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International audience; Summer mortalities of the adult Pacific oysters Crassostrea gigas are an important economic concern. In 2015 and 2016, the mortality of hatchery origin spat comprising 4 diploid batches and 9 triploid batches as well as 12 batches of wild caught diploids was followed at 3 sites in Normandy. Abnormal mortalities (> 20%) were observed at one site only and were significantly higher in triploid animals (P < .05). Triploid oysters are believed to be partially sterile but a high level of gametogenesis was observed in all monitored triploid batches. In 2015, preliminary results revealed that triploid oysters underwent strong gametogenesis with mature gametes (stage 3) and 50% of them were non-perturbed (i.e. named alpha). In 2016, 42% of triploid oysters were alpha. This study revealed that triploid oyster maturation occurred in September, which corresponded to the time of high mortalities in Normandy.

International audience; Summer mortality episodes in adult Pacific oysters have been described since the 1950s in various farming areas. Starting in 2012, a recrudescence of mortalities in commercial-sized oysters was first observed in France and then in Italy, with seasonality extension and translation later in the year. Moribund individuals collected during an event in Italy in December 2014 showed yellowish lesions of the mantle and adductor muscle. Histological examination revealed filamentous bacteria associated with necrotic areas. Quantitative PCRs targeting OsHV-1 and Vibrio aestuarianus detected only high loads of the pathogenic bacteria in tissues of symptomatic individuals. A lower diversity of the hemolymph microbiota was also evidenced in moribund individuals, with a predominance of Vibrio and Arcobacter species. A strain of Flavobacteriaceae was isolated from all the symptomatic individuals. Sequence analysis of the 16S rRNA gene identified the strain as Tenacibaculum soleae. When strain pathogenicity was tested by injection in adult individuals, it induced mortality rates of up to 45%, even in the absence of V. aestuarianus. As mortality occurred only 11 days post-infection, further investigation is needed to determine its effective virulence in natural conditions. This is the first description of a Tenacibaculum strain associated with bivalve mortalities.

International audience; The ostreid herpesvirus 1 (OsHV-1) and variants were implicated in mass mortality affecting the young Pacific cupped oysters, Crassostrea gigas, in European countries and those around the world. From 2008 onwards, oyster mortality had greatly increased on the French coast and was associated with the detection of a new OsHV-1 variant, entitled OsHV-1 μVar. The OsHV-1 μVar is predominant in oysters; however, other OsHV-1 variants have been detected in samples collected during mortality periods or collected out of mortality periods in France, Ireland, Spain, Portugal, Italy, Mexico, United States, South Korea, Australia, and New Zealand. A retrospective study conducted on 1047 OsHV-1 specimens sampled mainly in France between 2009 and 2012, revealed 17 undescribed OsHV-1 variants found in 65 oyster samples. These specimens presented point mutations situated downstream and upstream from the microsatellite area in the C region (ORF 4/5) which were different from the OsHV-1 reference and the OsHV-1 μVar. In the present work, investigation was performed to further characterize these OsHV-1 specimens by sequencing two habitually targeted regions to study genetic polymorphism of the virus: ORF 41/42 and ORF 35-38. An OsHV-1 variant detected in six oyster samples, contained a nucleotide substitution in the C region which impacted the amino acid sequence and might modify the function of the unknown protein encoding by ORF 4. For the ORF 41/42 region, only two specimens presented a synonymous mutation in comparison with the OsHV-1 μVar. All specimens contained the same deletion with the OsHV-1 μVar in ORF 35-38. Then, a phylogenetic analysis based on the C region was performed to investigate the distribution of undescribed specimens among 21 OsHV-1 DNA sequences notified in GenBank and collected from different countries (France, Japan, New Zealand, China, Ireland, and United States) between 1995 and 2012. All analyzed samples and the OsHV-1 μVar were placed in the same group, excepted for a Japan specimen. Our results contribute to improve the description of the genetic diversity of the OsHV-1 and the C region (ORF 4/5) appears to be a better target than ORF 42/42 and 35-38 to distinguish variants between themselves.

