Patent Number: 052395634
Section: description

DETAILED DESCRIPTION OF THE INVENTION As depicted in the schematic drawing of FIG. 1, momentum flux of the plasma stream on target 10 is transferred via quartz rod 22 and carbon plug 12 to transducer 28, comprised of diaphragm 29 and capacitor 30. Quartz rod 22 is suspended by pairs of wires 14 and 16 from adjustable supports 18 and 20, respectively. Metal shield 34 protects pairs of wires 14 and 16 from plasma bombardment. Leads 24 and 26 allow for biasing and monitoring the temperature of target 10. Carbon target 10 is manufactured from ATJ graphite. It is shaped as two coaxial disks, roughly 1 and 3.5 cm in diameter, each 0.5 cm thick. The smaller disk has a coaxial hole 5 mm in diameter that allows it to be securely placed onto quartz rod 22. The diameter of the larger disk was chosen to exceed the diameter of the plasma jet by about 1 cm. Those skilled in the art will recognize that targets of different materials, shapes, and orientations may be substituted for target 10. Quartz rod 22 has an i.d. of 3 mm, an o.d. of 5 mm, and a length of 68.5 cm. Quartz is used because of its electrical insulating properties, light weight, rigidity even at elevated temperatures, and low thermal conductivity. Calculations show that, even after 1 hour of plasma bombardment of target 10 at 10 W/cm.sup.2, the temperature rise of quartz rod 22 at diaphragm 29 is less than 30 degrees centrigrade and that the increase in length of rod 22 will not alter calibration. Hollow quartz rod 22 provides a channel to run wires to target 10. This allows for electrical biasing of target 10 or monitoring the temperature of target 10 with a thermocouple (not shown) through leads 24 and 26. In applications where a magnetic field is present, it is advisable to orient leads 24 and 26 parallel to the field to avoid magnetic forces. Quartz rod 22 has a carbon plug 12 on the end opposite target 10 that presses against diaphragm 29. This distributes force evenly over diaphragm 29, preventing plastic distortion. Between carbon plug 12 and diaphragm 29 is cushion 27 which reduces bouncing of quartz rod 22 off of diaphragm 29 when an impulsive load is applied. Transducer 28 is comprised of diaphragm 29 and capacitor 30. In the preferred embodiment, transducer 28 is a Baratron.RTM., commonly used as a pressure sensor in the range 10.sup.-6 to 10.sup.4 mTorr for room-temperature gases. The Baratron.RTM. is a capacitance manometer, i.e. a pressure differential causes elastic deflection of a thin, tensioned, circular, metallic diaphragm 29 which is one electrode of a capacitor 30. From the specifications of the head used, forces in the range of 10.sup.-5 -10.sup.-1 N can be measured with an accuracy of 1%, once the conversion from pressure to force has been obtained. Use of Baratron.RTM. heads with higher pressure capabilities, to 10.sup.4 Torr, allows extension of the measurable force range to .about.10.sup.3 N. Other pressure transducers, such as piezoelectric or strain gauges or torsion pendula, may be used. However, they are less suitable for application in an intense magnetic field an pulsed plasma environment. Quartz rod 22 is maintained in a horizontal position by means of two pairs of .about.3-mil chromel wires 14 and 16 that suspend quartz rod 22 at points about 10 and 30 cm from target 10 and carbon plug 12, respectively. The exact location of each support pair is chosen to avoid integral and fractional length resonances in tube bending. At each suspension point each wire within pairs of wires 14 and 16 is at 45.degree. with respect to the vertical. Each pair of wires 14 and 16 is coplanar and perpendicular to the axis of rod 22. The length of each wire, controlled by adjustable micrometer-type mounts 18 and 20, is about 10 cm. This suspension allows only longitudinal, axial displacement of quartz rod 22. A thin metal shield 34 protects pairs of wires 14 and 16 from plasma bombardment. Quartz rod 22 and pairs of wires 14 and 16 are mounted in a stainless-steel vacuum housing 32 with port crosses for mounts 18 and 20. Diaphragm 29 is attached to one end of housing 32 by use of stainless-steel welded bellows 31. The length of bellows 31 is set with micrometer mount 33. This is used to control contact between carbon plug 12, damping cushion 27, and diaphragm 29 and is an essential element in the calibration routine. The pressure data output by transducer 28 (.chi.), the force F.sub..chi. exerted on target 10 by the plasma column, and the plasma momentum flux .PHI., are related by EQU .PHI.=F.sub..chi. /S.sub.plasma =.chi.S.sub.eff /S.sub.plasma(1) where S.sub.eff is the effective interaction area of diaphragm 29 and S.sub.plasma is the plasma cross section. For a circular tensioned diaphragm, deformed by a uniformly applied force, S.sub.eff can be easily calculated by analytic methods. But, because the exact shape of deformation caused by the contact of carbon plug 12 with diaphragm 29 is not readily measured, the apparatus needs to be calibrated to experimentally determine S.sub.eff. Calibration may be accomplished by several different methods, many of which rely on gravity. Referring to FIG. 1, in the preferred embodiment of the present invention calibration is accomplished by moving diaphragm 29 axially relative to quartz rod 22 using bellows 31. When diaphragm 29 is moved towards quartz rod 22, the angle .theta. of pairs of wires 14 and 16 with respect to vertical increases. The resulting increment in gravitational force .DELTA.F.sub.g, is related to displacement, .DELTA.x, of quartz rod 22 by EQU .DELTA.F.sub.g =M.omega..sub.0.sup.2 .DELTA.x=Mg sin .theta.