Astrophys Space Sci (2007) 307:29–34

DOI 10.1007/s10509-006-9238-1

O R I G I NA L A RT I C L E

Jet Deflection by a Quasi-Steady-State Side Windin the LaboratoryDavid J. Ampleford · Andrea Ciardi · Sergey V. Lebedev · Simon N. Bland ·Simon C. Bott · Jeremy P. Chittenden · Gareth N. Hall · Adam Frank · Eric Blackman

Received: 16 May 2006 / Accepted: 18 August 2006C© Springer Science + Business Media B.V. 2006

Abstract We present experimental data on the steady state

deflection of a highly supersonic jet by a side-wind in the lab-

oratory. The use of a long interaction region enables internal

shocks to fully cross the jet, leading to the development of

significantly more structure in the jet than in previous work

with a similar setup (Lebedev et al., 2004). The ability to

control the length of the interaction region in the laboratory

allows the switch between a regime representing a clumpy

jet or wind and a regime similar to a slowly varying mass

loss rate. The results indicate that multiple internal oblique

shocks develop in the jet and the possible formation of a sec-

ond working surface as the jet attempts to tunnel through the

ambient medium.

Keywords Hydrodynamics . ISM . Herbig . Haro objects .

Methods . Laboratory . Stars . Winds . Outflows

1 Introduction

Astrophysical observations have shown that some jets pro-

duced by protostars are not straight, and instead exhibit a

steady curvature over a significant fraction of their length

D. J. Ampleford (�)Sandia National Laboratories, Albuquerque, NM 87123-1106,USAe-mail: damplef@sandia.gov

A. CiardiObservatoire de Paris, LUTH, Meudon, 92195, France

S. V. Lebedev · S. N. Bland · S. C. Bott · J. P. Chittenden ·G. N. HallBlackett Laboratory, Imperial College, London SW7 2BW, UK

A. Frank · E. BlackmanDepartment of Physics and Astronomy, Laboratory for LaserEnergetics, University of Rochester, Rochester, NY 14627, US

(many jet radii). Deflected jets normally occur as a pair of

counter-propagating jets from a common source. These de-

flected bipolar jets fall into two categories – those with S-

shaped (Reipurth et al., 1997) and those with C-shaped sym-

metries (Bally and Reipurth, 2001). The mechanisms behind

the deflection of the C-shaped jets has been the subject of var-

ious studies; these studies have indicated that the deflection

of the many of these jets cannot be explained by an ambi-

ent magnetic field (Hurka et al., 1999), photo-ablation of the

surface of the jet (Bally and Reipurth, 2001), or a pressure

gradient in the ISM (Canto and Raga, 1996). It has emerged

that the most likely explanation for the deflection of these

jets is the effect of a ram pressure due to a side-wind as dis-

cussed by Balsara and Norman (1992) and Canto and Raga

(1995). For protostellar jets such a wind may be produced

by differential motion of the source star and the surrounding

interstellar medium. This is substantiated by observations

which show that within a nebula many C-shaped jet struc-

tures are present, each with the jets deflected back towards

the central star forming region, hence the effective wind is

produced by the motion of the stars outward through the ISM

(Bally and Reipurth, 2001).

In previous experiments we have studied the deflection of

highly supersonic jets in the laboratory using conical wire ar-

ray z-pinches and a photo-ablated CH foil (Ampleford et al.,

2002; Lebedev et al., 2004; Frank et al., 2005). The previous

work indicated that these experiments are in the correct pa-

rameter regime to study the propagation of astrophysical jets

in a side-wind, similar to the mechanism for deflection of C-

shaped jets (the experiments aim to model the propagation of

one of the jets far from the source; the formation mechanism

and other jet are neglected). An important feature observed

in the previous experiments was the presence of shocks in

the jet during the deflection (as also shown by simulations

utilizing astrophysical codes (Frank et al., 2005; Lim and

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30 Astrophys Space Sci (2007) 307:29–34

Fig. 1 (a) The experimentalsetup used by Lebedev et al.(2004), (b) illustration ofrequirements for a shock to crossthe jet and (c) the setup used inthis paper. The target is longcompared to the jet diameter,and angled to provide a uniformwind density on the jet

Raga, 1998)). In this paper we will use a modification of the

experimental setup used by Lebedev et al. (2004) to study

the deflection of a supersonic radiatively cooled jet by a side

wind that is steady state on the typical time scale of the jet;

shocks are allowed to fully evolve within the jet while the jet

is still subjected to a constant side wind.