International audience; Infections with Ostreid herpesvirus 1 (OsHV-1) microvariants in young Pacific oysters are associated with massive mortality events and significant economic losses. Previous studies, focusing on few regions of the genome, have revealed the genomic diversity of these genotypes with respect to the reference type. We used a NGS process to sequence the whole genome of the OsHV-1 µVar in infected individuals, collected during mortality events in France and Ireland. The final genome length of OsHV-1 µVar was approximately 205kbp, shorter than the reference genotype and the overall genome organisation resembled herpes simplex viruses. 94.4% similarity was observed with the OsHV-1 reference genotype. Large indels, including five deletions and three insertions were found to induce the loss and the addition of several ORFs, summed with codon substitutions in 64% of genes shared with the reference type. This diversity raises the question of the exact origin and evolution of OsHV-1 µVar.

One of the most widely European farmed mollusc, the mussel Mytilus sp., has been subjected to massive mortalities located in Charente-Maritime (France) in spring 2014. The national surveillance network for mollusc health has reported a systematic detection of V. splendidus in all dying batches. V. splendidus is the type species of a clade composed of almost 20 known strains with variable pathogenicity on bivalves. In our study, we first developed a Multi Locus Sequence Analysis (MLSA), specific to V. splendidus group, based on fragments of 5 housekeeping genes: atpA , ftsZ , mreB , rpoD and topA . This tool was validated on reference strains and compared with individual gene analyses. It allowed a useful and reliable classification of V. splendidus closely-related strains. Thanks to MLSA, we then tried to classify genetically 23 strains isolated from healthy or dying mussels. 21 were classified within the Splendidus clade: in splendidus cluster (38%), in tasmaniensis cluster (24%), in artabrorum cluster (24%) and on distinct branches (14%). All of them were tested by injection to healthy adult mussels to identify possible pathogenic strains. Experimental trials revealed the presence of a strain called M3H, allied with the splendidus cluster and able to induce mortality in mussels with rates up to 80%. The M3H virulence was demonstrated by the recovery of the injected strain in dying animals, resulting from repeated experimental infections. Further work should be now conducted to explore the pathogenicity of the M3H strain towards different mussel batches and under various conditions. Statement of relevance Our MLSA is useful to identify related V. splendidus strains

International audience; Ostreid herpesvirus 1 (OsHV-1) is a significant pathogen affecting the young Pacific oyster Crassostrea gigas, worldwide. A new variant, OsHV-1 μVar, has been associated with recurrent mortality events in Europe since 2008. Epidemiological data collection is key for global risk assessment; however little is known about health status and genotypes present in European wild oyster beds. Most studies to date have involved only cultivated individuals during mortality events, and reported low genotype diversity. With this study, conducted along the Italian coasts, we investigated for the first time the presence of OsHV-1 in European natural oyster beds. Analysis of three genomic regions revealed the presence of at least nine different genotypes, including two variants close to the OsHV-1 reference, known since the early 1990s but with no European record reported since 2010, and highlights relevant genotype diversity in natural environment. Phylogenetic analysis distinguished two distinct clusters and geographical distribution of genotypes, with the exception of a variant very closely related to the μVar, which appeared the single genotype present in all the Adriatic sites. Interestingly, these wild symptom free populations could represent, in Europe, an accessible alternative to the import of OsHV-1-resistant oyster strains from the East Pacific, the native area of C. gigas, avoiding the high-risk of non-native marine species and new pathogen introductions.

Since the summer of 2008, the mortality of young Pacific oysters Crassotrea gigas has been greatly increased on the French coast and may be linked to a microvariant of the Ostreid Herpesvirus 1 (OsHV-1), named OsHV-1 μVar. This variant is mainly characterized by a deletion of 12 consecutive nucleotides followed by a deletion of one adenine in a microsatellite area located in the C region. The present study is the continuity of a previous study that was conducted on oysters collected from July 2008 to June 2010 (Martenot et al., 2011). The purpose of the current survey is to determine the genotype of oyster samples collected from July 2010 to August 2011 and samples taken before 2008. The OsHV-1 μVar was detected in 98% of characterized samples and the two new forms of microvariants were found in the remaining 2%. The first genotype has 15 bp successive deletions and the second genotype is similar to the OsHV-1 reference with an insertion of three nucleotides. The OsHV-1 μVar Δ9 previously described in the literature was detected in a sample originating in Brittany. These observations reinforce the possibility of the emergence of different OsHV-1 variants.