(2) where g is the gravitational acceleration, M is the total mass of quartz rod 22 and attachment, e.g., target 10, and pairs of wires 14 and 16, and .omega..sub.0 is the free oscillation frequency of the pendulum. Mass is measured using a precision microbalance; natural frequency is measured using a pickup coil near target 10 and a small (&lt;0.05 g) magnet, temporarily attached to target 10. The total mass of the pendulum and the pendulum frequency were found to be 34.225.+-.0.001 g and 16.16.+-.0.02 Hz, respectively. The force increment is related to transducer 28 output by EQU .DELTA.F.sub.g =.DELTA.XS.sub.eff. (3) Defining the output of transducer 28 as EQU R=.DELTA.X/.DELTA.x (4) from a gravitational loading, giving EQU S.sub.eff =M.omega..sub.0.sup.2 /R. (5) From a least squares fit of the pressure data versus diaphragm displacement shown in FIG. 2, we get R=103.06 mTorr/mm (with .sigma..sub.R =0.47 mTorr/mm) or R=1.374.times.10.sup.3 dyne/cm.sup.3 (with .sigma..sub.R =6 dyne/cm.sup.3), from which S.sub.eff =6.51.+-.0.04 cm.sup.2. (The geometrical area of diaphragm 29 is about 20 cm.sup.2.) This calibration yields the conversion of gauge pressure to applied force (both in practical units), EQU F.sub..chi. (grams)=8.86.+-.0.05.times.10.sup.-3 .chi.(mTorr).(6) To obtain the momentum flux, the cross-sectional area of the plasma must be known [Eq. (1)]. For a plasma which is uniform, measurements of momentum flux are averages over the plasma cross section. Transducer pressure data in FIG. 2 start at .about.30 mTorr. This value is a fixed offset applied to transducer 28 of the preferred embodiment by shortening the length of bellows 31 by use of micrometer mount 33. It is useful to preload cushion 27 between carbon plug 12 and diaphragm 29 to reduce compression during subsequent loads because motion of quartz rod 22 causes a reduction in force applied to transducer 28 by an amount Mg sin .theta.. Smaller preloads are to be used for detecting smaller forces. This method of calibration allows in situ recalibrations, without the need to vent housing 32 to air. Also, calibrations done with the apparatus exposed to air require a pumpable chamber on the back face of diaphragm 29. Without this pumpable chamber, exposure to atmospheric pressure causes the diaphragm to seat itself on the counter electrode. In the preferred embodiment the variable capacitor 30 comprising transducer 28 is part of an RF capacitance bridge that operates at 10 kHz. The high accuracy of transducer 28 requires an electronic averaging time of typically 10 cycles. This gives an intrinsic response time of 1 ms. (The response of the Baratron.RTM. to gas pressure changes can be longer due to vacuum conductances.) The response time of the present invention is longer, however, due to the natural frequency and mass of quartz rod 22, the natural frequency of diaphragm 29, and the small, but finite, compression of cushion 27. The compression of cushion 27 is approximately linear with applied force at low forces, but rapidly saturates at high forces. From the nonlinearity of the data in FIG. 2, at .chi..about.-55.35 mm, it appears that compression saturates at a F.sub.s &lt;0.001 g, corresponding to a displacement of (F.sub.s /M.omega..sub.0).about.0.001 cm. This will add less than 1/4 an oscillation period to the response time of the apparatus, corresponding to .about.75 ms for the undamped pendulum and &lt;30 ms for the damped pendulum. FIG. 3a is an idealized plot of response time of the present invention for the impulse of an He gas plasma onto a negatively biased target. The response time of the apparatus was measured using plasma pulses of varying duration from a linear plasma device. Short duration plasma pulses, &lt;20 ms, provide a current to target 10. The force is essentially impulsive, followed by a decay time of .about.10 ms. The apparatus output rises to a peak about 25 ms after the impulse and then decays in a similar time. Plasma pulses of longer duration have a more complicated time evolution. Again there is a rapid rise, .about.1 ms in plasma current to target 10. This is followed by the same .about.10-ms decay to a valley roughly equal to 1/4 the peak value. This is followed by a rise in plasma current in 30 ms to a plateau that is about twice the amplitude of the valley. Measuring from the time of breakdown of the gas, the formation of the plasma takes about 50 ms to stabilize at the plateau level. When the plasma is terminated, the current drops to zero in about 1 ms. From that instant, the output signal falls to 1/e of its peak in 50 ms, as shown in FIG. 3b, for plasma pulses of two different durations. As depicted in the preferred embodiment, then, this invention enables measurement of momentum flux from an intense plasma stream onto surfaces defining the plasma boundary. It is particularly useful when the plasma stream is accompanied by high heat fluxes, in an intense magnetic field, and in a pulsed plasma environment. In other embodiments, the apparatus of the present invention may be applied as well to determine other forces in equally inhospitable environments, for example, the force of intense pulsed laser beams on material surfaces, or the force due to chemical reactions on a surface. The apparatus of the present invention may be used to measure static, slowly varying, and/or pulsed forces. The foregoing description of a preferred embodiment of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and obviously many modifications and variations are possible in light of the above teaching. The embodiments described explain the principles of the invention and practical applications and should enable others skilled in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.