2 Criteria for producing a steady state deflection andexperimental setup

In order to determine whether the interaction of a jet in a side

wind is steady state it is useful to consider an oblique shock

in the jet. If the jet is still influenced by the wind for the full

spatial scale required to allow a shock to fully cross the jet

then the interaction can be considered steady state. A shock

will cross the jet in a time

tcross = φ j

vs, (1)

where φ j is the jet diameter and vs is the transverse velocity of

the shock (see Fig. 1b for the setup and parameters discussed).

The maximum time that the jet is influenced by the side-

wind (of axial extent L) is Lv j

. Hence for a shock to be allowed

to cross the jet (and potentially be reflected or break-out) the

transit time of the shock should be less than the time that the

jet is influenced by the wind:

L

v j>

φ j

vs(2)

� φ j

cs(3)

where it has been assumed that the transverse shock in the

jet is weakly driven, so the shock velocity vs can be approx-

imated as the sound speed cs . This can be reformulated to

incorporate the definition of the internal Mach number of the

jet (the axial Mach number) M = v j

cs:

L

φ j� M (4)

Satisfying Equation (4) guarantees that the interaction is

steady state (it should be noted that not satisfying Eq. (4)

does not necessarily indicate that the interaction is not steady

state). Depending on the clumpiness of the jet and wind, it

is possible that C-shaped protostellar jets could fall into the

steady-state and non-steady-state regimes. For the case dis-

cussed by Lebedev et al. (2004), assuming the jet remains

in a constant wind density for the full length of the foil

(L ∼ 5 mm), then the length of the interaction was ∼10 jet

diameters, however the jet Mach number was �20 (the ac-

tual Mach number depending on heating of the jet during the

interaction). This does not satisfy Equation (4), so shocks

were unlikely to be able to cross the jet, and the experimen-

tal data suggests that they did not (Lebedev et al., 2004). To

explore a steady state interaction a longer interaction region

is required.

The overall experimental setup used in this paper is

broadly similar to that used by Lebedev et al. (2004). Current

produced by the MAGPIE generator (1MA, 240ns described

by Mitchell et al. (1996)) is passed through a conical arrange-

ment of 16 fine tungsten wires (each 18 μm in diameter). The

current and self-generated magnetic field of the array produce

a J × B force that acts on the low density coronal plasma

which surrounds each static wire producing a steady flow of

plasma (Lebedev et al., 2002a). This Lorentz J × B force has

components which are both radial and axial (Fig. 1a). The

formation of a conical shock on the array axis thermalizes the

kinetic energy associated with the radial component of the ve-

locity, leaving the axial component unaffected (Canto et al.,

1988). At the top of this conical shock a pressure gradient is

present which accelerates the flow; strong radiative cooling

enables the formation of a highly supersonic (Mach number

M � 30), well collimated outflow (Lebedev et al., 2002b).

Data from two diagnostics will be discussed in this pa-

per. A 532 nm, 0.4 ns Nd-YAG laser is used for laser shad-

owgraphy, with a schlieren cut-off of 1 × 1020 cm−3. An

XUV imaging system which is sensitive to photon energies

hν > 30 eV and has an integration time of 3 ns (Bland et al.,

2004) is also fielded.

Following the previous discussion of the ability of shocks

to cross the jet in a characteristic time-scale, we note that the

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Astrophys Space Sci (2007) 307:29–34 31

jet production process continues for many shock crossing

times (i.e. the jet itself can be considered steady-state if no

interaction occurs). Previously jets produced by this method

have been used to explore various aspects relevant to the un-

derstanding of protostellar jets, such as the effect of radiative

cooling and the effect of symmetry of convergent flows on

jet production (Lebedev et al., 2002b; Ciardi et al., 2002),

the effect of angular momentum on the jet (Ampleford et al.,

2006a), the effect of an ambient medium on jet propagation

(Ampleford et al., 2005) and the effect of a side-wind on the

jet (Lebedev et al., 2004). To impose such a side wind on the

jet a CH foil is photo-ablated by soft X-ray emission from the

wire array; the expansion of the foil causes the wind to im-

pact on the jet, as discussed in more detail by Lebedev et al.