French oyster farming has been experiencing severe mortalities since 2008 in Crassostrea gigas spat and juveniles mainly during the summer months. This recent phenomenon can be explained by the involvement of many factors including pathogenic agents such as the Ostreid Herpesvirus 1 (OsHV-1). From July 2008 to September 2010, 424 oyster samples received from French oyster farmers were tested to follow the concentration of OsHV-1 by quantitative real-time PCR. Comparisons of OsHV-1 loads were also made according to oyster age, ploidy, origin (hatchery-bred or wild-collected) and sample status (healthy or dying). The survey revealed that high viral loads detected in spat and juvenile oyster tissues were associated with summer mortality occurrence. No significant differences were observed in pathogen detection in relation to oyster ploidy and origin. Finally, this study allowed us to define a viral load threshold of 8.8 × 103 of OsHV-1 DNA per mg of tissue, which can be useful to oyster farmers in evaluating mortality risk during the different stages of oyster handling.

The radiofrequency quadrupole linear ion trap is a widely used device in physics and chemistry. When used for trapping of large ion clouds, the presence of anharmonic terms in the radiofrequency potential limits the total number of stored ions. In this paper, we have studied the anharmonic content of the trapping potential for different implementations of a quadrupole trap, searching for the geometry best suited for the trapping of large ion clouds. This is done by calculating the potential of a real trap using SIMION8.0, followed by a fit, which allows us to obtain the evolution of anharmonic terms for a large part of the inner volume of the trap., Accepted for publication at International Journal of Mass Spectrometry

The manipulation of trapped charged particles by electric fields is an accurate, robust, and reliable technique for many applications or experiments in high-precision spectroscopy. The transfer of an ion sample between multiple traps allows the use of a tailored environment in quantum information, cold chemistry, or frequency metrology experiments. In this article, we experimentally study the transport of ion clouds of up to $80\phantom{\rule{0.16em}{0ex}}000$ ions over a distance of 20 mm inside a linear radio-frequency trap. Ion transport is controlled by a transfer function, which is designed taking into account the local electric potentials. We observe that the ion response is very sensitive to the details of the description of the electric potential. Nevertheless, we show that fast transport---with a total duration of 100 $\ensuremath{\mu}s$---results in transport efficiencies attaining values higher than 90% of the ion number, even with large ion clouds. For clouds smaller than 2000 ions, a 100% transfer efficiency is observed. Transport induced heating, which depends on the transport duration, is also analyzed.

International audience; The Ostreid herpesvirus 1 (OsHV-1) is associated with the mortality of young oysters Crassostrea gigas in France. The virus could infect different marine bivalve species and OsHV-1 DNA was previously detected in crustacean, marine gastropod, and sediment. Nevertheless, the effect of the temperature on the stability of infectious viral particles in sea water remains unpublished. During a mortality event, dead oysters may release virus in the sea water such as free viral particles, viral particles included in cells or attached to a support like plankton, suspended particulate matters, and microalgae. Oysters placed around the infectious source may filtrate the virus and then be infected by OsHV-1. In the present study, we investigated the OsHV-1 μVar virulence in sea water at 16 °C and 25 °C. Filtered infected tissue homogenates were kept at 16 °C or 25 °C prior to injection challenge of uninfected spats. A real-time reverse transcription PCR targeting the OsHV-1 polymerase gene has been developed to confirm the death cause oysters collected during the experiment. The RNA detection of herpesvirus in their hosts is a sign of viral replication and indicates that the virus is certainly infectious. Results showed that the OsHV-1 μVar remained infectious and induced mortality after 33 h at 25 °C and 54 h at 16 °C in controlled conditions. The OsHV-1 μVar is thus able to persist in sea water and high temperatures seem to reduce its infectivity. However, the OsHV-1 μVar virulence may be modulated by biological, physical and chemical factors which are present in sea water.

A theoretical model of semiconductor lasers mounted in external cavities is developed. The modes' frequencies and gains are calculated. We study the frequency of the oscillating mode versus the laser injection current. We explain its hysteresis behavior by introducing in the model the coupling between the semiconductor gain and its index of refraction induced by fluctuations in carrier density. We also determine the influence of the laser diode output facet reflection coefficient on mode hops and frequency tuning. Theoretical simulations coincide well with experimental observations.

Parmi les differents ions proposes comme etalons de frequence dans le domaine optique, l'ion calcium est un candidat tres attractif pour la metrologie du temps et des frequences, les longueurs d'ondes concernees se trouvent en effet dans le visible ou le proche infrarouge. Il est donc possible de concevoir la partie optique de l'etalon entierement composee de lasers a diodes, l'autre partie etant formee par un piege radiofrequence confinant l'ion.

