The ExaVolt Antenna: Concept and Development Updates

A flux of ultrahigh energy neutrinos is expected both directly from sources and from interactions between ultrahigh energy cosmic rays and the cosmic microwave background. Using the cost-effective radio Cherenkov technique to search for these neutrinos, the ExaVolt Antenna (EVA) is a mission concept that aims to build on the capabilities of earlier radio-based balloon-borne neutrino detectors and increase the sensitivity to lower energies and fluxes. The novel EVA design exploits the surface of the balloon to provide a focusing reflector that aims to provide a signal gain of ∼ 30 dBi (compared to 10 dBi on ANITA). This increase in gain when combined with a large instantaneous viewing angle will yield a 10-fold increase in sensitivity and will allow this balloon-borne experiment to probe the expected low neutrino fluxes even at energies greater than 1019 eV. This contribution will present an overview of the mission concept, recent technology developments, and the results of a hang test of a 1:20-scale model which demonstrates the effectiveness of the design.


Introduction
A flux of ultra-high energy (UHE) neutrinos is expected due to interactions between UHE cosmic rays and the cosmic microwave background, as predicted by Berezinsky and Zatsepin [1].Several experiments are currently pursuing the detection of the cosmogenic UHE neutrino flux using radio detection techniques.Detectors embedded within polar ice caps either several kilometers deep [2,3] or at the surface [4] exploit the proximity to neutrino interactions in a large target volume.The ANITA [5] balloon-borne interferometer synoptically scans a significantly larger ice volume (∼ million km 3 ) providing high sensitivity to the high energy end of the expected neutrino spectra.
An experiment's sensitivity to UHE-induced radio emissions is determined by the combination of visible target volume and live-time.Ground-based arrays implement large numbers of high duty cycle stations (37 for ARA and ∼ 1000 for ARIANNA) but the visible volume is limited by the refractive and attennuative properties of the ice.A balloon-borne detector, on the other hand, has a large visible target volume with a limited operational duty cycle.For balloon-borne detectors, the only practical improvement is to increase the antenna gain.The ExaVolt Antenna (EVA) [6] is a mission concept that builds on the success of three ANITA flights.EVA aims to implement a ∼ 30 dBi gain antenna, exploiting the surface of a super-pressure balloon (SPB) to improve sensitivity to neutrino-induced radio-impulsive transients by a factor of 100 over ANITA.

ExaVolt Antenna Design Concept
The proposed EVA design will be the largest-aperture radio telescope ever flown on a balloon payload and will improve the sensitivity to neutrino-induced impulsive radio-frequency (RF) signals by a factor 100 or more over any previous experiment.This large improvement in gain can be obtained by using a portion of the balloon surface itself as part of the radio detector with a toroidal geometry as indicated in Fig. (1), where an RF-reflective film is applied to a section of the balloon surface 10-m high about the equator.An incoming plane wave impulse enters the balloon below the lower reflector rim opposite the active focusing area that will apply for that direction.Once reaching the opposite side of the balloon reflector surface, the incoming plane wave impulse is focused onto a focal plane inside the balloon.A set of patch feed antennas will be positioned at the focal plane of the reflector to receive the focused plane wave.

S(ICRC2015)1151
geometry as indicated in Fig. 2, where an approximately 10 m high equatorial section of the balloon surface is covered with RF reflective film.An incoming plane wave impulse enters the balloon below the lower reflector rim opposite the active focusing area that will apply for that direction.
Once reaching the opposite side of the balloon reflector surface (a geometry that is locally an offaxis segment of a spheroidal mirror), the incoming plane wave impulse is focused onto a focal plane inside the balloon.Although only a portion of the area of the reflective region contributes to radio signal collection from any given direction, this area is still of order 100 square meters or more, equivalent to an 11 m diameter single-dish with uniform azimuth coverage, and a usable range of 10-15 degrees in elevation angle.
The proposed design centers on a novel use of a toroidal reflector as outlined above in Fig.

