Page 2 - Fast imaging diagnostics on the C-2U advanced beam-driven field-reversed configuration device

11D428-2 Griswold et al.
Rev. Sci. Instrum. 87, 11D428 (2016)
FIG. 1. Magnetic field surface contour lines and plasma density (color) in C-2U calculated by a 2-D magnetohydrodynamic numerical simulation.6
A conservative estimate of the maximum current density that can flow in this gap without being limited by space charge is given by the one-dimensional Child-Langmuir limit,
FIG. 3. The energy analyzer consists of a series of mesh electrodes in front of a current collector: #1 attenuation electrode, #2 shielding electrode, #3 ion-repelling electrode, #4 electron-repelling electrode.
where is the pyroelectric coe cient, c and ⇢ are the specific heat and density of the crystal, is the width of the crystal (Fig. 4), and Q is the power density that the crystal absorbs.
The LiTaO3 material has a broad frequency response (100-200 kHz) and can withstand high heat loads (20 MW/m2).11 Our design closely followed that of Reference 11, with the addition of springs to isolate the crystal from vibration (Fig. 4). This is necessary because, in addition to its pyroelectric response, the crystal also has a piezoelectric response that produces a large noise signal due to the vibration caused by pulsed magnets.
IV. ELECTRONICS
The ELA is mounted to the divertor electrode, so the cases of the analyzer elements as well as the signal ground float at Ud which can be as much as 2 kV below machine ground. The vacuum side cables are insulated with high voltage PEEK shrink and the air side cables travel through corrugated plastic piping to the electronics cabinet, which has its inside sur- faces insulated by PVC. Power supplies and amplifiers inside the cabinet are powered through an isolation amplifier with 10 kV rating, and their case ground floats at Ud. The signal is digitized inside the cabinet and sent out over fiber-optic cable.
V. SAMPLE DATA
The ELA was installed toward the end of operation of C-2U and all data were taken while the machine operated in a non-standard configuration. As a result, our conclusions about electron heat transport are limited. Nevertheless, the
FIG.4. Pyroelectricbolometerwiththecrystalmountedonspringstoisolate it from vibration. Incident power changes the temperature of the pyroelectric crystal, causing a polarization current to flow.
4 r2q(V+E/q)3/2 JCL=✏0i,(1)
where JC L is the Child-Langmuir current density,9 q and mi are the charge and mass of a hydrogen ion, d is the distance be- tween electrodes #3 and #4 (3 mm), V is the minimum voltage drop between these electrodes (250 V), and Ei (200 eV) is the drift energy of ions that enter the analyzer. Evaluating Eq. (1) gives a limit of JC L = 3.5 mA/cm2. The first two electrodes attenuate by a combined factor of T1 ⇤ T2 = (0.016) ⇥ (0.68) = 0.011 which reduces the maximum expected input current Ji = 250 mA/cm2 to just below JC L.
Electrode #3 is operated in two modes: to measure the ion energy distribution, a bias is applied and swept from 0 to +2 kV with respect to Ud, and to measure total ion current it is simply held at Ud. In both cases, electrode #4 is held at 250 V with respect to Ud to repel plasma electrons and to prevent the escape of secondary electrons from the collector.
III. BOLOMETER
The pyrobolometer is based around a pyroelectric LiTaO3 crystal. Pyroelectric materials have a permanent polarization that varies with temperature, causing a polarization current Ip to flow in an external circuit (Fig. 4),10
Ip= ⇥Q, (2) c⇢
FIG. 2. The ELA, consisting of two bolometers and two gridded energy analyzers, is shown mounted on the divertor electrode after operation.
9 mi d2