(2004). In this paper we expand on our previous discussion

of jet deflection experiments, with the aim of investigating

the dynamics of jet deflection in a regime that is more suited

to some astrophysical jets, namely in a configuration which

allows shocks to propagate across the jet whilst the jet is still

under the influence of the side-wind. To increase the axial

extent of the wind the size of the foil is increased, however

to ensure that the jet is propagating through a near-constant

wind density it is necessary to angle the foil with respect to

the initial jet axis (Fig. 1c). This alteration to the foil angle

also changes the position of the stagnation point (the point

where the velocities of the of jet and wind are perpendicular)

so it can be better diagnosed. The jet and wind parameters are

expected to be broadly similar to those discussed by Lebedev

et al. (2004).

3 Dynamics of jet propagation in a side-wind

Figure 2a shows a schlieren image of the deflection of a jet

in this modified configuration. In the image the jet is seen

propagating vertically from array, which is below the base of

the image. The side-wind is produced by photo-ablation of

the CH foil and propagates right to left (away from the foil),

with a small downward component. As the jet is subjected

to the side wind the jet is steadily deflected in the direction

of the wind motion, as drawn on Fig. 2b (see Lebedev et al.,

2004 for a more detailed discussion of the basic deflection).

The plasma jet in these experiments is highly supersonic,

hence any perturbation to it, such as the ram pressure due

to the side wind should generate strong shocks in the flow

(as was observed by (Lebedev et al., 2004)). The schlieren

diagnostic used in Fig. 2a is sensitive to density gradients

in the plasma, such as those produced by these strong

shocks. Correlation of these structures with increased XUV

emission (Fig. 2c) is consistent with the thermalization of

kinetic energy in these shocks.

The interaction of the jet is much more complex than was

seen in the previous study using a shorter wind (Lebedev

et al., 2004), with numerous structures now present between

the jet and foil. For clarity this image has been repeated in

Fig. 2b, with the many different features that will be discussed

drawn and labelled. The axial position of the tip of the curved

portion of the jet (at the left of the interaction) corresponds

to the expected axial position of the tip of a jet propagating

in vacuum.

At the base of the target we expect a downward component

to the wind (due to the angle of the foil and divergence). On

the schlieren image (Fig. 2a) two shocks are present where

the expanding wind meets the upwards travelling halo plasma

surrounding the jet as it exits the wire array (labelled Haloshocks in Fig. 2b). The lower of these two shocks is a shock in

the halo and the upper is a reverse shock in the wind (they are

marked Halo shock and Wind shock respectively in Fig. 2d).

In the next three sections we will describe the other struc-

tures observed in the interaction.

3.1 Internal oblique shock formation

On the high magnification image (Fig. 2d) we see that there

is an internal shock in the centre of the jet (labelled OS1).

Fig. 2 Shocks within the jet shown in both (a) low and (d) high mag-nification schlieren images (both at 343 ns). (b) is a repeat of (a) withlabels on the image which are discussed in the text. (c) shows an XUVemission image (Bland et al., 2004) at 380 ns

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32 Astrophys Space Sci (2007) 307:29–34

As with the earlier experiments with a shorter interaction

this is likely to be the oblique internal shock responsible

for the initial deflection. We see that this internal shock

is not straight, but instead bends each way by a few de-

grees. It is interesting to note that the most pronounced of

these bends coincides with a continuation of the shock in

the halo flow. Thus it is likely that this bend in the inter-

nal shock is associated with a change in the wind density

and hence ram pressure – further experiments would be re-

quired to investigate the effect of variations in the ambient

density.

Further along the jet-wind interaction on the low mag-

nification image we see that more structure is present; one

obvious shock is labelled OS2. In this image it is unclear

what the significance of this shocks is, however we can un-

derstand this better if we look at XUV emission. Figure 2c

shows a gated XUV emission image from the same exper-

iment, however 40 ns after the schlieren image. This image

was taken at 22.5◦ from the plane containing the laser probe

beam and foil, hence some emission from the surface of the

foil can be seen in the XUV image. The structure seen in the

XUV image is broadly similar to that in the schlieren image,

however these shock features have developed slightly. Again

we see the shock previously labelled OS2; it appears that this

is static in time, and remains almost parallel to the jet, so is

likely to be a second internal oblique shock in the jet (OS2),

further deflecting the jet.