Scale Model Hang Test
In order to test the feasibility of the concept, the EVA collaboration performed a hang test of a 5.7m-diameter SPB constructed to our specifications by Aerostar International.The test was performed at NASA Wallops Flight Facility in September 2014.This balloon included 50-cm high aluminizedmylar reflective panels attached to the balloon at the equator by the vendor using polyethylene adhesive tape.A view of the fully-deployed balloon during the test is shown in Fig. (2).
Prior to inflation of the balloon, we inserted a scale-model of the antenna feed array membrane through the top end-plate of the balloon.In order to accommodate the insertion, the feed array was folded and was attached to internal support lines which were used to expand the array from its folded state during inflation and which provided support for the array upon final deployment.The feed array membrane was instrumented with dual polarization sinuous patch antennas over a portion of its circumference, and four of these antennas were also instrumented with microwave receivers coupled to RF-over-fiber transceivers which inserted the RF signals into optical fiber, as shown in Fig. (2).
The two primary goals of the test were to demonstrate (1) that it was possible to deploy a feed membrane antenna array within a super-pressure balloon, and (2) that the response of the balloon and patch antennas would function as predicted by antenna models when subject to an external impulsive plane-wave signal.To create this external signal, we used a 2.6 meter offset parabolic dish as an RF collimator, fed by a broadband dual-ridge horn emitting microwave impulses.We made several compromises on the scalability for the sake of the test: the number of gores was set at 28 for this  ed a scalehrough the ith a 1 m modate inines which ation, and ht outward he feed arolarization ts circumlso instruo RF-overals into oped through al receiver, nals at the on losses.ate that it enna array e optics of ion as preexternal impulsive plane-wave stimulus.To create this as used as an RF collimator, with a broadband dualave impulses.The impulse used was short enough that e feed patch antenna initially, and then as it returned nels.We made several compromises on the scalability was set at 28 for this balloon, rather than 280, to keep ollimation dish was much smaller than the receiving ts of portability of the dish.These compromises were The results of the microwave test are summarized in litudes are seen, both for the low-gain feed, and after pane shows the corresponding intensity of the signals.he arriving impulse is preserved in the focused pulse.odel estimates within about 2 dB, a solid validation of e have thus improved the credibility of the estimates hnology.The results of the hang test confirm both the effectiveness of the concept of using the balloon as a focusing reflector and of the agreement of the measured data with expectations from simulation.Using the expected gain in sensitivity from these results, one can predict the improvement in sensitivity to neutrino-induced RF signals and thus to the neutrino flux itself.This flux sensitivity is shown in Fig. (5) and predicts that the EVA concept will be a competitive and valuable contributor to the detection of UHE neutrinos.

2 .
Radio reflector antennas based on a toroidal geometry were first described by Kelleher and Hibbs in 1953 [20], and Peeler and Archer in 1954 [21] and summary analyses may be found in several modern antenna textbooks [22].Further details of our design process can be found in [9] and [23].

Figure 2 :
Figure 2: 3D schematic model of the EVA fullscale 18.7 Mcft super-pressure balloon.The reflector panels (light blue) are shown as a cutaway in the front portion, to reveal the internal feed array.

2 DOIFigure 2 .
Figure 2. Left: The fully inflated hang test balloon.Photogrammetry was used to determine the geometry of the ballon and reflector after inflation.Center: One panel of the feed array.Four of these sinuous patch antennas were instrumented with electronics.Right: The hang test setup with the test signal emitter dish shown in the background.

Figure 8 :
Figure 8: Direct and focused pulse as measured in our 1/20th scale EVA test, both received voltage (top) and intensity (bottom).

Figure 3 .
Figure 3. On the left is shown the incident and reflected pulse as measured during the hang test.The upper plot shows the recorded voltage and the lower shows the intensity.Shown on the right is the voltage for the incident (top) and reflected (bottom) pulse as predicted by the simulation program XFdtd [7] for the hang test geometry.

Figure 4 .
Figure 4. GRASP model power pattern for a 18.7 Mcft EVA model, using a feed with a response matched to the reflector.By using feeds matched to the antenna response of the reflector in the GRASP simualtions, the predictions of the gain can be improved up to 32 dBi as shown here.

Figure 5 .
Figure 5.Estimated flux limits for a full-scale EVA detector after 150 days of flight time.Also shown are estimated sensitivity curves for other UHE neutrino detection experiments and several flux models.