We report on a lithium triborate (LBO) second harmonic generator delivering up to 6-μW usable power at 397 nm out of 33 mW of a 794-nm diode laser radiation. The LBO crystal is placed in an external ring cavity, compensated for astigmatism and coma, to enhance the fundamental power. Three cavity input couplers were tested for impedance-matching. With the best cavity, the enhancement factor of 1.5 is measured for 794-nm radiation, leading to an enhancement factor of 225 for the 397-nm beam.

New external-cavity semiconductor lasers with self aligning techniques are described. These new techniques assure insensitivity to mechanical instabilities and permit long term reliable operation. They were tested on three different structures that we compare. Additionally we describe a −80°C cooled external-cavity laser at 729 nm and show its performances.

A STIRAP-like scheme is proposed to exploit a three-photon resonance taking place in alkaline-earth-metal ions. This scheme is designed for state transfer between the two fine structure components of the metastable D-state which are two excited states that can serve as optical or THz qu-bit. The advantage of a coherent three-photon process compared to two-photon STIRAP lies in the possibility of exact cancellation of the first order Doppler shift which opens the way for an application to a sample composed of many ions. The transfer efficiency and its dependence with experimental parameters are analyzed by numerical simulations. This efficiency is shown to reach a fidelity as high as $(1-8.10^{-5})$ with realistic parameters. The scheme is also extended to the synthesis of a linear combination of three stable or metastable states., Comment: Journal of Physics B: Atomic, Molecular and Optical Physics (2013) a paraitre

4 pages; International audience; The signal-to-noise ratio in the detection of a single ion is one of the determining factors in the speed of the detection of the ion's internal states. Its value defines the necessary interrogation time to distinguish between an ion in the "on" and an ion in the "off" state. The improvement of the detection signal by means of active optics is a new approach in single-ion experiments. They accompany further progress in the stabilization of the clock laser for the single ion interrogation.

An optical frequency/wavelength standard using the high line Q/sup 2/S/sub 1/2/-/sup 2/D/sub 5/2/ transition of Ca/sup +/ confined in a RF trap and laser cooled is discussed according to the following points: optical excitation scheme to cool the ions and probe the clock transition of an odd isotope of Ca/sup +/ and feasibility of the production of the optical excitations from laser diodes. >

The real-time polymerase chain reaction (PCR) is considered to be a suitable tool for nucleic acid quantitation because it is accurate, rapid and reliable. The reference protocol for quantitation of ostreid herpesvirus 1 in Pacific oysters Crassostrea gigas is based on a Sybr(®) Green real-time PCR developed by the IFREMER laboratory. The Frank Duncombe Departmental Laboratory has developed an alternative protocol based on TaqMan(®) chemistry (alternative technique). The quantitation limits were 1000 and 18UG/mg of tissues for the reference method and alternative protocols, respectively, and the latter protocol has a detection limit of 6UG/mg of tissues. The aim of this study was to compare the two protocols using DNA samples obtained from 210 spat. The kappa index (0.41) indicated a moderate concordance between the protocols, according to the measures of Landis and Koch. All samples that were positive by the reference protocol were also positive by the alternative protocol. Of the 76 samples that were negative by the reference protocol, 49 were positives by the alternative protocol. In conclusion, the alternative protocol is an improvement of the reference protocol in terms of sensitivity, specificity and rapidity (

International audience; A ring crystal of ions trapped in a linear multipole trap is studied as a basis for an optical frequency standard. The equilibrium conditions and cooling possibilities are discussed through an analytical model and molecular dynamics simulations. A configuration which reduces the frequency sensitivity to the fluctuations of the number of trapped ions is proposed. The systematic shifts for the electric quadrupole transition of calcium ions are evaluated for this ring configuration. This study shows that a ring of 10 or 20 ions allows to reach a short term stability better than for a single ion without introducing limiting long term fluctuations.

Ultrahigh-resolution spectroscopy is limited by the spectral quality of lasers. We describe a servo system that stabilizes a dye laser by reflection in an optical resonator, which provides an error signal. We analyze the error signal to determine the spectral performance of the dye laser, which is shown to have a Gaussian-shaped spectrum with a width of less than 4 kHz.

High finesse optical cavities are used either in reflection or in transmission for the frequency stabilization of lasers. In this paper, we calculate the transient response of the Fabry–Perot cavity after a small frequency jump of the laser in both cases. We compare the static gains and the transfer functions for small frequency fluctuations, and explain partially the good experimental observed performances of the Fabry–Perot used in reflection.