3.2 Formation of a new working surface

The nature of the shock WS2 becomes clear if we look at

simulations of a jet in a side wind. Figure 3 shows a 2D slice

taken from a 3D HD simulation of a jet propagating in a side-

wind. For simplicity this simulation has a constant mass flux

in the jet, constant jet injection velocity and uniform wind

density and velocity. In these simulations we see that as the

jet propagates the upwind surface becomes unstable and a

second (and in the last frame a third) working surface begins

to form. This is similar to what is observed in Fig. 2a and c –

the feature labelled WS2 is likely to be the formation of this

secondary working surface (the first working surface being

at the head of the jet, labelled WS1). The development of

this structure with time can be seen experimentally in Fig. 4,

which shows a series of gated XUV images (for a different

experiment). The development of a second working surface

has also been observed for a different setup using a conical

wire array (Ampleford et al., 2005).

Fig. 3 Simulations of a jet in propagating in a side-wind. 2D slice from a 3D HD simulation (Chittenden et al., 2004) with uniform jet and wind(i.e. different from the experiments)

Fig. 4 Development of the jet-wind interaction with time is shown experimentally by time resolved XUV emission (hν > 30 eV)

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Astrophys Space Sci (2007) 307:29–34 33

Fig. 5 (a) Low and (b) high magnification schlieren images showingthe interaction of the low density, un-collapsed tip of the jet (from adifferent experiment to all other images)

3.3 Interaction of a low density jet with a denser wind

If we look above the tip of the jet on the earlier schlieren

image (Fig. 2a) we see that more shocks have formed. The

axial position of this material implies that it was ejected be-

fore the well formed jet that has previously been discussed

(Lebedev et al., 2002b), and instead consists of material that

reached the axis before the conical shock was well formed

(Bott et al., 2006).

On a different experiment we can see this interaction in

more detail on a high magnification schlieren image (Fig. 5).

This image shows the low density jet through shadowgra-

phy, and shocks through schlieren effect. We see that there

are actually two shocks present. The shock furthest from the

foil is an internal shock in the jet, producing yet another re-

gion of deflection. The closest shock to the foil is a reverse

shock forming in the wind. It is believed that when this jet

material passed through the lower area of wind the ambi-

ent material was of sufficiently low density that either there

was not enough momentum in the wind at that time or the

mean free path of the jet was too long to be deflected (i.e. a

particle effect that cannot be modeled using a hydrodynamic

simulation). Also on this experiment the low magnification

schlieren image shows a well defined reverse shock in the

wind near the first deflection of the jet.

4 Conclusions

We have discussed experimental data for the deflection of

highly supersonic jets by a cross wind where the cross wind is

effectively continuous in relation to the typical spatial scales

of the jet. Such a configuration could be of interest in mod-

eling the propagation of a jet in a side-wind that is neither

clumpy or gusty (experiments that reach the inverse regime

were discussed in Lebedev et al. (2004)). The data has shown

that many different shocks are formed in the interaction. It is

interesting to note that experiments utilizing two very differ-

ent ambient configurations (here and Ampleford et al. (2005))

both lead to the formation of secondary working surfaces in

the jet. A laboratory 3D HD code has recovered many of the

features of the present experiments; the data should provide

a useful testbed for astrophysical computer simulations of

such a case. Future experiments will aim to follow the evolu-

tion of shocks more closely and attempt to evaluate the shock

jump conditions.

Acknowledgements This research was sponsored by the NNSA underDOE Cooperative Agreement DE-F03-02NA00057 and in part by theEuropean Communitys Marie Curie Actions – Human resource andmobility within the JETSET (Jet Simulations, Experiments and Theory)network under contract MRTN-CT-2004 005592. Sandia is a multi-program laboratory operated by Sandia Corporation, a Lockheed MartinCompany, for the US DOE’s National Nuclear Security Administrationunder contract DE-AC04-94AL85000.

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