēnā, he wāhanga e pā ana ki te haeretanga mai o Aotea waka, ā, ka tirohia hoki tētehi kōrero tawhito mō te wehenga o ngā iwi o Aotea.
Mai i ngā pakiaka ka piki ake ki te tinana o te rākau. Koinei te wāhanga e pā ana ki ngā korero whenua o Ngā Ruahine. Kei te ūpoko tuarua nei, ka āta tirohia ngā kōrero mō ngā tongi o te rohe o Ngā Ruahine, me ngā take kōrero e whakawhanaunga ana i te tangata ki te whenua. Kei te ūpoko tuarua anō hoki ētehi whakaaturanga whenua mō te rohe o Ngā Ruahine, hei āwhina i te kaipānui.  
Mai i te tinana, ka aro atu ki ngā peka o te rākau i raro i te ūpoko tuatoru. Koinei te wāhanga nui o tēnei tuhingaroa, nā te tokomaha o ngā toronga o ngā peka nei. I roto i te ūpoko tuatoru, ka tirohia ētehi kōrero mō ia hapū o Ngā Ruahine. Mai i ngā kōrero mō ngā rohe whenua o ia hapū, ki ngā kōrero mō ngā marae me ngā pā tawhito o ērā hapū, ā, tae atu hoki ki ngā tūpuna rongonui me ā rātou mahi whakaari.  
Hei te mutunga, ka herea katoatia ngā kōrero o te tuhinga nei i raro i te ūpoko tuawhā, kia kite ai , he aha ngā hua e pūawai ai i tēnei tuhinga. Koinei te whakamutunga o ngā kōrero, arā ko te whakakapīnga o ngā kōrero.  
Ko tēnei tuhingaroa, he whakaemi kōrero mai i ngā kaumātua o Ngā Ruahine, ā, he mea tango mai hoki ētehi kōrero i ngā pukapuka, ko ēnei kōrero kua whakatakotoria i roto i tēnei tuhingaroa hei whakawhāriki iho ki te aroaro o te tini. Ko te tūmanako, he tuhinga rangahau tēnei hei āwhina i ngā rangatahi o Ngā Ruahine, kia rongo ai rātou i ngā kōrero kāore i te tino rangona i ēnei rā, kia mau hoki ngā kōrero a ngā kaumātua o Ngā Ruahine, kei ngaro. Erangi, he whakamōhio tēnei, he tīmatanga anahe tēnei tuhingaroa mō te kaupapa nei. Ko te tūmanako, he hua anō ka puta i tēnei rangahau, hei whāngai i ngā manu hiakai o Ngā Ruahine.

year: 1998