The electric quadrupole transition between the ground state and the first metastable state of a single Ca ion is an attractive choice for a frequency standard in the optical domain. Probing of the clock transition is carried out using quantum jump statistics which requires cycle times of several seconds. The spectral linewidth of the probe laser (local oscillator) must reach the hertz level for a duration at least as long as these cycle times. To reduce the TiSa laser linewidth at 729 nm, we use a KD*P electro-optic modulator (EOM) inside the laser cavity. This device shows a couple of mechanical resonances induced by piezo-electric ringing at frequencies larger than 50 kHz, limiting notably the use of the device for frequency corrections. Actual performances (Deltav < 1 kHz for tau < 1 ms) of our laser set-up allow the observation of a preliminary spectrum with Zeeman splitting of the 729 nm transition of a single trapped Ca+ ion.

Among the ions proposed for a frequency standard in the optical domain, Ca/sup +/ is one of the most favorable candidates due to its wavelengths accessible with all-solid-state laser systems. This allows to imagine an ultimate experimental setup being simple and compact. The 4S/sub 1/2/-3D/sub 5/2/ electric quadrupole transition at 729 nm with a natural linewidth below 1 Hz serves as clock transition, while laser cooling on one of the resonance lines is carried out at 397 nm. To obtain the small spectral linewidths necessary for a frequency standard performance, it is essential to reduce the residual Doppler broadening caused by the RF heating in the Paul trap. The first-order Doppler broadening of a single ion can be eliminated in the Lamb-Dicke regime where the ion's amplitude of motion is inferior to the emitted wavelength. The main condition for the access of this frequency modulation regime is a high motional frequency of the trapped ion. This is more easily obtained in a low capacitance device and thus traps with reduced dimensions are required. We have set up a miniature ion trap of the Paul-Straubel-type. The stored ions are observed by a photomultiplier in the photon-counting mode as well as by an intensified CCD camera, permitting the visualization of their spatial behavior. Laser cooling of the Ca/sup +/ ions is carried out on the 4S/sub 1/2/-4P/sup 1/2/ resonance line at 397 nm. Progress on the characterization of the trapped single ion as well as on the generation of the involved wavelengths exclusively by solid-state lasers is reported.

The purpose of this paper was to correlate the magnetic resonance imaging (MRI) characteristics of a mature brain abscess in a horse with histopathologic alterations of brain tissue. Eight months after the onset of clinical signs, MRI of the brain of a 10-month-old filly was performed. A large space-occupying lesion in the right cerebral hemisphere was identified. This space-occupying lesion was delineated by a thick and well-defined capsule that was isointense to brain parenchyma on the T1-weighted images and with a markedly hypointense on the T2-weighted images. The identification of such a capsule is highly diagnostic of a mature brain abscess. The lesion seen on MR images was confirmed at necropsy where a large abscess of the right hemisphere was observed. Streptococcus zooepidemicus and Pseudomonas aeruginosa were isolated from the abscess. Based on histopathologic examination, the signal characteristics of the capsule on T1-weighted and T2-weighted images were found to be due to the presence of numerous hemosiderin-laden macrophages. These results are in agreement with previous studies on human patients. This report confirms the value of MRI in the diagnosis of equine brain diseases.

Diode lasers with a power output superior to 100 mW are in widespread use in medical as well as research applications. However, for such diodes lasing oscillation generally occurs simultaneously in several longitudinal and transverse modes that are unsuitable for high-resolution spectroscopy. We spectrally narrow a 100-mW broad-area diode laser by first using an extended cavity and then an electrical feedback produced by a Pound-Drever-Hall stabilization on a low-finesse reference cavity. Reduction of the linewidth by more than 6 orders of magnitude is achieved (the output linewidth is narrowed from 1 THz to less than 500 kHz), making possible its use for high-resolution spectroscopy. The power and the spectral qualities of this diode laser allow us to induce quantum jumps toward the D5/2 metastable level of single Ca+ ions.

We present a 729 nm laser diode placed in a self-aligned external cavity and stabilized on a high finesse Fabry-Perot cavity in order to probe the high Q /sup 2/S/sub 1/2 /-/sup 2/D/sub 5/2/ Ca ion transition.

