Why current-carrying magnetic flux tubes gobble up plasma and become thin as a result
Why current-carrying magnetic flux tubes gobble up plasma and become thin as a result
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为什么载流磁通量管会吞噬等离子体并因此变薄
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发表时间：
2002
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通讯作者：
P. Bellan
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作者：
P. Bellan
假设电流I沿着磁通量管流动，该磁通量管具有极向磁通量psi和半径a = a（z），其中z是沿着磁通量管的轴向位置沿着。该电流产生环形磁场Bphi。结果表明，在这种情况下，非线性、非保守的J×B力使等离子体从小α区轴向加速到大α区，且加速速度正比于[偏导数] I ~ 2/[偏导数]z。因此，如果载流通量管在z = 0处凸起，在z = ±h处收缩，则等离子体将从z = ±h向z = 0加速，导致类似于两个水射流彼此指向的情况。被摄入的等离子体对流嵌入，冻结在从z = ±h到z = 0的环形磁通量中。反向流动在z = 0处碰撞并停滞，这样一来，（i）将其平移动能转化为热量，（ii）增加z[近似]0处的等离子体密度，以及（iii）增加z[近似]0处的嵌入环形通量密度。z[近似]0处的环形磁通密度的增加增加了Bphi，因此增加了z[近似]0处的磁箍缩力，从而导致a（0）的减小。因此，通量管形成轴向均匀的横截面、减小的体积、增加的密度和增加的温度。这个模型被提出来作为一个可能的假设，以解释为什么太阳日冕环被观测到轴向均匀，炎热和明亮的长期谜团。此外，有人认为，少量的尾部粒子之间的反弹接近逆流等离子体射流应费米加速到极端的能量。最后，Grad-Shafranov方程的解析解预测，当吸入的等离子体变得足够热和稠密，达到2µ 0 nkappaT/B pol 2 =（µ 0 Ia（0）/psi）2/2时，通量管会变得轴向均匀;观测到的日冕环参数与这个关系相当吻合，类似于托卡马克中的betapol = 1。
Suppose an electric current I flows along a magnetic flux tube that has poloidal flux psi and radius a = a(z), where z is the axial position along the flux tube. This current creates a toroidal magnetic field Bphi. It is shown that, in such a case, nonlinear, nonconservative J×B forces accelerate plasma axially from regions of small a to regions of large a and that this acceleration is proportional to [partial-derivative]I2/[partial-derivative]z. Thus, if a current-carrying flux tube is bulged at, say, z = 0 and constricted at, say, z = ±h, then plasma will be accelerated from z = ±h towards z = 0 resulting in a situation similar to two water jets pointed at each other. The ingested plasma convects embedded, frozen-in toroidal magnetic flux from z = ±h to z = 0. The counterdirected flows collide and stagnate at z = 0 and in so doing (i) convert their translational kinetic energy into heat, (ii) increase the plasma density at z[approximate]0, and (iii) increase the embedded toroidal flux density at z[approximate]0. The increase in toroidal flux density at z[approximate]0 increases Bphi and hence increases the magnetic pinch force at z[approximate]0 and so causes a reduction of a(0). Thus, the flux tube develops an axially uniform cross section, a decreased volume, an increased density, and an increased temperature. This model is proposed as a likely hypothesis for the long-standing mystery of why solar coronal loops are observed to be axially uniform, hot, and bright. It is furthermore argued that a small number of tail particles bouncing between the approaching counterstreaming plasma jets should be Fermi accelerated to extreme energies. Finally, analytic solution of the Grad–Shafranov equation predicts that a flux tube becomes axially uniform when the ingested plasma becomes hot and dense enough to have 2µ0nkappaT/B pol 2 = (µ0Ia(0)/psi)2/2; observed coronal loop parameters are in reasonable agreement with this relationship which is analogous to having betapol = 1 in a tokamak.