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Advice on lab techniques (tinkering tips)

  1. Aug 27, 2011 #1
    So I am about to start an undergrad Thesis lab on investing waves in a glow discharge tube. I am seeking advice from experts tinkerers, which I know are on this board, for advice to build up my whole apparatus as soon as possible. According to my professor it will take most of the first semester, but I would like to do it before mid-terms (I have a reason for doing this). This is probably a stretch, but I'm sure it is possible given a smart mind with two active hands and help. Anyways, I was looking for some general tinkering advice, but specific ones pertaining to my lab would be very appreciated. Without further ado, I am uploading the schematics for my glow discharge lab:

    http://s616.photobucket.com/albums/tt247/zheng89120/?action=view&current=Apparatus-1.jpg

    The title and the abstract of this lab is provided:

    Discrete Modes in the Ion Acoustic
    Range of Frequencies in a Glow Discharge Plasma Column

    Abstract
    The frequency landscape of waves in the ion acoustic range of frequencies
    (IARF) was studied as a function of voltage and pressure in a constricted DC
    glow discharge plasma. Capacitive antennae were used to measure density
    and temperature oscillations in the plasma. Four different gases (helium,
    neon, argon and air) were examined. Discrete modes, with harmonics up to
    n = 26, were observed at frequencies in the ion acoustic range, 1-100 KHz.
    The optical spectra emitted by the discharges show that air contamination
    is a possible cause of the wave. For some pressures and voltages, the normal
    modes transition to chaos though a period doubling route. The possibility
    of an ionization wave parametrically decaying into other ionization waves or
    into an ion acoustic wave was explored by adding a small audio frequency
    (AF) component to the high voltage. Sideband frequencies around each naturally
    occurring discrete harmonic were observed. The turbulence caused
    by parametric excitation is compared with the period-doubling phenomena
    leading to chaos. The AF component succeeded in stabilizing a semi-chaotic
    signal for a small window of the driving frequency. Thresholds for driving
    the plasma with the AF component were measured.


    PS. Oops, the most important part of this, the Lab Setup is as follow:

    2.1 Experimental Setup
    Experiments were performed in a pyrex glass and steel vacuum system, as
    seen in Fig. 2.1. The cathode is a stainless steel disk with a 6 cm diameter.
    The glass discharge tube used for the bulk of the data was 38 cm long and
    had a 5.515 cm outer diameter and a 5.14 cm inner diameter. The pyrex
    discharge tube is sealed to the stainless steel sections with two o-rings at
    its top and bottom. For most of the data taken, the stainless steel vacuum
    tubing beneath the glass discharge tube acted as the anode. The vacuum
    tubing had an internal diameter of 3.8 cm. Late in the data-taking process,
    a flat stainless steel plate with four drilled holes was added at the anode.
    The metal plate was 3 mm thick with a 7.7 cm diameter. The metal plate
    allowed the gas to flow through, but defined the plane of the anode, instead
    of the vacuum tubing acting as the anode. The discharge tube was cleaned
    43
    twice during the data taking process, to remove a thin film build-up at the
    cathode end of the discharge tube.
    A turbo pump was used to evacuate the tubing and discharge tube to
    a base pressure of 1 0 - 6 Torr. The gas flowed through copper tubing from
    the compressed gas cylinders to the apparatus. A different gas could be
    connected after evacuating the tubing with a small foreline pump. The tubing
    was always flushed four times to remove any impurities and the after a
    change in gas, the system was flushed for 15 minutes. Pressures were measured
    in all experiments on the MKS Baratron pressure gauge, which has an
    accuracy to 0.0001 Torr. Gases of 99.9999% purity were used, and Swagelock
    fittings were used for all gas-feed connections. If the cathode plate was
    not sufficiently tightened an air leak would deposit carbon impurities at the
    top of the discharge tube. The gas pressure could be controlled with an
    on/off valve, a needle valve and by closing the valve to the turbo pump.
    The pressures were sometimes difficult to stabilize and great diligence was
    needed to maintain pressures to an accuracy of 0.0001 torr.
    Two different high voltage sources were separately used to apply up to
    1500 V to the cathode. One was a Bertran with an accuracy and stability to
    0.1 V and a maximum current of 10 mA; the other was a Hipoelectronic with
    an accuracy of 5 V but a maximum current of 100 mA. An audio-frequency
    (AF) voltage could be added to the high voltage with a Hitran iron core
    transformer. A coil of wire with approximately 600 turns was added as an
    inductor between the transformer and the power source to resist backcurrents
    and large AF currents from reaching the power source. The inductor
    has an inner diameter of 2.25 cm and an outer of 6.5 cm. The measured
    44
    inductance was 91 mH on an inductance meter, in agreement with the calculated
    value of 81.8 mH. The sinusoidal signal of the desired frequency from
    a Hewlett Packard function generator was amplified by a Samson audio amplifier
    and then coupled to the transformer. The function generator's wave
    frequency can be measured on a universal counter to an accuracy of 0.01
    Hz. The maximum amplitude from the HP function generator is 11 V and
    the Samson studio amplifier allowed amplification to a maximum of ±23 V,
    or ±20.5 V on the transformed high voltage. The power supply will be discussed
    in the next three section, while diagnostics of the spectrum analyzer
    and transformer can be found in Appendix E.

    If You have additional questions, than i would be happily try to answer.
     
    Last edited: Aug 28, 2011
  2. jcsd
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