Singly-ionized calcium confined in a rf trap has become an ion of choice for widespread applications in spectroscopy, metrology and quantum computing [1,2]. In fact, the wavelengths of the transitions between the lowest energy levels lie in the visible and nearinfrared range, easily accessible by all-solid-state laser systems. Laser-cooling may be carried out on a strong electric-dipole transition at 397nm, while the natural linewidth of the 729nm transition which interconnects the metastable 3D5/2-state and the 4S1/2 ground-state is inferior to one Hertz. To obtain the small spectral linewidths necessary for a frequency standard performance, it is essential to access the Lamb-Dicke regime where the ion's amplitude of motion is inferior to the emitted wavelength. We have set up and thoroughly characterized the trapping performance of a miniature ion trap of the Paul-Straubel-type [3]. Two different techniques have been applied to a small, laser-cooled ion cloud: measurement of the frequencies of motion of the confined particles by application of a resonant "tickle" frequency, and scanning of the UDC-potential. The stored ions are observed by a photomultiplier in the photon-counting mode as well as by an intensified CCD camera, permitting the visualization of their spatial behavior. Our aim is to localize a single ion as precisely as possible in the trapping device. In a first step, we have used the observation of the spatial distribution of the ion's fluorescence as a monitor to reduce its micromotion in the trap. This allows for a rough compensation of static electric stray fields in the plane of observation. Correlation measurement of the emitted photons with the rf trapping frequency has been chosen for fine-tuning of the potentials applied to the compensation electrodes. In a final stage of our experiment, the laser-cooling and probing of the ions in the trap will be carried out exclusively by diode lasers. Laser cooling of the Ca ions is made on the 4S1/2-4P1/2 resonance line at 397 nm. One way to obtain this wavelength is frequency doubling of a 794 nm laser with a non-linear SHG crystal. We have set up an injection system using a broad-area laser diode (500mW) as slave laser. About 120 mW of spectrally and spatially single-mode output power at 794 nm could be reached [4]. To produce radiation at 729 nm we use a high-power broad-area laser diode (BAL) put into an external cavity. The initial spectral linewidth of the free-running laser is about 2 nm, in a Littrow-configuration cavity the linewidth is reduced to a couple of MHz. Making use of the Pound-Drever-stabilization method on an low finesse cavity (F=300), the laser linewidth is reduced to less than 400 kHz. The next step will be the stabilization using the same technique on a high finesse ULE-cavity (F=15000) which is thermally and mechanically controlled.

A self-aligned cavity is presented which shows significant improvements, in particular much less sensitivity to mechanical perturbations. Among the two studied configurations, the unfolded cavity is the more suitable regarding beam quality and reliability. Continuous tunability depends greatly on the quality of the AR coating of the diode output facet.

Publisher Summary This chapter discusses two-photon transition between the fundamental state 5d106s2S1/2 and the metastable state 5d96s22S5/2 in mercury ions confined in a radiofrequency trap. The lifetime of the metastable state is about 0,1s, and so this transition is attractive as an optical frequency standard. The chapter describes the energetic properties of the confined particles. It illustrates the calculated profile of the line, broadened and red shifted by second order Doppler effect, for different temperatures of the ions in the trap with a monochromatic laser. The chapter further illustrates width at half maximum of the observed signal in function of the temperature of the ions for different linewidths of the laser.

The purpose of our experiment is to observe the two-photon transition between the fundamental state 5d106s2S1/2 and the metastable state 5d96s22D5/2 in stored Hg+. This transition is attractive as an optical frequency standard for two reasons. First, the radiative lifetime of the 2D5/2 state is about 0.1s and second, the first-order Doppler effect cancels for a two-photon absorption. Thus the transition line consists of a very narrow carrier and we can study the second-order Doppler effect (red shift and broadening, which is estimated to be a few kHz).

Very long, one-dimensional (1D) ion chains are the basis for many applications, in particular in quantum information processing and reliable diagnostics are needed to quantify them. To that purpose, we have experimentally validated Dubin's model for very long ion chains [Phys. Rev. Lett. 71, 2753 (1993)]. This diagnostic allows to precisely determine the number of trapped ion with an accuracy of almost 1% without counting them, by measuring the ion-ion distance of the innermost particles, as well as the trapping potential along the ion chain direction. In our experiment, based on a 155 ion chain, the central 30 ions are measured to be equidistant to better than 2%, and we can determine the total number of trapped ions with a 4.5% uncertainty, completely dominated by a conservative estimation of the experimental characterisation of the trap.