Beam Multiwire

Collector large and small

Rogowski Coils Thomson Scattering Langmuir Probe Visible Spectroscopy rf b probe Electron Gun Visible Light RF Antennae 8mm Interferometer DC and RF Rogowski Inner PFC+HFC

Diamagnetic Loop FIR Interteromete

2mm Microwave Polarimeter rf b probe

Figure 3.2: Top view of H-l showing the locations of different diagnostic systems. The multi-wire grid detector is at 0 = 85° and the electron gun is at 0 = 120°.

6 omm

trias

stainless steel tube

*

M

5.0 mm cross-section

thoriated tugsten

tinned copper wire

a^K

alumina tube

Gun Wall _filament Gun wall bias ^ supply"'^ 0 - 1 5 0 V ■ = - 0 - 5 V detector filament power supply

Figure 3.4: Circuit connection for the electron gun.

shows the the electron gun circuit. The emission current (collected by the tube) is monitored with a, DVM during scans and held constant at values between 5 and

30 i i A and the electron beam energy is around 30 eV. At 8 /iA emission current,

the current escaping from the the exit hole ( “beam current") is estimated (from the total current in the wire-grid, adding all the wire currents, for a particular detector orientation) to be around 1.1/r A when B0 = 0.1T and the background pressure is lxlO-6 tone Under these conditions, deterioration of the filament is

a few

reduced and filament life is ~ weeks of operation.

3.2

W i r e - G r i d D e t e c t o r

The wire-grid assembly is designed to stay permanently inside the vacuum tank. It is installed in a port at (f> = 85° where the greater TFC coil spacing and clear­

ance from other H-l structures allows the mounting of the support frame for the wire-grid. During plasma experiments the wires are parked away from the plasma

region and will not perturb the results, and the support frame does not act as a plasma limiter. To protect the wires from any RF induced fields, the park posi­ tion is shielded with stainless steel plates and the wires are connected to ground. Both the support frame and the grid frame are machined from inch-thick stainless steel. A segment, held in place by dowel pins and screws, can be removed from

f n/i-V^ \

both to allow them to be assembled,Uinking the PFC.JFigure 3.5 shows an overall view of the the detector assembly.

The grid frame is provided with four steel ball bearings that ride on the V-groove of the support frame. The ball bearing is captured in a teflon cup in the rotating frame which reduces the friction and allows easy height adjustment by using a short jacking screw behind the cup. (See figure 3.6.) The wire frame has dowel pins, which serve as precision gear teeth, inserted near the rim and driven by a sprocket gear. The coupling rod is axially driven by a stepping motor. In addition to the electrically-switched limit contacts provided, the end positions of the grid frame are fitted with teflon stops to prevent accidental disengagement of the drive sprocket from the pins.

CD C JZ x j- CD " O CD CD C L_ □ D O " O

>

Cross-sectional View

frame _{stainless steel}

cheesehead screw molybdenum wire

teflon-coated

teflon _gold-plated

inserts feedthrough

Figure 3.7: The wires are mounted by twisting it around a steel screw inside teflon inserts.

There are 64 molybdenum wires(0.15 mm-dia) in the grid used as the beam collector. These are mounted onto the grid frame using 2mm x 10 mm stainless steel screws and insulated by teflon inserts (see figure 3.7). Holes are provided in the screws to hold the molybdenum wire and the gold-plated pins that connect to teflon-insulated wires. The teflon-insulated wires are gathered and formed into a cable, shielded with a copper braid. The complete cable wraps around a series of teflon pulleys on the support frame as the grid turns.

The wires are designed to be tensioned so that small (< 1 mm) changes in their length, or in the dimensions of the grid frame as it turns, will not result in any appreciable sagging under gravity, or limpness. The latter was a significant problem in the SHEILA prototype experiment[5], producing uncertainties of up to 1 mm (3% of the plasma radius) in the wire positions caused by fractional millimetre changes in the roundness of the wire frame as it turned.

The tension in the wires is adjusted to achieve a transverse vibrational frequency of 80 — 100 Hz to avoid problems that may arise from the wire vibrations when

the grid is rotating or caused by other sources, and to produce a known tension in the wire. This tension causes an elastic elongation of the wire in the order of 1 mm for the longest wires, which is adequate to avoid sagging. The shorter wires are subject to smaller length changes, and therefore require less elastic elongation, keeping their resonant frequency approximately constant. To ensure that vibrational motion of the wire will not degrade the quality of the data, the grid is rotated at intervals of one second or more.