Page 3 - A mass resolved, high resolution neutral particle analyzer for C-2U
P. 3

configuration because they first have to exit through the mirror loss cone and then travel across the decreasing magnetic field of the expander. As a result, the ions reach the analyzer with an angle θa, that is limited by Eq. 1[8]:
θa = atan(  Ba/(Bm − Ba − qφ/μ) ≤ atan(  Ba/(Bm − Ba)), (1)
where Ba (100−500G) and Bm (≈ 1.3T ) are the magnetic field at the GEA and mirror plug, respectively, q is the proton charge, φ is the plasma potential inside the mirror plugs, and μ is the magnetic moment of a given ion. Neglecting the last term in the denominator gives the upper bound that ions will enter the analyzer with θa < 12◦. This allows us to approximate the transparency of each individual grid as a single value: TA ≈ .015 and TM ≈ 0.85.
The combined transparency of a series of meshes in sequence is found by multiplying the transparency of each individual mesh. This does not depend on the relative alignment of mesh or attenuation plate holes because the spacing between these holes (380/30μm) was chosen to be smaller than twice the local electron gyroradius (2ρe ≈ 500μm) so that the plasma electrons and ions will expand into a uniform plasma in between meshes rather than being cut into locally high density beamlets. The combined transparency, TT of each GEA is given by Eqs. 2 and 3:
TT (GEA #1) = (TA) × (TM)7 = 4.8 × 10−3, (2) TT (GEA #2) = (TA) × (TM)4 = 7.8 × 10−3. (3)
The GEA electrodes (Fig. 2) are built by spot welding a mesh to either one or both sides of the electrode frame, which provides flexibility in setting the total transparency of the GEA without significantly affecting its operation. GEA #1 and #2 were deliberately constructed with different numbers of meshes in order to check the optical mea- surement of TM and the calculation of the compound transparency. Figure 3 (c) shows the ion current measured by each GEA calculated using values for total transparency listed in Eqs. 2 and 3. The close agreement of these curves demonstrates the accuracy of the calculation.
Power Density
FIGURE 4: (Color online). Pyroelectric bolometer with the crystal mounted on springs to isolate it from vibration. Incident power changes the temperature of the pyroelectric crystal, causing a polarization current to flow.
Power density was measured by a pyrobolometer that uses a pyroelectric LiTaO3 crystal. Our design closely followed that of reference [9], with the addition of springs (Fig.4) to isolate the crystal from vibration caused by pulsed magnets that can introduce noise through the piezoelectric response of the crystal. Pyroelectric materials have a permanent polarization that varies with temperature, causing a polarization current Ip to flow in an external circuit [10]:
Ip= γ ×Q, (4) cρδ
where γ is the pyroelectric coefficient, c and ρ are the specific heat and density of the crystal, δ is the width of the crystal (Fig. 4) and Q is the power density that the crystal absorbs. The absolute sensitivity of the bolometer was calculated using Eq. 4 and the crystal’s technical data.
Representative data from the bolometer for a single shot is presented in Fig. 5 (a) and data for many shots, time-averaged over the main part of each shot (0.5-1.5 ms) is plotted fas a function of Vbias in Fig. 5 (b). Note that

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