Experimental Design

Temporally resolved measurements on RF time scales (7MHz frequency or T = 143ns period) are not feasible with high-speed cameras. However when the desired signal is periodically reproducible, as is the case here, intensified cameras can be used to gradually build up an image on the camera sensor over 1000s of ∼ 50ns intensifier gatings. The accumulated image can then be digitised with sufficient signal to overcome readout noise. A low voltage duplicate of the 7MHz H-1 antenna waveform was accessible and used to trigger the camera at the same point in the RF cycle, as depicted in Fig. 5.2. The Princeton Instruments PI-MAX3 1024i intensified CCD camera used for the experiments has a maximum intensifier gating repetition rate of 1MHz. To give the most reliable camera performance a downsampling counter was developed in Labview to produce a rising edge for the camera every 8th cycle of the RF (875kHz) as there were repeatability issues when pushing the limit of every 7th cycle. The counter has a sampling rate of 80 MS/s that causes a 12.5ns jitter in the intensifier gating window. This jitter is averaged over ∼50 000 intensifier gatings and is therefore insignificant. For each image the delay between the rising edge and intensifier gate opening are carefully selected in the camera software (Lightfield) to target a particular window in the RF cycle. Only a single phase in the RF cycle was sampled for each plasma shot as the≈40ms full-frame readout time of the camera is comparable to the 100ms duration of typical H-1 plasma shots. H-1 shots are usually sufficiently repeatable that a full set of phases in the RF cycle can be targeted in the subsequent shots. Typically 4 (as in Fig. 5.2) or 8 equally spaced phases in the RF cycle were used to obtain a time resolved sequence. It is advantageous to measure 8 or more equally spaced phases in the RF cycle when higher order harmonics are important, however if only the first harmonic is of interest there are no SNR disadvantages in taking two sets of 4 phases instead of a single set of 8. Indeed in rare cases where the shots are not repeatable it is often advantageous to take a smaller number of phases per set so any outliers can be discarded without fracturing the completeness of the sequence. The optimal intensifier gate width can be determined by maximising the value of (assuming only shot noise and that the desired signal only contains a first harmonic component)

sin(2πTgate/T)

p

Tgate/T

. (5.3)

The solution is a gate width ofTgate= 0.371T = 53ns in which case there is some overlap

of the gatings for each phase, as shown in Fig. 5.2. In the presence of camera readout noise the optimal gate width will be slightly longer but shorter gate widths are necessary if it is also desirable to measure the second and higher order harmonics.

The linear polarisation sensitive imaging polarimeter used for the RF Stark effect measurement is shown in the left of Fig. 5.3 and is comparable to that in the left of Fig. 3.11, only with the inclusion of delay plates to increase the delay without increasing the carrier fringe frequency. For the H-1 measurements two 15mm lithium niobate delay plates and a 2.5mm αBBO Savart plate were used. The delay plates are crossed with an intervening half-wave plate at 45◦ to deliver a more uniform delay across the field of view [79]. This field-widening is more critical for this measurement, compared with the IMSE measurements, because more delay is required to achieve the optimal contrast due to the weaker electric field and shorter focal length lens. A ‘reentrant’ port (port 113), shown in the right of Fig. 5.3, was designed to provide an optimised view of the H-1 antenna. To relay the image to the camera it was necessary to incorporate a mirror and optical

0 100 200 300 400 500 Time(ns)

RF Signal

Intensifier Phase for 1st Image Intensifier Phase for 2nd Image Intensifier Phase for 3rd Image Intensifier Phase for 4th Image

Figure 5.2: Timing of the intensifier gatings relative to the RF waveform for synchronous imaging. Each image is built up by opening the intensifier at the same point in the RF cycle before the phase is offset for subsequent images to build up a complete phase resolved measurement (in this example using 4 different phases). In reality only every 8thRF cycle was sampled, yielding a duty cycle of∼4%.

imaging fibre bundle into the assembly. The dielectric mirror in the system is optimised for polarisation preservation at 660nm so any possible coupling between linear and circular polarisation is expected to be small. To measure the polarisation information it is essential for the polarimeter to be located before the imaging optical fibre bundle resulting in some degradation of the carrier fringe fidelity.

Antenna Plasma Polarimeter ICCD Camera Gas Puffer Imaging Optical Fibre

Figure 5.3: (Left) Polarimeter used for the linear polarisation measurement. The delay plate is field-widened to produce a more uniform delay across the image. In reality a Savart plate was used instead of the displacer as it delivered the best carrier fringe frequency of the available plates, but in the most straightforward implementation a displacer is sufficient. The lenses, interference filter and camera sensor are implied but not shown. (Right) Viewing geometry used for the RF Stark measurement. The plasma is shown in yellow and the edge of the vacuum vessel is depicted with the curved black line. A negative lens (−60mm) near the window of the reentrant assembly enhances the field of view. The plasma light is then reflected by a dielectric mirror and collimated with a 150mm lens. The polarisation information is encoded by the polarimeter before it is focused onto the 8mm×10mm imaging optical fibre bundle with a 35mm C-mount lens. The fibre bundle relays the light to the ICCD camera outside the H-1 vacuum vessel where the light is refocused onto the camera with a 50mm and 85mm F-mount lens combination and the

A supersonic gas puffer was installed on H-1 specifically for this RF electric field measurement[80]. The gas puffer is located above the antenna and injects any desired gas species into the region under the antenna with a FWHM cone angle of 33◦. The gas species can be the same as the background plasma to enhance the light intensity (eg hydrogen puff into hydrogen plasma) or an alternative species to enhance the localisation of the measurement (eg neon into hydrogen plasma).